BACKGROUND
in the mouse olfactory system, each olfactory sensory neuron  extends an axon from the epithelium to the olfactory bulb to form synapses within structures called glomeruli  <cit> .
the pattern of glomerular convergence is remarkably stereotyped  <cit>  and is strongly dependent upon the specific odorant receptor expressed by the osn  <cit> .
but despite the number of cues that have been found that are expressed by osns or along the path traveled by osns to the bulb, surprisingly little is known about the role of the olfactory bulb itself in this process  <cit> .
transplantation and bulbectomy studies have observed the formation of glomerular-like structures by osns within cortical tissue  <cit> .
genetic ablation of mitral and tufted projection neurons showed that topographically correct convergence by p2-expressing osns still occurred in the absence of these target cells  <cit> .
disruption of the laminar nature of the mitral layer also did not affect convergence by p2-expressing osns  <cit> .
these results have suggested that the olfactory bulb plays a minimal role, if any, in targeting  <cit> .
on the other hand, studies ablating most of the bulb, as opposed to eliminating only projection neurons, have observed topographically incorrect convergence by p2-expressing osns, suggesting a role for the bulb in determining the ultimate location of osns  <cit> .
tracing studies have long suggested the existence of zone-to-zone mapping of osns onto corresponding domains within the bulb  <cit> .
in drosophila, dscam expression in the antennal lobe is critical for proper olfactory receptor neuron targeting  <cit> , providing the first data that the target structure is important for convergence.
these experiments in drosophila suggest that the mammalian bulb may also provide guidance information to osns  <cit> .
part of the uncertainty surrounding the role of the bulb in guidance is that it is unclear what other bulbar cells in addition to mitral and tufted projection neurons could provide targeting information.
while radial glia have long been suggested as one possibility  <cit> , cells in the external plexiform  and glomerular  layers also possess many attractive characteristics.
first, in addition to projection neurons, other cells in the epl and gl form synapses with osns  <cit> .
second, recent studies point to a diversity of cell-types within these layers  <cit> , and electrophysiological studies have shown the presence of different types of interneurons within the epl  <cit> .
third, some glomerular cell populations migrate into the bulb much earlier than previously thought  <cit> .
and finally, it has been shown that external tufted cells indirectly connect glomeruli that are innervated by osns expressing a common receptor  <cit> .
collectively, these data suggest that little is known about the different sub-types and developmental profiles of these cells.
in one model, a subset of these epl and/or gl cells could provide guidance information to osns.
if this hypothesis is correct, then it should be possible to identify genes expressed by cells within these layers that are important for guidance and targeting.
here we have undertaken a high resolution screen for genes that spatially define distinct regions of the epl and gl.
we used laser microdissection  coupled with rna amplification, microarrays, bioinformatics, and in situ hybridization to search for genes that are differentially expressed by cells within these layers.
we identified  <dig> such cdnas and show that the superficial layers of the bulb can be subdivided into large regions based upon the various expression patterns.
the patterns demonstrate the existence of gradients, localized expression domains, and medial-lateral symmetry within the olfactory bulb.
moreover, these patterns are maintained in cells that do not express tbr <dig>  consistent with the interpretation that they are expressed in non-projection neurons.
we further show, using a chemical ablation paradigm, evidence consistent with the interpretation that these genes are expressed in cells that interact with osns.
RESULTS
spatial reconstruction of gene expression using laser microdissection
we performed our screen at embryonic day  <dig>  , as this is the first time that convergence is observed in vivo  <cit> .
as the epl and gl are not distinguishable at this stage  <cit>  our samples contained cells from both developing layers .
we will refer to the regions where these samples were taken as the nascent epl/gl.
only sections up to the front of the accessory olfactory bulb were used.
these ~  <dig> coronal sections were binned into three groups corresponding to anterior, middle, and posterior bulb .
as no prior genes have been identified as being spatially differentially expressed within the nascent epl/gl, the size of each lmd sample, orientation, and location was chosen based on the ability to easily separate individual lmd samples.
we also considered the known pattern of glomerular innervation by osns, which is shifted ~ 45° from vertical.
we sampled cells from the dorsolateral , dorsomedial , ventrolateral , and ventromedial  nascent epl/gl within each section .
three sections were chosen from each group and four lmd isolates were obtained per section for a total of  <dig> isolates.
rna was isolated from the ~ 30– <dig> cells within each sample, amplified, and applied to microarrays .
we synthesized the pool of microarray data into a three-dimensional representation of gene expression within the bulb .
as a result, all  <dig>  cdnas on the array could be queried to identify those whose expression patterns indicate differential expression within the bulb.
for example, we examined whether or not expression for any given gene was higher or lower in the vm aspect relative to the remainder of the bulb.
genes were grouped into four categories  and the top  <dig> from each were tested by in situ hybridization.
of the  <dig> cdnas, nine did not have any detectable signal.
of the remaining  <dig>   <dig> were spatially differentially expressed .
in one category , 83% were spatially differentially expressed.
these results speak to the sensitivity of our analysis, as all samples differ essentially only by position.
we also examined a small subset of genes chosen based on annotation.
these genes, which were all ranked within the top  <dig>  were selected because they correspond to known, transmembrane-bound molecules.
as such, they would be more likely to directly interact with osns and influence targeting.
in total,  <dig> cdnas were found to be spatially varied in expression.
as expected, all identified cdnas were differentially expressed within the nascent epl/gl, as confirmed by dapi labeling and double-label in situ hybridization .
expression was not necessarily exclusive to these layers, and was often observed in the mitral layer as well.
spatial differential expression of candidate cues at e <dig>
we used gap <dig> expression  <cit>  and dapi staining as controls for our initial experiments.
we found gap <dig> to be evenly distributed throughout the mitral and epl layers .
dapi staining showed that slightly more cells are observed laterally in the anterior bulb, but in the middle and posterior bulb the distribution of cells appears uniform .
these controls served to distinguish whether or not any of our observed in situ patterns simply reflected the presence of increased numbers of cells at a given location within the bulb.
we identified a diversity of patterns that are spatially differentially expressed within the nascent epl/gl.
we found that many of our positive cdnas corresponded to genes that are either secreted or are transmembrane-bound proteins.
gicerin is an immunoglobulin superfamily member that is a homophilic cell-adhesion molecule  <cit> .
gicerin is expressed differentially in the lateral aspect of the anterior bulb .
however, not all cells in this lateral domain express gicerin, as the pattern appears stippled.
expression is also observed in the nerve layer.
strikingly, in the middle bulb, expression of gicerin appears symmetric in some areas about the ml and dv axes, with high levels observed in the dm and vl bulb .
in more posterior bulb, these expression domains gradually become connected with one another along the dl surface, forming a crescent-shaped expression domain .
expression in the vm aspect, however, remains low throughout the ap extent of the bulb.
connective tissue growth factor, or ctgf  <cit> , is a secreted protein that is a member of a small family of genes which interact with the extracellular matrix and growth factors, and are important for angiogenesis  <cit> .
in the anterior bulb, ctgf is localized to the lateral aspect .
like gicerin, this expression is not uniform and significantly higher expression occurs within a subset of these cells.
in the middle and posterior bulb, as with gicerin, expression is symmetric as higher ctgf expression is detected along the dm and vl surfaces .
ctgf is weaker in the dl aspect and noticeably reduced in the vm bulb.
ctgf is also expressed in cartilage and connective tissue surrounding the bulb.
zinc finger csl-type containing  <dig>  is a protein of unknown function  <cit> .
throughout the ap axis, zcsl <dig> is expressed in a gradient-like pattern that extends from the dm to the vl bulb , although expression tends to shift more ventrally in the middle bulb .
protocadherins have been shown to possess both homophilic and heterophilic adhesion properties  <cit> .
protocadherin  <dig>  is expressed throughout the bulb, but is enhanced in the anterior bulb along the vm aspect in a small subset of cells that extend into the epl .
in the middle bulb, a small subset of cells in the vl nerve layer expresses this gene .
protocadherin  <dig>  is expressed anteriorly most strongly along the medial aspect, although some signal can be seen laterally .
just beyond the anterior bulb, this pattern is reversed and pcdh <dig> becomes strongest laterally, although some expression can still be detected medially .
this pattern is maintained throughout the posterior bulb .
jagged <dig>  a ligand for notch, has been shown to be required for proper endothelial branching during angiogenesis  <cit> .
in the anterior bulb, jagged <dig> is symmetrically expressed about the ml axis .
expression of jagged <dig> in these domains appears more even than that of gicerin or ctgf, and no obvious high or low-expressing cells are observed within the expression domain.
in the middle bulb, jagged <dig> maintains this ml symmetry, but weaker expression connects the two subdomains .
a similar pattern is observed in posterior bulb .
although many of our genes are expressed in the nascent mitral layer, we emphasize that at least some expressing cells are located within the developing epl.
all of our in situ hybridization experiments were counterstained with dapi so that we could correlate the location of the observed signal with the position of the nascent mitral and external plexiform layers.
delta <dig>  another ligand for notch  <cit> , is expressed primarily within the mitral layer, and does not extend into the epl .
this can be seen both by dapi staining and by the relatively compact expression pattern for this gene .
jagged <dig>  on the other hand, extends beyond the mitral layer into the epl, and its expression domain is broader than that of delta <dig> .
we further demonstrate that these genes are expressed in the nascent epl/gl using double-label experiments and temporal analysis of their expression patterns, as described below.
these expression patterns underscore the strong spatial heterogeneity within the external plexiform layer.
moreover, they emphasize that there is variation along all three axes of the bulb.
they also show for the first time that the bulb does indeed express genes that appear symmetric about the ml axis.
a spatially differentially expressed putative microrna
among the remaining  <dig> cdnas, significantly enhanced expression was observed along the dv axis.
the patterns observed among all of these positives were remarkably similar, and one representative example is shown in figure  <dig>  within the nascent epl/gl, these genes are expressed in a punctate pattern .
this punctate expression occurs throughout the nascent epl/gl, but significantly fewer cells were labeled in the vm aspect .
bioinformatic analysis showed that all  <dig> cdnas contained intronic or 3' untranslated sequence .
pair-wise alignment identified a  <dig> basepair common element that contained two microrna  sequences, mir- <dig> and mir- <dig> .
the remainder of the  <dig> bp element was ~ 95% identical to sequence immediately upstream of the mir- <dig> sequence  <cit> , corresponding to what we assume is part of the primary mirna  sequence for mir- <dig> .
the expression pattern we observed may therefore reflect the expression of mir- <dig> pri-mirna.
we obtained locked nucleic acid -modified oligos corresponding to mir- <dig> and mir- <dig> .
we performed in situ hybridization with the lna oligos as well as with the  <dig> basepair common element.
hybridization patterns using the common element were identical to that observed with our  <dig> cdnas .
although the signal for the mir- <dig> probe was weaker than that of the common element, punctate signal was also observed in the nascent epl/gl in a similar pattern as well .
these results contrast with those for the mir- <dig> probe, which bound weakly and uniformly to the nerve layer and epl .
the mirna in situ patterns that have been published in the mouse generally show uniform expression within a given layer or structure , and/or are detected in specific cell-types.
the pattern of the mir- <dig> probe suggests, however, that mirnas may also be present selectively within subsets of cells in a spatially distinct fashion.
additional studies are required to determine the possible targets of this putative mirna.
three-dimensional reconstruction of gene expression
our identified expression patterns show that the bulb can be subdivided into different domains that are distinguished molecularly by the expression of these candidate genes.
these domains have not been previously described, as only a handful of genes are known to be differentially expressed within a given layer of the bulb  <cit> .
we were surprised by the complexity of the observed patterns, as prior mapping studies had suggested the existence of dv cues in the bulb  <cit> , and theoretical models had suggested the existence of symmetric cues to label medial and lateral bulb  <cit> .
our observed expression patterns, however, do not readily fit any of these prior suggested models.
for example, gicerin symmetrically labels the dm and vl bulb, but symmetry about these axes has not been previously predicted.
to obtain a more detailed understanding of the expression domains distinguished by these individual genes, we generated a three-dimensional reconstruction of gene expression for these genes within the bulb.
to produce these recreations, coronal sections from e <dig>  bulbs were obtained every 60– <dig> μm along the ap extent of the bulb for every gene.
the images of the hybridized sections were combined using imagej .
the projection of the stacked images was then superimposed upon a cartoon representation of the olfactory bulb for ease of visualization.
a subset of the images used to create the jagged <dig> and ctgf stacks are shown in figure 4h–i, and the corresponding reconstructions are shown in figure 4e–f.
reconstructions of the other genes are also shown in figure  <dig>  these reconstructions show that domains of the bulb along the ap, dv, and ml axes can be uniquely specified by the combinatorial expression of these genes.
these data support a model where molecular heterogeneity in the nascent epl/gl would provide spatial guidance information to osn axons as they converge within the olfactory bulb.
temporally dynamic expression of candidate cues during development – e14
the first osns are known to extend an axon towards the developing bulb at e11– <dig>  and "wait" at the periphery as the bulb develops  <cit> .
sema3a expression at the periphery of the bulb, expressed by ensheathing glia, plays a role in maintaining osns outside of the bulb  <cit> .
it is known that mitral cell reorientation requires innervation by a small population of early osns  <cit>  that enter the bulb in a medial-to-lateral progression  <cit> .
why this innervation proceeds initially along the medial surface is not known.
one possibility would be that additional cues, in conjunction with sema3a expression, serve to guide osn axons to their position within the bulb.
we therefore examined the expression of our candidate cues at e <dig> for differential expression.
all of our identified genes are differentially expressed at e <dig> in the superficial mantle layer.
the putative mirna containing sequence, for example, is expressed strongly in the dl bulb, with weak expression in the vm aspect .
gicerin is expressed more highly in the dl bulb as well, with weak expression in vl bulb .
ctgf, on the other hand, is expressed more highly in the dm bulb .
jagged <dig> is expressed more strongly in the ventral aspect .
pcdh <dig> and pcdh <dig> label complementary domains .
pcdh <dig> strongly labels two populations of cells in the vm bulb, with lighter, more uniform staining observed internally, and stronger, more selective staining externally.
faint staining is also observed in the dl bulb, symmetrically opposing the vm expression.
pcdh <dig>  in contrast, strongly labels a large number of cells throughout the bulb except those along the vm surface.
several of the observed patterns at e <dig> mimic those seen at e <dig> .
for example, the putative mirna is weakly expressed in the vm bulb at both stages.
similarly, the dual-layer expression of pcdh <dig> in the vm bulb at e <dig> is reflected at e <dig> .
jagged <dig> is ventrally expressed at both stages.
on the other hand, other genes change in expression between e <dig> and e <dig> .
gicerin, for example, is expressed most strongly at e <dig> in dl bulb, but at e <dig>  is present more strongly in the dm and vl bulb.
ctgf also shifts in expression along different axes from e <dig> to e <dig> , increasing in expression laterally and anteriorly as development proceeds.
temporally dynamic expression of candidate cues during development – p3
we next examined the expression of these various candidate cues at postnatal stages.
while convergence has been shown to occur at e <dig>  for osns expressing the p <dig> odorant receptor, other osn populations do not converge until later stages, up to postnatal day  <dig>  <cit> .
the existence of relatively late converging osns prompted us to examine the expression of these genes at p <dig>
at p <dig>  gicerin is expressed in the anterior bulb more highly in the lateral aspect .
in the middle bulb, expression can be seen in a complementary, symmetric pattern .
ctgf is also expressed differentially in the lateral aspect of the anterior bulb , but in the middle and posterior bulb, this expression becomes more uniform .
in contrast to gicerin and ctgf, the putative mirna is now expressed uniformly within the epl .
similarly, zcsl <dig> , pcdh <dig>  and pcdh <dig> , are uniformly expressed.
however, jagged <dig> is still differentially expressed at p <dig>  but unlike it's expression at e <dig> and e <dig> , higher expression is observed along the dorsal aspect .
as with the data regarding expression at e <dig>  these patterns indicate the temporally dynamic nature of expression by these various genes.
gicerin, for example, is expressed most highly in the dl bulb at e <dig>  but is dm and vl in both the e <dig>  and p <dig> bulb.
pcdh <dig> is vm and dl at e <dig>  vm at e <dig>  and uniform at p <dig>  we note that, consistent with our initial observations at e <dig> , zcsl <dig>  the putative mirna, jagged <dig>  pcdh <dig>  and pcdh <dig> expression can clearly be detected in the epl, gl, and mitral layers.
lack of precise colocalization with tbr1
we have hypothesized that cells within the epl express guidance cues for osns.
genetic studies using the tbr <dig> mutant mouse resulted in the ablation of most projection neurons in the bulb  <cit> .
projection neurons include mitral cells as well as internal and middle tufted cells  <cit> .
as targeting is unaffected in tbr <dig> mutant mice, a major problem has been to determine what cell-types within the bulb remained that could be important for this process.
if our genes are expressed in cells that provide guidance information to osns, we would predict that at least some cells that express our candidate genes do not express tbr <dig>  and would not be ablated in the mutant.
prior studies have shown that tbr <dig> is expressed in cells distributed from the ventricle to the nascent mitral/tufted layer at e <dig>   <cit> .
we found a similar distribution of tbr1-positive cells at e <dig> .
we performed double-label experiments with tbr <dig> and with our genes to determine what overlap exists in expression.
zcsl <dig> is enhanced in expression laterally .
in contrast, tbr <dig> expression  in this same section is widespread in the granule and mitral/tufted layer , as previously described  <cit> .
merging the two images shows that expression of zcsl <dig> occurs in cells that extend into the nascent epl/gl which do not express tbr <dig> .
similarly, pcdh <dig> is enhanced in vm cells in the anterior bulb .
a subset of these cells do not overlap in expression with tbr <dig> .
the putative mirna identified in our screen is also expressed in nascent epl/gl cells that do not express tbr <dig> .
finally, a subset of ctgf  and gicerin  expression also occurs within cells that do not express tbr <dig>  thus, while overlap with tbr1-expressing cells does occur, tbr1-negative cells that express our genes are present within the bulb.
perhaps most interestingly, these tbr1-negative cells are primarily located in the regions where the differential expression of our genes appears strongest.
for example, zcsl <dig> is spatially differentially expressed in the lateral bulb, and many of these cells have no detectable tbr <dig> expression .
expression of candidate cues are affected in chemically ablated mice
while we have shown the existence of a number of genes demarcating different domains within the bulb, it remains unknown whether or not they are functionally relevant for osn guidance.
unfortunately, no in vitro system exists in the olfactory system to test this hypothesis, and genetic analysis has remained the primary means for assessing function.
to more rapidly determine the potential role of these genes in targeting, we decided to employ a chemical ablation approach.
the olfactory epithelium is continuously being regenerated during adult life.
prior studies have shown that tyrosine hydroxylase  expression is drastically reduced following intranasal ablation with zinc sulfate  <cit> .
as regeneration proceeds, th expression gradually recovers.
although th is an intracellular enzyme, it has been proposed that this recovery is dependent upon afferent input from regenerating osns.
this process, although poorly understood, is thought to be a result of transneuronal regulation of glomerular neurons by osns  <cit> .
we decided to ask whether or not this ablation paradigm could be used to determine whether our candidate genes are expressed in cells that interact with osns.
we would expect that if they were, then their expression, like that of th, would change in response to osn deafferentation.
we ablated osns in one nares of eight-week old mice with a solution of zinc sulfate, leaving the other nares alone.
we determined the success of the ablation based on expression of omp in the epithelium.
as shown in the right half of fig.
8a, at  <dig> days post-ablation, essentially no omp is observed in the ablated epithelium.
small, isolated pockets of omp expression were occasionally detected in some ablated epithelia, presumably due to a lack of access to these regions by the zinc sulfate solution .
by  <dig> days post-ablation, omp expression has started to recover, although this expression is still somewhat thinner when compared to the control side .
we did not examine later timepoints.
we found that the rate of omp recovery was somewhat variable, with some epithelia showing omp expression by  <dig> days, while others showed little or no expression.
we therefore based our experiments in the bulb on the pattern of omp expression in the epithelium, and not by the number of days post-ablation.
we found the bulb on the ablated side is smaller as previously reported  <cit>  due to a significant reduction in the size of the nerve and glomerular layers .
based on dapi staining, the number of cells in the glomerular layer did not appear to be reduced , consistent with previous studies showing that little or no cell death results from the ablation process  <cit> .
we found that glur <dig> expression appeared to be reduced in the glomerular layer and increased in the mitral layer at  <dig> days  but was equal in expression between ablated and control bulbs by  <dig> days .
this is reminiscent of alterations in glur <dig> expression in naris occluded animals  <cit> .
we confirmed that th was absent from the ablated side at  <dig> days  but recovered by  <dig> days post-ablation .
we then tested a subset of our genes on these ablated animals.
interestingly, we found a variety of different responses to the ablation.
ctgf is expressed in the adult in the glomerular and mitral layers .
this expression is absent in bulbs on the ablated side at  <dig> days  but recovers by  <dig> days .
pcdh <dig> is expressed in the glomerular layer in adults, with weaker, more scattered expression in the mitral layer .
like ctgf, both layers are affected in the ablated side at  <dig> days , but expression recovers by  <dig> days post-ablation .
pcdh <dig> is also expressed in the glomerular and mitral layers .
however, in the ablated side, only the glomerular layer expression of pcdh <dig> is affected, while mitral layer expression remains constant .
expression in the glomerular layer returns to control levels by  <dig> days .
we did not find any differences in expression for the putative mirna or zcsl <dig> in bulbs in ablated animals .
based on prior studies showing th regulation by osn innervation, we conclude that several of our genes are likely to be expressed in cells that interact directly with osns.
pcdh <dig> and pcdh <dig> are expressed in subsets of osns
two of our genes, pcdh <dig> and pcdh <dig>  are members of the cadherin superfamily.
protocadherins are known to possess weak homophilic adhesion properties in vitro  <cit> .
in one model, protocadherins expressed by osns would bind their cognate partner expressed by nascent epl/gl cells within the bulb.
the specificity of these interactions would contribute towards the final glomerular convergence pattern.
we therefore examined pcdh <dig> and pcdh <dig> expression in the epithelium.
interestingly, we found both genes are expressed in subsets of osns .
double-label fluorescent in situ hybridization shows that, although some cells express both pcdh <dig> and pcdh <dig>  others express one or the other protocadherin .
within the bulb, fluorescent double-label in situ experiments show a similar phenotype, as pcdh <dig> expression overlaps with that of pcdh <dig>  but distinct domains of the bulb are uniquely labelled with one or the other protocadherin .
the expression of these cadherins in both the epithelium and in the bulb is consistent with a model in which these molecules contribute towards the establishment of the topographic map.
discussion
to our knowledge, no cell-type or gene expressed within the bulb has been functionally proven to provide spatial guidance information to osns.
sema3a, for example, has been genetically shown to affect osn targeting, but this is due to its expression in ensheathing glia  <cit>  which migrate to the nerve layer from the epithelium.
a number of genes are uniformly expressed within cells derived from the bulb , but it is difficult to understand how, given this uniform expression, these genes would guide osn axons to different domains within the bulb.
the lack of viable potential guidance cues has led instead to genetic approaches that ablated either whole classes of cell-types within the bulb  or eliminated much of the bulb entirely .
the absence of suitable molecular candidates that could potentially guide osn axons to subdomains of the bulb has strongly inhibited any study of the role of the bulb in guidance.
in contrast, studies manipulating odorant receptor expression have led to models where the odorant receptor plays a significant role in guidance  <cit> .
the recent discovery that the kirrel 2/ <dig> and ephrin/eph a <dig> expression are controlled by odorant receptor activity provides new insight into how receptor choice influences glomerular positioning.
however, in all of these receptor swap experiments, it is clear that the receptor alone does not mediate all aspects of glomerular targeting.
the dorsal-ventral position of the osn within the epithelium is thought to provide position-dependent information as well  <cit> .
but what genes comprise these position-dependent determinants, and what potential partners are bound by such determinants are still unknown.
moreover, why osns innervate both a medial and a lateral glomerulus has not been explained by studies manipulating odorant receptor expression.
the presence of gradients  <cit>  or of mirror-symmetric expression patterns within the bulb  <cit>  have been suggested as potential explanations for how topographic osn projection patterns are established.
however, considerable disagreement exists over the role of the bulb in guidance  <cit> .
the role of bulbar projection neurons in guiding osn axons has been examined using different genetic paradigms  <cit> .
these studies have suggested that these cells are not necessary for topographic convergence of osns.
while these analyses only examined osns that express the p <dig> odorant receptor, mitral cell tracing studies also do not necessarily support a role for these cells in guidance.
mitral cell dendritic arbors initially are distributed among several glomeruli, but are refined during the first week of postnatal development to a single glomerulus  <cit> .
it is difficult to understand, if mitral cells do indeed express the primary cues needed for osn guidance, why such cues would be distributed among multiple glomeruli initially, and the dendritic arbor refined only after convergence has been completed.
but despite the fact that these studies indicate that mitral cells may not play a primary role in guidance, genetic ablation studies removing the entire bulb  <cit>  have shown that topographically correct convergence by p2-expressing osns does not occur.
one interpretion of these results is that other cell-types within the bulb are important for providing guidance information to osns.
we theorized that these cells correspond to those in the epl and gl.
in this hypothesis-driven microarray-based experiment, we show that the nascent epl/gl of the bulb can be subdivided into multiple domains based on the expression of our identified genes.
at e <dig> , the first time that convergence is observed  <cit> , the anterior bulb can be distinguished molecularly both laterally  and medially .
in the middle and posterior bulb, zcsl <dig> is expressed in a gradient that extends from dorsal to ventrolateral, while a probe containing the mir- <dig> sequence is expressed in a subset of cells that span a similar region.
perhaps most interestingly, we find that gicerin, ctgf, and jagged <dig> are expressed in a symmetric manner.
while it is well-known that osns project to medial and lateral glomeruli, the existence of mirror-symmetric cues has, to date, only been hypothesized  <cit> .
however, our patterns suggest that this medial-lateral symmetry is more complex than previously thought.
gicerin and ctgf are roughly symmetric about both the dv and ml axes, while jagged <dig> appears symmetric only around the ml axis.
thus, each hemi-bulb can be further distinguished based on dorsal or ventral expression of gicerin and ctgf.
several lines of evidence are consistent with the expression of these candidate cues within cells of the epl.
first, we show by counterstaining with dapi that expression of these genes does not colocalize precisely with the nascent mitral layer.
second, we show at p <dig> and in adult that many of these genes are expressed in the epl and gl, consistent with their expression at e <dig>  in the nascent epl/gl.
finally, we show that there are populations of cells at e <dig>  that express our candidate genes but do not express tbr <dig>  a marker for projection neurons.
thus, we have shown using a variety of approaches that, in addition to mitral layer expression, nascent epl/gl cells also express these genes.
interestingly, in addition to the fact that not all cells that express our genes express tbr <dig>  the cells that do not express tbr <dig> are those that show most clearly the differential expression patterns observed with our genes.
for example, pcdh <dig> is strongly and selectively expressed in a subset of cells in the vm aspect of the anterior bulb.
these same cells do not express tbr <dig>  thus, in tbr <dig> mutant animals, we predict that these cells would not only be present, but that the observed differential expression patterns would also still be present.
these cells may therefore provide a potential solution to the problem of which cells within the bulb could express guidance cues important for osn guidance.
mitral cells are known to mature in a medial-to-lateral fashion from e14–e <dig>  <cit> , and a rostral-caudal wave of development occurs during glomerular formation  <cit> .
however, these developmental events cannot readily explain our various observed patterns.
for example, gicerin and ctgf are expressed in subsets of cells in the lateral domain in anterior bulb, while jagged <dig> and pcdh <dig> are expressed medially.
several of our genes are both medially and laterally expressed.
it seems unlikely that these patterns can be explained solely on the basis of neuronal maturation events during development.
are any of our identified cdnas involved in guiding osns to the bulb?
an alternative interpretation would be that these genes may be important for establishing intrabulbar circuits.
external tufted cells are known to indirectly connect glomeruli innervated by a common population of osns  <cit> .
thus, these genes may act to establish these connections.
yet another potential function of these genes can be suggested based on their expression in the mitral layer.
these genes may act to mediate connectivity between projection neurons and higher cortical synaptic partners.
finally, these genes may not be involved in axon guidance per se, but instead are important for events surrounding synaptogenesis.
to provide support for the hypothesis these genes are important for guiding osn axons, we used a chemical ablation approach  <cit> .
expression of tyrosine hydroxylase  is known to be down-regulated initially in ablated animals, but reappears as reinnervation of the bulb proceeds.
thus, th expression is influenced by osn innervation.
similarly, this approach has also been used to study sema3a expression in axotomized bulbs  <cit> .
in these experiments, normal expression of sema3a is greatly reduced in axotomized bulbs.
expression is instead upregulated near the site of axotomy.
two months after ablation, sema3a expression reappeared at its normal location.
these results indicate that cells which interact with osns can display molecular alterations in their expression profile upon deafferentation.
we showed that ctgf, pcdh <dig>  and pcdh <dig> were altered in expression in response to ablation.
based on prior studies of th, we conclude that these genes are expressed in cells that interact with osns.
we note that several of our candidate cues correspond to genes known to be important for guiding axons elsewhere in the nervous system , or are involved in angiogenesis .
ctgf and gicerin are known to mediate cell-cell adhesion in vitro.
cadherins have been shown to be play multiple roles during development, from neurite outgrowth to synaptogenesis  <cit> .
the changing patterns of expression of pcdh <dig> and pcdh <dig> during development may signal multiple roles as osn innervation of the bulb proceeds.
pcdh <dig> and pcdh <dig> may operate initially as guidance cues based on their differential expression within the bulb and epithelium.
after growth of osns into the bulb, they may then act secondarily in synaptogenesis.
while definitive proof requires genetic manipulation of the individual candidates, these genes provide novel targets that will enable new approaches towards determining the role of the bulb in guidance.
CONCLUSIONS
we have identified several genes whose expression patterns define various subdomains within the developing mouse olfactory bulb.
we show that these genes are expressed in cells within the nascent epl/gl and ml, and that some of these cells do not express tbr <dig>  our regeneration paradigm suggests that these genes are expressed in cells that interact with osns.
these genes therefore represent novel candidate guidance cues that may guide osn axons to appropriate locations within the bulb.

 in the mouse olfactory system, each olfactory sensory neuron extends an axon from the epithelium to the olfactory bulb to form synapses within structures called glomeruli. the pattern of glomerular convergence is remarkably stereotyped and is strongly dependent upon the specific odorant receptor expressed by the osn. but despite the number of cues that have been found that are expressed by osns or along the path traveled by osns to the bulb, surprisingly little is known about the role of the olfactory bulb itself in this process. transplantation and bulbectomy studies have observed the formation of glomerular-like structures by osns within cortical tissue. genetic ablation of mitral and tufted projection neurons showed that topographically correct convergence by p2-expressing osns still occurred in the absence of these target cells. disruption of the laminar nature of the mitral layer also did not affect convergence by p2-expressing osns. these results have suggested that the olfactory bulb plays a minimal role, if any, in targeting. on the other hand, studies ablating most of the bulb, as opposed to eliminating only projection neurons, have observed topographically incorrect convergence by p2-expressing osns, suggesting a role for the bulb in determining the ultimate location of osns. tracing studies have long suggested the existence of zone-to-zone mapping of osns onto corresponding domains within the bulb. in drosophila, dscam expression in the antennal lobe is critical for proper olfactory receptor neuron targeting, providing the first data that the target structure is important for convergence. these experiments in drosophila suggest that the mammalian bulb may also provide guidance information to osns. part of the uncertainty surrounding the role of the bulb in guidance is that it is unclear what other bulbar cells in addition to mitral and tufted projection neurons could provide targeting information. while radial glia have long been suggested as one possibility, cells in the external plexiform and glomerular layers also possess many attractive characteristics. first, in addition to projection neurons, other cells in the epl and gl form synapses with osns. second, recent studies point to a diversity of cell-types within these layers, and electrophysiological studies have shown the presence of different types of interneurons within the epl. third, some glomerular cell populations migrate into the bulb much earlier than previously thought. and finally, it has been shown that external tufted cells indirectly connect glomeruli that are innervated by osns expressing a common receptor. collectively, these data suggest that little is known about the different sub-types and developmental profiles of these cells. in one model, a subset of these epl and/or gl cells could provide guidance information to osns. if this hypothesis is correct, then it should be possible to identify genes expressed by cells within these layers that are important for guidance and targeting. here we have undertaken a high resolution screen for genes that spatially define distinct regions of the epl and gl. we used laser microdissection coupled with rna amplification, microarrays, bioinformatics, and in situ hybridization to search for genes that are differentially expressed by cells within these layers. we identified such cdnas and show that the superficial layers of the bulb can be subdivided into large regions based upon the various expression patterns. the patterns demonstrate the existence of gradients, localized expression domains, and medial-lateral symmetry within the olfactory bulb. moreover, these patterns are maintained in cells that do not express tbr consistent with the interpretation that they are expressed in non-projection neurons. we further show, using a chemical ablation paradigm, evidence consistent with the interpretation that these genes are expressed in cells that interact with osns.  spatial reconstruction of gene expression using laser microdissection we performed our screen at embryonic day, as this is the first time that convergence is observed in vivo. as the epl and gl are not distinguishable at this stage our samples contained cells from both developing layers. we will refer to the regions where these samples were taken as the nascent epl/gl. only sections up to the front of the accessory olfactory bulb were used. these ~ coronal sections were binned into three groups corresponding to anterior, middle, and posterior bulb. as no prior genes have been identified as being spatially differentially expressed within the nascent epl/gl, the size of each lmd sample, orientation, and location was chosen based on the ability to easily separate individual lmd samples. we also considered the known pattern of glomerular innervation by osns, which is shifted ~ 45° from vertical. we sampled cells from the dorsolateral, dorsomedial, ventrolateral, and ventromedial nascent epl/gl within each section. three sections were chosen from each group and four lmd isolates were obtained per section for a total of isolates. rna was isolated from the ~ 30– cells within each sample, amplified, and applied to microarrays. we synthesized the pool of microarray data into a three-dimensional representation of gene expression within the bulb. as a result, all cdnas on the array could be queried to identify those whose expression patterns indicate differential expression within the bulb. for example, we examined whether or not expression for any given gene was higher or lower in the vm aspect relative to the remainder of the bulb. genes were grouped into four categories and the top from each were tested by in situ hybridization. of the cdnas, nine did not have any detectable signal. of the remaining were spatially differentially expressed. in one category, 83% were spatially differentially expressed. these results speak to the sensitivity of our analysis, as all samples differ essentially only by position. we also examined a small subset of genes chosen based on annotation. these genes, which were all ranked within the top were selected because they correspond to known, transmembrane-bound molecules. as such, they would be more likely to directly interact with osns and influence targeting. in total, cdnas were found to be spatially varied in expression. as expected, all identified cdnas were differentially expressed within the nascent epl/gl, as confirmed by dapi labeling and double-label in situ hybridization. expression was not necessarily exclusive to these layers, and was often observed in the mitral layer as well. spatial differential expression of candidate cues at e we used gap expression and dapi staining as controls for our initial experiments. we found gap to be evenly distributed throughout the mitral and epl layers. dapi staining showed that slightly more cells are observed laterally in the anterior bulb, but in the middle and posterior bulb the distribution of cells appears uniform. these controls served to distinguish whether or not any of our observed in situ patterns simply reflected the presence of increased numbers of cells at a given location within the bulb. we identified a diversity of patterns that are spatially differentially expressed within the nascent epl/gl. we found that many of our positive cdnas corresponded to genes that are either secreted or are transmembrane-bound proteins. gicerin is an immunoglobulin superfamily member that is a homophilic cell-adhesion molecule. gicerin is expressed differentially in the lateral aspect of the anterior bulb. however, not all cells in this lateral domain express gicerin, as the pattern appears stippled. expression is also observed in the nerve layer. strikingly, in the middle bulb, expression of gicerin appears symmetric in some areas about the ml and dv axes, with high levels observed in the dm and vl bulb. in more posterior bulb, these expression domains gradually become connected with one another along the dl surface, forming a crescent-shaped expression domain. expression in the vm aspect, however, remains low throughout the ap extent of the bulb. connective tissue growth factor, or ctgf, is a secreted protein that is a member of a small family of genes which interact with the extracellular matrix and growth factors, and are important for angiogenesis. in the anterior bulb, ctgf is localized to the lateral aspect. like gicerin, this expression is not uniform and significantly higher expression occurs within a subset of these cells. in the middle and posterior bulb, as with gicerin, expression is symmetric as higher ctgf expression is detected along the dm and vl surfaces. ctgf is weaker in the dl aspect and noticeably reduced in the vm bulb. ctgf is also expressed in cartilage and connective tissue surrounding the bulb. zinc finger csl-type containing is a protein of unknown function. throughout the ap axis, zcsl is expressed in a gradient-like pattern that extends from the dm to the vl bulb, although expression tends to shift more ventrally in the middle bulb. protocadherins have been shown to possess both homophilic and heterophilic adhesion properties. protocadherin is expressed throughout the bulb, but is enhanced in the anterior bulb along the vm aspect in a small subset of cells that extend into the epl. in the middle bulb, a small subset of cells in the vl nerve layer expresses this gene. protocadherin is expressed anteriorly most strongly along the medial aspect, although some signal can be seen laterally. just beyond the anterior bulb, this pattern is reversed and pcdh becomes strongest laterally, although some expression can still be detected medially. this pattern is maintained throughout the posterior bulb. jagged a ligand for notch, has been shown to be required for proper endothelial branching during angiogenesis. in the anterior bulb, jagged is symmetrically expressed about the ml axis. expression of jagged in these domains appears more even than that of gicerin or ctgf, and no obvious high or low-expressing cells are observed within the expression domain. in the middle bulb, jagged maintains this ml symmetry, but weaker expression connects the two subdomains. a similar pattern is observed in posterior bulb. although many of our genes are expressed in the nascent mitral layer, we emphasize that at least some expressing cells are located within the developing epl. all of our in situ hybridization experiments were counterstained with dapi so that we could correlate the location of the observed signal with the position of the nascent mitral and external plexiform layers. delta another ligand for notch, is expressed primarily within the mitral layer, and does not extend into the epl. this can be seen both by dapi staining and by the relatively compact expression pattern for this gene. jagged on the other hand, extends beyond the mitral layer into the epl, and its expression domain is broader than that of delta. we further demonstrate that these genes are expressed in the nascent epl/gl using double-label experiments and temporal analysis of their expression patterns, as described below. these expression patterns underscore the strong spatial heterogeneity within the external plexiform layer. moreover, they emphasize that there is variation along all three axes of the bulb. they also show for the first time that the bulb does indeed express genes that appear symmetric about the ml axis. a spatially differentially expressed putative microrna among the remaining cdnas, significantly enhanced expression was observed along the dv axis. the patterns observed among all of these positives were remarkably similar, and one representative example is shown in figure within the nascent epl/gl, these genes are expressed in a punctate pattern. this punctate expression occurs throughout the nascent epl/gl, but significantly fewer cells were labeled in the vm aspect. bioinformatic analysis showed that all cdnas contained intronic or 3' untranslated sequence. pair-wise alignment identified a basepair common element that contained two microrna sequences, mir- and mir-. the remainder of the bp element was ~ 95% identical to sequence immediately upstream of the mir- sequence, corresponding to what we assume is part of the primary mirna sequence for mir-. the expression pattern we observed may therefore reflect the expression of mir- pri-mirna. we obtained locked nucleic acid -modified oligos corresponding to mir- and mir-. we performed in situ hybridization with the lna oligos as well as with the basepair common element. hybridization patterns using the common element were identical to that observed with our cdnas. although the signal for the mir- probe was weaker than that of the common element, punctate signal was also observed in the nascent epl/gl in a similar pattern as well. these results contrast with those for the mir- probe, which bound weakly and uniformly to the nerve layer and epl. the mirna in situ patterns that have been published in the mouse generally show uniform expression within a given layer or structure, and/or are detected in specific cell-types. the pattern of the mir- probe suggests, however, that mirnas may also be present selectively within subsets of cells in a spatially distinct fashion. additional studies are required to determine the possible targets of this putative mirna. three-dimensional reconstruction of gene expression our identified expression patterns show that the bulb can be subdivided into different domains that are distinguished molecularly by the expression of these candidate genes. these domains have not been previously described, as only a handful of genes are known to be differentially expressed within a given layer of the bulb. we were surprised by the complexity of the observed patterns, as prior mapping studies had suggested the existence of dv cues in the bulb, and theoretical models had suggested the existence of symmetric cues to label medial and lateral bulb. our observed expression patterns, however, do not readily fit any of these prior suggested models. for example, gicerin symmetrically labels the dm and vl bulb, but symmetry about these axes has not been previously predicted. to obtain a more detailed understanding of the expression domains distinguished by these individual genes, we generated a three-dimensional reconstruction of gene expression for these genes within the bulb. to produce these recreations, coronal sections from e bulbs were obtained every 60– μm along the ap extent of the bulb for every gene. the images of the hybridized sections were combined using imagej. the projection of the stacked images was then superimposed upon a cartoon representation of the olfactory bulb for ease of visualization. a subset of the images used to create the jagged and ctgf stacks are shown in figure 4h–i, and the corresponding reconstructions are shown in figure 4e–f. reconstructions of the other genes are also shown in figure these reconstructions show that domains of the bulb along the ap, dv, and ml axes can be uniquely specified by the combinatorial expression of these genes. these data support a model where molecular heterogeneity in the nascent epl/gl would provide spatial guidance information to osn axons as they converge within the olfactory bulb. temporally dynamic expression of candidate cues during development – e14 the first osns are known to extend an axon towards the developing bulb at e11– and "wait" at the periphery as the bulb develops. sema3a expression at the periphery of the bulb, expressed by ensheathing glia, plays a role in maintaining osns outside of the bulb. it is known that mitral cell reorientation requires innervation by a small population of early osns that enter the bulb in a medial-to-lateral progression. why this innervation proceeds initially along the medial surface is not known. one possibility would be that additional cues, in conjunction with sema3a expression, serve to guide osn axons to their position within the bulb. we therefore examined the expression of our candidate cues at e for differential expression. all of our identified genes are differentially expressed at e in the superficial mantle layer. the putative mirna containing sequence, for example, is expressed strongly in the dl bulb, with weak expression in the vm aspect. gicerin is expressed more highly in the dl bulb as well, with weak expression in vl bulb. ctgf, on the other hand, is expressed more highly in the dm bulb. jagged is expressed more strongly in the ventral aspect. pcdh and pcdh label complementary domains. pcdh strongly labels two populations of cells in the vm bulb, with lighter, more uniform staining observed internally, and stronger, more selective staining externally. faint staining is also observed in the dl bulb, symmetrically opposing the vm expression. pcdh in contrast, strongly labels a large number of cells throughout the bulb except those along the vm surface. several of the observed patterns at e mimic those seen at e. for example, the putative mirna is weakly expressed in the vm bulb at both stages. similarly, the dual-layer expression of pcdh in the vm bulb at e is reflected at e. jagged is ventrally expressed at both stages. on the other hand, other genes change in expression between e and e. gicerin, for example, is expressed most strongly at e in dl bulb, but at e is present more strongly in the dm and vl bulb. ctgf also shifts in expression along different axes from e to e, increasing in expression laterally and anteriorly as development proceeds. temporally dynamic expression of candidate cues during development – p3 we next examined the expression of these various candidate cues at postnatal stages. while convergence has been shown to occur at e for osns expressing the p odorant receptor, other osn populations do not converge until later stages, up to postnatal day. the existence of relatively late converging osns prompted us to examine the expression of these genes at p at p gicerin is expressed in the anterior bulb more highly in the lateral aspect. in the middle bulb, expression can be seen in a complementary, symmetric pattern. ctgf is also expressed differentially in the lateral aspect of the anterior bulb, but in the middle and posterior bulb, this expression becomes more uniform. in contrast to gicerin and ctgf, the putative mirna is now expressed uniformly within the epl. similarly, zcsl, pcdh and pcdh, are uniformly expressed. however, jagged is still differentially expressed at p but unlike it's expression at e and e, higher expression is observed along the dorsal aspect. as with the data regarding expression at e these patterns indicate the temporally dynamic nature of expression by these various genes. gicerin, for example, is expressed most highly in the dl bulb at e but is dm and vl in both the e and p bulb. pcdh is vm and dl at e vm at e and uniform at p we note that, consistent with our initial observations at e, zcsl the putative mirna, jagged pcdh and pcdh expression can clearly be detected in the epl, gl, and mitral layers. lack of precise colocalization with tbr1 we have hypothesized that cells within the epl express guidance cues for osns. genetic studies using the tbr mutant mouse resulted in the ablation of most projection neurons in the bulb. projection neurons include mitral cells as well as internal and middle tufted cells. as targeting is unaffected in tbr mutant mice, a major problem has been to determine what cell-types within the bulb remained that could be important for this process. if our genes are expressed in cells that provide guidance information to osns, we would predict that at least some cells that express our candidate genes do not express tbr and would not be ablated in the mutant. prior studies have shown that tbr is expressed in cells distributed from the ventricle to the nascent mitral/tufted layer at e. we found a similar distribution of tbr1-positive cells at e. we performed double-label experiments with tbr and with our genes to determine what overlap exists in expression. zcsl is enhanced in expression laterally. in contrast, tbr expression in this same section is widespread in the granule and mitral/tufted layer, as previously described. merging the two images shows that expression of zcsl occurs in cells that extend into the nascent epl/gl which do not express tbr. similarly, pcdh is enhanced in vm cells in the anterior bulb. a subset of these cells do not overlap in expression with tbr. the putative mirna identified in our screen is also expressed in nascent epl/gl cells that do not express tbr. finally, a subset of ctgf and gicerin expression also occurs within cells that do not express tbr thus, while overlap with tbr1-expressing cells does occur, tbr1-negative cells that express our genes are present within the bulb. perhaps most interestingly, these tbr1-negative cells are primarily located in the regions where the differential expression of our genes appears strongest. for example, zcsl is spatially differentially expressed in the lateral bulb, and many of these cells have no detectable tbr expression. expression of candidate cues are affected in chemically ablated mice while we have shown the existence of a number of genes demarcating different domains within the bulb, it remains unknown whether or not they are functionally relevant for osn guidance. unfortunately, no in vitro system exists in the olfactory system to test this hypothesis, and genetic analysis has remained the primary means for assessing function. to more rapidly determine the potential role of these genes in targeting, we decided to employ a chemical ablation approach. the olfactory epithelium is continuously being regenerated during adult life. prior studies have shown that tyrosine hydroxylase expression is drastically reduced following intranasal ablation with zinc sulfate. as regeneration proceeds, th expression gradually recovers. although th is an intracellular enzyme, it has been proposed that this recovery is dependent upon afferent input from regenerating osns. this process, although poorly understood, is thought to be a result of transneuronal regulation of glomerular neurons by osns. we decided to ask whether or not this ablation paradigm could be used to determine whether our candidate genes are expressed in cells that interact with osns. we would expect that if they were, then their expression, like that of th, would change in response to osn deafferentation. we ablated osns in one nares of eight-week old mice with a solution of zinc sulfate, leaving the other nares alone. we determined the success of the ablation based on expression of omp in the epithelium. as shown in the right half of fig. 8a, at days post-ablation, essentially no omp is observed in the ablated epithelium. small, isolated pockets of omp expression were occasionally detected in some ablated epithelia, presumably due to a lack of access to these regions by the zinc sulfate solution. by days post-ablation, omp expression has started to recover, although this expression is still somewhat thinner when compared to the control side. we did not examine later timepoints. we found that the rate of omp recovery was somewhat variable, with some epithelia showing omp expression by days, while others showed little or no expression. we therefore based our experiments in the bulb on the pattern of omp expression in the epithelium, and not by the number of days post-ablation. we found the bulb on the ablated side is smaller as previously reported due to a significant reduction in the size of the nerve and glomerular layers. based on dapi staining, the number of cells in the glomerular layer did not appear to be reduced, consistent with previous studies showing that little or no cell death results from the ablation process. we found that glur expression appeared to be reduced in the glomerular layer and increased in the mitral layer at days but was equal in expression between ablated and control bulbs by days. this is reminiscent of alterations in glur expression in naris occluded animals. we confirmed that th was absent from the ablated side at days but recovered by days post-ablation. we then tested a subset of our genes on these ablated animals. interestingly, we found a variety of different responses to the ablation. ctgf is expressed in the adult in the glomerular and mitral layers. this expression is absent in bulbs on the ablated side at days but recovers by days. pcdh is expressed in the glomerular layer in adults, with weaker, more scattered expression in the mitral layer. like ctgf, both layers are affected in the ablated side at days, but expression recovers by days post-ablation. pcdh is also expressed in the glomerular and mitral layers. however, in the ablated side, only the glomerular layer expression of pcdh is affected, while mitral layer expression remains constant. expression in the glomerular layer returns to control levels by days. we did not find any differences in expression for the putative mirna or zcsl in bulbs in ablated animals. based on prior studies showing th regulation by osn innervation, we conclude that several of our genes are likely to be expressed in cells that interact directly with osns. pcdh and pcdh are expressed in subsets of osns two of our genes, pcdh and pcdh are members of the cadherin superfamily. protocadherins are known to possess weak homophilic adhesion properties in vitro. in one model, protocadherins expressed by osns would bind their cognate partner expressed by nascent epl/gl cells within the bulb. the specificity of these interactions would contribute towards the final glomerular convergence pattern. we therefore examined pcdh and pcdh expression in the epithelium. interestingly, we found both genes are expressed in subsets of osns. double-label fluorescent in situ hybridization shows that, although some cells express both pcdh and pcdh others express one or the other protocadherin. within the bulb, fluorescent double-label in situ experiments show a similar phenotype, as pcdh expression overlaps with that of pcdh but distinct domains of the bulb are uniquely labelled with one or the other protocadherin. the expression of these cadherins in both the epithelium and in the bulb is consistent with a model in which these molecules contribute towards the establishment of the topographic map. discussion to our knowledge, no cell-type or gene expressed within the bulb has been functionally proven to provide spatial guidance information to osns. sema3a, for example, has been genetically shown to affect osn targeting, but this is due to its expression in ensheathing glia which migrate to the nerve layer from the epithelium. a number of genes are uniformly expressed within cells derived from the bulb, but it is difficult to understand how, given this uniform expression, these genes would guide osn axons to different domains within the bulb. the lack of viable potential guidance cues has led instead to genetic approaches that ablated either whole classes of cell-types within the bulb or eliminated much of the bulb entirely. the absence of suitable molecular candidates that could potentially guide osn axons to subdomains of the bulb has strongly inhibited any study of the role of the bulb in guidance. in contrast, studies manipulating odorant receptor expression have led to models where the odorant receptor plays a significant role in guidance. the recent discovery that the kirrel 2/ and ephrin/eph a expression are controlled by odorant receptor activity provides new insight into how receptor choice influences glomerular positioning. however, in all of these receptor swap experiments, it is clear that the receptor alone does not mediate all aspects of glomerular targeting. the dorsal-ventral position of the osn within the epithelium is thought to provide position-dependent information as well. but what genes comprise these position-dependent determinants, and what potential partners are bound by such determinants are still unknown. moreover, why osns innervate both a medial and a lateral glomerulus has not been explained by studies manipulating odorant receptor expression. the presence of gradients or of mirror-symmetric expression patterns within the bulb have been suggested as potential explanations for how topographic osn projection patterns are established. however, considerable disagreement exists over the role of the bulb in guidance. the role of bulbar projection neurons in guiding osn axons has been examined using different genetic paradigms. these studies have suggested that these cells are not necessary for topographic convergence of osns. while these analyses only examined osns that express the p odorant receptor, mitral cell tracing studies also do not necessarily support a role for these cells in guidance. mitral cell dendritic arbors initially are distributed among several glomeruli, but are refined during the first week of postnatal development to a single glomerulus. it is difficult to understand, if mitral cells do indeed express the primary cues needed for osn guidance, why such cues would be distributed among multiple glomeruli initially, and the dendritic arbor refined only after convergence has been completed. but despite the fact that these studies indicate that mitral cells may not play a primary role in guidance, genetic ablation studies removing the entire bulb have shown that topographically correct convergence by p2-expressing osns does not occur. one interpretion of these results is that other cell-types within the bulb are important for providing guidance information to osns. we theorized that these cells correspond to those in the epl and gl. in this hypothesis-driven microarray-based experiment, we show that the nascent epl/gl of the bulb can be subdivided into multiple domains based on the expression of our identified genes. at e, the first time that convergence is observed, the anterior bulb can be distinguished molecularly both laterally and medially. in the middle and posterior bulb, zcsl is expressed in a gradient that extends from dorsal to ventrolateral, while a probe containing the mir- sequence is expressed in a subset of cells that span a similar region. perhaps most interestingly, we find that gicerin, ctgf, and jagged are expressed in a symmetric manner. while it is well-known that osns project to medial and lateral glomeruli, the existence of mirror-symmetric cues has, to date, only been hypothesized. however, our patterns suggest that this medial-lateral symmetry is more complex than previously thought. gicerin and ctgf are roughly symmetric about both the dv and ml axes, while jagged appears symmetric only around the ml axis. thus, each hemi-bulb can be further distinguished based on dorsal or ventral expression of gicerin and ctgf. several lines of evidence are consistent with the expression of these candidate cues within cells of the epl. first, we show by counterstaining with dapi that expression of these genes does not colocalize precisely with the nascent mitral layer. second, we show at p and in adult that many of these genes are expressed in the epl and gl, consistent with their expression at e in the nascent epl/gl. finally, we show that there are populations of cells at e that express our candidate genes but do not express tbr a marker for projection neurons. thus, we have shown using a variety of approaches that, in addition to mitral layer expression, nascent epl/gl cells also express these genes. interestingly, in addition to the fact that not all cells that express our genes express tbr the cells that do not express tbr are those that show most clearly the differential expression patterns observed with our genes. for example, pcdh is strongly and selectively expressed in a subset of cells in the vm aspect of the anterior bulb. these same cells do not express tbr thus, in tbr mutant animals, we predict that these cells would not only be present, but that the observed differential expression patterns would also still be present. these cells may therefore provide a potential solution to the problem of which cells within the bulb could express guidance cues important for osn guidance. mitral cells are known to mature in a medial-to-lateral fashion from e14–e, and a rostral-caudal wave of development occurs during glomerular formation. however, these developmental events cannot readily explain our various observed patterns. for example, gicerin and ctgf are expressed in subsets of cells in the lateral domain in anterior bulb, while jagged and pcdh are expressed medially. several of our genes are both medially and laterally expressed. it seems unlikely that these patterns can be explained solely on the basis of neuronal maturation events during development. are any of our identified cdnas involved in guiding osns to the bulb? an alternative interpretation would be that these genes may be important for establishing intrabulbar circuits. external tufted cells are known to indirectly connect glomeruli innervated by a common population of osns. thus, these genes may act to establish these connections. yet another potential function of these genes can be suggested based on their expression in the mitral layer. these genes may act to mediate connectivity between projection neurons and higher cortical synaptic partners. finally, these genes may not be involved in axon guidance per se, but instead are important for events surrounding synaptogenesis. to provide support for the hypothesis these genes are important for guiding osn axons, we used a chemical ablation approach. expression of tyrosine hydroxylase is known to be down-regulated initially in ablated animals, but reappears as reinnervation of the bulb proceeds. thus, th expression is influenced by osn innervation. similarly, this approach has also been used to study sema3a expression in axotomized bulbs. in these experiments, normal expression of sema3a is greatly reduced in axotomized bulbs. expression is instead upregulated near the site of axotomy. two months after ablation, sema3a expression reappeared at its normal location. these results indicate that cells which interact with osns can display molecular alterations in their expression profile upon deafferentation. we showed that ctgf, pcdh and pcdh were altered in expression in response to ablation. based on prior studies of th, we conclude that these genes are expressed in cells that interact with osns. we note that several of our candidate cues correspond to genes known to be important for guiding axons elsewhere in the nervous system, or are involved in angiogenesis. ctgf and gicerin are known to mediate cell-cell adhesion in vitro. cadherins have been shown to be play multiple roles during development, from neurite outgrowth to synaptogenesis. the changing patterns of expression of pcdh and pcdh during development may signal multiple roles as osn innervation of the bulb proceeds. pcdh and pcdh may operate initially as guidance cues based on their differential expression within the bulb and epithelium. after growth of osns into the bulb, they may then act secondarily in synaptogenesis. while definitive proof requires genetic manipulation of the individual candidates, these genes provide novel targets that will enable new approaches towards determining the role of the bulb in guidance.  we have identified several genes whose expression patterns define various subdomains within the developing mouse olfactory bulb. we show that these genes are expressed in cells within the nascent epl/gl and ml, and that some of these cells do not express tbr our regeneration paradigm suggests that these genes are expressed in cells that interact with osns. these genes therefore represent novel candidate guidance cues that may guide osn axons to appropriate locations within the bulb. 
in the mouse olfactory system, the role of the olfactory bulb in guiding olfactory sensory neuron axons to their targets is poorly understood. we demonstrate that the nascent external plexiform and glomerular layers of the bulb can be divided into multiple domains based on the expression of these genes, several of which are known to function in axon guidance, synaptogenesis, and angiogenesis. the expression patterns of these genes divide the bulb into previously unrecognized subdomains. although projection neurons are not required, other cell-types within the external plexiform and glomerular layers also form synapses with osns. we hypothesized that these cells are important for targeting, and express spatially differentially expressed guidance cues that act to guide osn axons within the bulb. these genes represent candidate guidance cues that may act to guide osn axons within the bulb during targeting. interestingly, some genes are expressed in both the medial and lateral bulb, showing for the first time the existence of symmetric expression along this axis. we used laser microdissection and microarray analysis to find genes that are differentially expressed along the dorsal-ventral, medial-lateral, and anterior-posterior axes of the bulb.
 in the mouse olfactory system, the role of the olfactory bulb in guiding olfactory sensory neuron axons to their targets is poorly understood. what cell types within the bulb are necessary for targeting is unknown. what genes are important for this process is also unknown. although projection neurons are not required, other cell-types within the external plexiform and glomerular layers also form synapses with osns. we hypothesized that these cells are important for targeting, and express spatially differentially expressed guidance cues that act to guide osn axons within the bulb.  we used laser microdissection and microarray analysis to find genes that are differentially expressed along the dorsal-ventral, medial-lateral, and anterior-posterior axes of the bulb. the expression patterns of these genes divide the bulb into previously unrecognized subdomains. interestingly, some genes are expressed in both the medial and lateral bulb, showing for the first time the existence of symmetric expression along this axis. we use a regeneration paradigm to show that several of these genes are altered in expression in response to deafferentation, consistent with the interpretation that they are expressed in cells that interact with osns.  we demonstrate that the nascent external plexiform and glomerular layers of the bulb can be divided into multiple domains based on the expression of these genes, several of which are known to function in axon guidance, synaptogenesis, and angiogenesis. these genes represent candidate guidance cues that may act to guide osn axons within the bulb during targeting. 