bluuebunny/crossref_metadata_2025 huggingface dataset:
MK: a pluripotential embryonic stem-cell-derived neuroregulatory factor
Authors: Nurcombe Victor , Fraser Neil , Herlaar Ellen , Heath John K.
Published: April 2021

MK is a gene encoding a secreted heparin-binding polypeptide originally isolated by differential screening for genes induced by retinoic acid (RA) in HM-1 embryonal carcinoma cells. Here we report that MK is expressed at high levels in both embryonal carcinoma and pluripotential embryonic stem cells and their dif-ferentiated derivatives. MK expression in these cell types is unaffected by the presence or absence of RA. Recombinant MK protein (rMK) was produced by tran-sient expression in COS cells and purified by heparin affinity chromatography. rMK is a weak mitogen for 10T1/2 fibroblast cells but inactive as a mitogen for Swiss 3T3 fibroblasts. rMK is a potent mitogen for neurectodermal precursor cell types generated by treat-ment of 1009 EC cells with RA but has no mitogenic or neurotrophic effects on more mature 1009-derived neuronal cell types. rMK is active as an in vitro neu-rotrophic factor for E12 chick sympathetic neurons and its activity is markedly potentiated by binding the factor to tissue-culture plastic in the presence of heparin. Stable 10T1/2 cells lines have been established which express MK. These cells do not exhibit any overt evidence of cell transformation but extracellular matrix preparations derived from these cells are a potent source of MK biological actvity. It is concluded that MK is a multifunctional neuroregulatory molecule whose biological activity depends upon association with com-ponents of the extracellular matrix.
Identification of RNA-binding proteins specific to Xenopus Eg maternal mRNAs: association with the portion of Eg2 mRNA that promotes deadenylation in embryos
Authors: Legagneux Vincent , Bouvet Philippe , Omilli Francis , Chevalier Stephane , Osborne H. Beverley
Published: April 2021

Maternal Xenopus Eg mRNAs have been previously identified as transcripts that are specifically deadenylated after fertilization and degraded after the mid blastula transition. Destabilizing cis sequences were previously localised in the 3 untranslated region of Eg2 mRNA. In order to characterize possible trans-acting factors which are involved in the post-transcriptional regulation of Eg mRNAs, gel-shift and u.v. cross-linking experiments were performed, which allowed the identification of a p53-p55 RNA-binding protein doublet specific for the 3 untranslated regions of Eg mRNAs. These p53-p55 proteins do not bind to the 3 untranslated regions of either ornithine decarboxylase or phosphatase 2Ac mRNAs, which remain polyadenylated in embryos. These novel RNA-binding proteins are distinct from the cytoplasmic polyadenylation element-binding protein that controls the polyadenylation of maternal mRNAs in maturing Xenopus oocytes, and from previously identified thermoresistant RNA-binding proteins present in oocyte mRNP storage particles. The p53-p55 bind a portion of the Eg2 mRNA 3 untranslated region, distinct from the previously identified destabilizing region, that is able to confer the postfertilization dead-enylation of CAT-coding chimeric mRNAs. This suggests that the p53-p55 RNA-binding proteins are good candidates for trans-acting factors involved in the deadenylation of Eg mRNAs in Xenopus embryos.
The function of the neurogenic genes during epithelial development in the Drosophila embryo
Authors: Hartenstein Amelia Y. , Rugendorff Astrid , Tepass Ulrich , Hartenstein Volker
Published: April 2021

The complex embryonic phenotype of the six neurogenic mutations Notch, mastermind, big brain, Delta, Enhancer of split and neuralized was analyzed by using different antibodies and PlacZ markers, which allowed us to label most of the known embryonic tissues. Our results demonstrate that all of the neurogenic mutants show abnormalities in many different organs derived from all three germ layers. Defects caused by the neurogenic mutations in ectodermally derived tissues fell into two categories. First, all cell types that delaminate from the ectoderm (neuroblasts, sensory neurons, peripheral glia cells and oenocytes) are increased in number. Secondly, ectodermal tissues that in the wild type form epithelial structures lose their epithelial phenotype and dissociate (optic lobe, stomatogastric nervous system) or show significant differentiative abnormalities (trachea, Malpighian tubules and salivary gland). Abnormalities in tissues derived from the mesoderm were observed in all six neurogenic mutations. Most importantly, somatic myoblasts do not fuse and/or form an aberrant muscle pattern. Cardioblasts (which form the embryonic heart) are increased in number and show differentiative abnormalities; other mesodermal cell types (fat body, pericardial cells) are significantly decreased. The development of the endoderm (midgut rudiments) is disrupted in most of the neurogenic mutations (Notch, Delta, Enhancer of split and neuralized) during at least two stages. Defects occur as early as during gastrulation when the invaginating midgut rudiments prematurely lose their epithelial characteristics. Later, the transition of the midgut rudiments to form the midgut epithelium does not occur. In addition, the number of adult midgut precursor cells that segregate from the midgut rudi- ments is strongly increased. We propose that, at least in the ectodermally and endodermally derived tissues, neu- rogenic gene function is primarily involved in interactions among cells that need to acquire or to maintain an epithelial phenotype.
Parental methylation patterns of a transgenic locus in adult somatic tissues are imprinted during gametogenesis
Authors: Ueda Takayuki , Yamazaki Kimie , Suzuki Rika , Fujimoto Hirokazu , Sasaki Hiroyuki , Sakaki Yoshiyuki , Higashinakagawa Toru
Published: April 2021

The methylation status of a mouse metallothionein- I/human transthyretin fusion gene was studied during gametogenesis in transgenic mice. In the adult tissues of this mouse line, the promoter region of the transgene on chromosome 11 is methylated when it is maternally inherited and undermethylated when it is paternally inherited. Germ cells from various developmental stages of gametogenesis were isolated, and their DNAs were assayed using methylation-sensitive restriction endonu- cleases and the polymerase chain reaction. Only low to nonexistent levels of transgene methylation were detected in germ cells from 14.5-day-old male and female fetuses irrespective of the parental origin of the transgene. This undermethylated state persisted in oocytes from newborn females as well as in testicular spermatogenic cells and sperm. By contrast, the trans- gene promoter was completely methylated in fully grown oocytes arrested at the first meiotic prophase. The endogenous metallothionein-I gene promoter, located on a different chromosome, remained under- methylated at all stages examined, consistent with pre- vious findings reported for a typical CpG island. Taken together, the results suggest that parental-specific adult patterns of transgene methylation are established during gametogenesis.
Ectopic expression of UBX and ABD-B proteins during Drosophila embryogenesis: competition, not a functional hierarchy, explains phenotypic suppression
Authors: Lamka Michele L. , Boulet Anne M. , Sakonju Shigeru
Published: April 2021

The Abdominal-B (Abd-B) gene, a member of the bithorax complex (BX-C), specifies the identities of parasegments (PS) 10-14 in Drosophila. Abd-B codes for two structurally related homeodomain proteins, ABD-B m and ABD-B r, that are expressed in PS10-13 and PS14-15, respectively. Although ABD-B m and r proteins have distinct developmental functions, ectopic expression of either protein during embryogenesis induces the development of filzkörper and associated spiracular hairs, structures normally located in PS13, at ectopic sites in the larval thorax and abdomen. These results suggest that other parasegmental differences contribute to the phenotype specified by ABD-B r activity in PS14.

Both ABD-B m and r repress the expression of other homeotic genes, such as Ubx and abd-A, in PS10-14. However, the importance of these and other cross-regulatory interactions among homeotic genes has been questioned. Since ectopic UBX protein apparently failed to transform abdominal segments, González-Reyes et al. (González-Reyes, A., Urquía, N., Gehring, W. J., Struhl, G. and Morata, G. (1990). Nature 344, 78-80) proposed a functional hierarchy in which ABD-A and ABD-B activities override UBX activity. We tested this model by expressing UBX and ABD-B m proteins ectopically in wild-type and BX-C-deficient embryos. Ectopic ABD-B m does not prevent transformations induced by ectopic UBX. Instead, ectopic UBX and ABD-B m proteins compete for the specification of segmental identities in a dose-dependent fashion. Our results support a quantitative competition among the homeotic proteins rather than the existence of a strict functional hierarchy. Therefore, we suggest that cross-regulatory interactions are not irrelevant but are important for repressing the expression of competing homeotic proteins.

To explain the apparent failure of ectopic UBX to transform the abdominal segments, we expressed UBX at different times during embryonic development. Our results show that ectopic UBX affects abdominal cuticular identities if expressed during early stages of embryogenesis. In later embryonic stages, abdominal segments become resistant to transformation by ectopic UBX while thoracic segments remain susceptible. Head segments also show a similar stage-dependent susceptibility to transformation by ectopic UBX in early embryogenesis but become resistant in later stages. These results suggest that abdominal and head identities are determined earlier than are thoracic identities.
Molecular markers for identified neuroblasts and ganglion mother cells in the Drosophila central nervous system
Authors: Doe Chris Q.
Published: April 2021

The first step in generating cellular diversity in the Drosophila central nervous system is the formation of a segmentally reiterated array of neural precursor cells, called neuroblasts. Subsequently, each neuroblast goes through an invariant cell lineage to generate neurons and/or glia. Using molecular lineage markers, I show that (1) each neuroblast forms at a stereotyped time and position; (2) the neuroblast pattern is indistinguishable between thoracic and abdominal segments; (3) the development of individual neuroblasts can be followed throughout early neurogenesis; (4) gene expression in a neuroblast can be reproducibly modulated during its cell lineage; (5) identified ganglion mother cells form at stereotyped times and positions; and (6) the cell lineage of four well-characterized neurons can be traced back to two identified neuroblasts. These results set the stage for investigating neuroblast specification and the mechanisms controlling neuroblast cell lineages.
Notochord grafts do not suppress formation of neural crest cells or commissural neurons
Authors: Artinger Kristin B. , Bronner-Fraser Marianne
Published: April 2021

Grafting experiments previously have established that the notochord affects dorsoventral polarity of the neural tube by inducing the formation of ventral structures such as motor neurons and the floor plate. Here, we examine if the notochord inhibits formation of dorsal structures by grafting a notochord within or adjacent to the dorsal neural tube prior to or shortly after tube closure. In all cases, neural crest cells emigrated from the neural tube adjacent to the ectopic notochord. When analyzed at stages after ganglion formation, the dorsal root ganglia appeared reduced in size and shifted in position in embryos receiving grafts. Another dorsal cell type, commissural neurons, identified by CRABP and neurofilament immunoreactivity, differentiated in the vicinity of the ectopic notochord. Numerous neuronal cell bodies and axonal processes were observed within the induced, but not endogenous, floor plate 1 to 2 days after implantation but appeared to be cleared with time. These results suggest that dorsally implanted noto-chords cannot prevent the formation of neural crest cells or commissural neurons, but can alter the size and posi-tion of neural crest-derived dorsal root ganglia.
Cell motility driving mediolateral intercalation in explants of Xenopus laevis
Authors: Shih John , Keller Ray
Published: April 2021

In Xenopus, convergence and extension are produced by active intercalation of the deep mesodermal cells between one another along the mediolateral axis (medi- olateral cell intercalation), to form a narrower, longer array. The cell motility driving this intercalation is poorly understood. A companion paper shows that the endodermal epithelium organizes the outermost meso- dermal cells immediately beneath it to undergo conver- gence and extension, and other evidence suggests that these deep cells are the most active participants in medi- olateral intercalation (Shih, J. and Keller, R. (1992) Development 116, 887-899). In this paper, we shave off the deeper layers of mesodermal cells, which allows us to observe the protrusive activity of the mesodermal cells next to the organizing epithelium with high reso- lution video microscopy. These mesodermal cells divide in the early gastrula and show rapid, randomly directed protrusive activity. At the early midgastrula stage, they begin to express a characteristic sequence of behaviors, called mediolateral intercalation behavior (MIB): (1) large, stable, filiform and lamelliform protrusions form in the lateral and medial directions, thus making the cells bipolar; (2) these protrusions are applied directly to adjacent cell surfaces and exert traction on them, without contact inhibition; (3) as a result, the cells elongate and align parallel to the mediolateral axis and perpendicular to the axis of extension; (4) the elongate, aligned cells intercalate between one another along the mediolateral axis, thus producing a longer, narrower array. Explants of essentially a single layer of deep mesodermal cells, made at stage 10.5, converge and extend by mediolateral intercalation. Thus by stage 10.5 (early midgastrula), expression of MIB among deep mesodermal cells is physiologically and mechanically independent of the organizing influence of the endoder- mal epithelium, described previously (Shih, J. and Keller, R. (1992) Development 116 887-899), and is the fundamental cell motility underlying mediolateral inter- calation and convergence and extension of the body axis.
Patterns of cell motility in the organizer and dorsal mesoderm of Xenopus laevis
Authors: Shih John , Keller Ray
Published: April 2021

In a companion paper (Shih, J. and Keller, R. (1992) Development 116, 901-914), we described a sequence of cell behaviors, called mediolateral intercalation behavior (MIB), that produces mediolateral cell intercalation, the process that drives convergence and extension of the axial and paraxial mesoderm of Xenopus. In this paper, we describe the pattern of expression of MIB in the mesoderm during gastrulation, using video image pro- cessing and recording of cell behavior in ‘shaved’, open- faced explants of the marginal zone. At midgastrula stage (10.5), MIB begins at two dorsolateral sites in the prospective anterior mesoderm and progresses medially along two arcs that lengthen toward and meet at the midline to form a single arc of cells expressing MIB, called the vegetal alignment zone (VgAZ). The noto- chordal-somitic mesodermal boundary forms within the VgAZ at stage 11, and then progresses animally and lat- erally, along the prospective anterior-posterior axis, eventually bounding a trapezoidal area the shape of the fate-mapped notochord. Meanwhile, from its origin in the VgAZ, MIB spreads in the prospective posterior direction along the lateral boundaries of both the noto- chordal and somitic mesoderm. From there it spreads medially in both tissues. Subsequently, vacuolation of notochord cells, and segmentation and expression of a somite-specific marker repeat the progression of medi- olateral intercalation behavior. Thus cells in the posterior, medial regions of the notochordal and the somitic territories are the last to express mediolateral intercalation behavior and subsequent tissue differentiations. In explants that do not converge, these cells neither express mediolateral intercalation behavior nor differentiate. These facts suggest that progressions of MIB in the anterior-posterior and lateral-medial directions may be organized by signals emanating from the lateral somitic and notochordal boundaries. These signals may have limited range and may be dependent on convergence, driven by mediolateral cell intercalation, to bring cells within their range. In the embryo, the posterior progression of MIB results in arcs of convergence, anchored in the vegetal endoderm at each end, acting on the inside of the blastoporal lip to produce involution of the IMZ.
ming is expressed in neuroblast sublineages and regulates gene expression in the Drosophila central nervous system
Authors: Cui Xuan , Doe Chris Q.
Published: April 2021

Cell diversity in the Drosophila central nervous system (CNS) is primarily generated by the invariant lineage of neural precursors called neuroblasts. We used an enhancer trap screen to identify the ming gene, which is transiently expressed in a subset of neuroblasts at reproducible points in their cell lineage (i.e. in neuro- blast ‘sublineages’), suggesting that neuroblast identity can be altered during its cell lineage. ming encodes a predicted zinc finger protein and loss of ming function results in precise alterations in CNS gene expression, defects in axonogenesis and embryonic lethality. We propose that ming controls cell fate within neuroblast cell lineages.
Increased levels of the Drosophila Abelson tyrosine kinase in nerves and muscles: subcellular localization and mutant phenotypes imply a role in cell-cell interactions
Authors: Bennett Randy L. , Hoffmann F. Michael
Published: April 2021

Mutations in the Drosophila Abelson tyrosine kinase have pleiotropic effects late in development that lead to pupal lethality or adults with a reduced life span, reduced fecundity and rough eyes. We have examined the expression of the abl protein throughout embryonic and pupal development and analyzed mutant pheno- types in some of the tissues expressing abl. abl protein, present in all cells of the early embryo as the product of maternally contributed mRNA, transiently localizes to the region below the plasma membrane cleavage fur- rows as cellularization initiates. The function of this expression is not yet known. Zygotic expression of abl is first detected in the post-mitotic cells of the develop- ing muscles and nervous system midway through embryogenesis. In later larval and pupal stages, abl protein levels are also highest in differentiating muscle and neural tissue including the photoreceptor cells of the eye. abl protein is localized subcellularly to the axons of the central nervous system, the embryonic somatic muscle attachment sites and the apical cell junctions of the imaginal disk epithelium. Evidence for abl function was obtained by analysis of mutant phenotypes in the embryonic somatic muscles and the eye imaginal disk. The expression patterns and mutant phenotypes indi- cate a role for abl in establishing and maintaining cell- cell interactions.
The fat facets gene is required for Drosophila eye and embryo development
Authors: Fischer-Vize Janice A. , Rubin Gerald M. , Lehmann Ruth
Published: April 2021

In a screen for mutations affecting Drosophila eye devel-opment, we have identified a gene called fat facets (faf) which is required for cell interactions that prevent par-ticular cells in the developing eye from becoming pho-toreceptors. Analysis of eyes mosaic for faf+ and faf cells shows that faf is required in cells near to, but out-side, normal developing photoreceptors and also outside of the ectopic photoreceptors in mutant facets. faf is also essential during oogenesis, and we show that a faf-lacZ hybrid protein is localized via the first 392 amino acids of faf to the posterior pole of oocytes. Posterior local-ization of faf-lacZ depends on oskar. oskar encodes a key organizer of the pole plasm, a specialized cytoplasm at the posterior pole of embryos. The pole plasm is required for germ cell formation and contains the deter-minant of posterior polarity, encoded by nanos. Although other pole plasm components are required for localization of nanos RNA or for nanos protein func-tion, faf is not. We have cloned the faf gene, and have shown that it encodes two similar large (∼300 103Mr) proteins that are unique with respect to other known proteins.
The patterning and functioning of protrusive activity during convergence and extension of the Xenopus organiser
Authors: Keller Ray , Shih John , Domingo Carmen
Published: April 2021

We discuss the cellular basis and tissue interactions regulating convergence and extension of the vertebrate body axis in early embryogenesis of Xenopus. Convergence and extension occur in the dorsal mesoderm (prospective notochord and somite) and in the posterior nervous system (prospective hindbrain and spinal cord) by sequential cell intercalations. Several layers of cells intercalate to form a thinner, longer array (radial intercalation) and then cells intercalate in the mediolateral orientation to form a longer, narrower array (mediolateral intercalation). Fluorescence microscopy of labeled mesodermal cells in explants shows that protrusive activity is rapid and randomly directed until the midgastrula stage, when it slows and is restricted to the medial and lateral ends of the cells. This bipolar protrusive activity results in elongation, alignment and mediolateral intercalation of the cells. Mediolateral intercalation behavior (MIB) is expressed in an anterior-posterior and lateral-medial progression in the mesoderm. MIB is first expressed laterally in both somitic and notochordal mesoderm. From its lateral origins in each tissue, MIB progresses medially. If convergence does not bring the lateral boundaries of the tissues closer to the medial cells in the notochordal and somitic territories, these cells do not express MIB. Expression of tissue-specific markers follows and parallels the expression of MIB. These facts argue that MIB and some aspects of tissue differentiation are induced by signals emanating from the lateral boundaries of the tissue territories and that convergence must bring medial cells and boundaries closer together for these signals to be effective. Grafts of dorsal marginal zone epithelium to the ventral sides of other embryos, to ventral explants and to UV-ventralized embryos show that it has a role in organising convergence and extension, and dorsal tissue differentiation among deep mesodermal cells. Grafts of involuting marginal zone to animal cap tissue of the early gastrula shows that convergence and extension of the hindbrain-spinal cord are induced by planar signals from the involuting marginal zone.
trithorax regulates multiple homeotic genes in the bithorax and Antennapedia complexes and exerts different tissue-specific, parasegment-specific and promoter-specific effects on each
Authors: Breen T. R. , Harte P. J.
Published: April 2021

The trithorax (trx) gene is required for normal development of the body plan in Drosophila embryos and adults. Mutations in trx cause homeotic transformations throughout the body. Genetic studies suggest that trx encodes a positive regulatory factor required throughout development for normal expression of multiple homeotic genes of the bithorax and Antennapedia complexes (BX-C and ANT-C). To determine how trx influences homeotic gene expression, we examined the expression of the BX-C genes Ultrabithorax, abdominal-A, Abdominal-B and the ANT-C genes Antennapedia, Sex combs reduced and Deformed in trx embryos. We show that trx does indeed exert its effects by positively regulating homeotic gene expression and that its effects on expression of individual homeotic genes are complex: each of the BX-C and ANT-C genes examined exhibits different tissue-specific, parasegment-specific and promoter-specific reductions in their expression. This implies that each of these genes have different requirements for trx in different spatial contexts in order to achieve normal expression levels, presumably depending on the promoters involved and the other regulatory factors bound at each of their multiple tissue- and parasegment-specific cis-regulatory sites in different regions of the embryo. These results also imply that those components of homeotic gene expression patterns for which trx is dispensable, require other factors, possibly those encoded by other trithorax-like genes.
Developmental origin of segmental identity in the leech mesoderm
Authors: Gleizer Lidia , Stent Gunther S.
Published: April 2021

Segmentation in the leech embryo is established by a stereotyped cell lineage. Each of the 32 segments arises from homologous, bilaterally symmetrical complements of mesodermal and ectodermal blast cell clones. Although segments are homologous, they are regionally differentiated along the longitudinal body axis. Various segments display idiosyncratic ensembles of features, which constitute discrete segmental identities. The differentiation of segment-specific features, such as the mesoderm-derived nephridia, genital primordia and identified Small Cardioactive Peptide immunoreactive neurons, reflects a diversification of the developmental fates of homologous blast cell clones. We have investigated whether segment-specific differentiation of homologous mesodermal blast cell clones depends on cellintrinsic mechanisms (based on the cells’ lineage history) or on cell-extrinsic mechanisms (based on the cells’ interactions with their environment) in embryos of Theromyzon rude. For this purpose, we first mapped the segment-specific fates of individual mesodermal blast cell clones, and then induced mesodermal clones to take part in the formation of segments for which they are not normally destined. Two types of ectopic segmental position were produced: one in which a mesodermal blast cell clone was out of register with all other consegmental cells and one in which a mesodermal blast cell clone was out of register with its overlying ectoderm, but was in normal register with the mesoderm and ectoderm on the other side of the embryo. Mesodermal blast cell clones that developed in either type of ectopic segmental position gave rise to segment-specific features characteristic of their original segmental fates rather than their ectopic positions. Thus, the development of segmental identity in the leech mesoderm is attributable to a cell-intrinsic mechanism and, either before or soon after their birth, mesodermal blast cells are autonomously committed to segment-specific fates.
The Tribolium homeotic gene Abdominal is homologous to abdominal-A of the Drosophila bithorax complex
Authors: Stuart Jeffrey J. , Brown Susan J. , Beeman Richard W. , Denell Robin E.
Published: April 2021

The Abdominal gene is a member of the single homeotic complex of the beetle, Tribolium castaneum. An integrated developmental genetic and molecular analysis shows that Abdominal is homologous to the abdominal- A gene of the bithorax complex of Drosophila. abdomi - nal-A mutant embryos display strong homeotic transformations of the anterior abdomen (parasegments 7-9) to PS6, whereas developmental commitments in the posterior abdomen depend primarily on Abdominal-B. In beetle embryos lacking Abdominal function, paraseg-ments throughout the abdomen are transformed to PS6. This observation demonstrates the general functional significance of parasegmental expression among insects and shows that the control of determinative decisions in the posterior abdomen by homeotic selector genes has undergone considerable evolutionary modification.
Capacity to form choroid plexus-like cells in vitro is restricted to specific regions of the mouse neural ectoderm
Authors: Thomas Tim , Dziadek Marie
Published: April 2021

Neural ectoderm was dissected from 9.5-day and 8.5-day gestation mouse embryos and divided into fore-brain, midbrain, hindbrain and spinal cord regions. Forebrain and hindbrain material from 9.5-day neural ectoderm was further divided into presumptive choroid plexus regions and regions that would normally form nervous tissue in vivo. All tissues were plated onto a basement membrane substratum for culture in vitro. It was found that explants of neural ectoderm that would normally form choroid plexus in vivo, readily differentiated to form choroid plexus-like cells in culture. Cells from hindbrain segments and forebrain regions, which would normally form nervous tissue, also had the potential to differentiate into cells resembling the choroid plexus epithelium in culture, provided that the normal cell-cell interactions were disrupted. Cells from the midbrain neuromeres of 9.5-day embryos, which do not form a choroid plexus in vivo, did not form this lineage in vitro. However, cells cultured from the earlier headfold stage midbrain neural ectoderm could develop into choroid plexus epithelium. There was no evidence that neural ectoderm from the spinal cord had the developmental potential to form choroid plexus epithelial cells at either of these two developmental stages. These studies show that the restrictions in the potential of neural ectoderm stem cells to form different lineages proceeds according to morphological divisions that appear along the anterior-posterior axis during the early stages of brain development. These results suggest that the division of neural ectoderm into segments which contain discrete stem cell populations may be a general feature of the early phase of development of the central nervous system.
A mRNA localized to the vegetal cortex of Xenopus oocytes encodes a protein with a nanos-like zinc finger domain
Authors: Mosquera Luis , Forristall Caryl , Zhou Yi , King Mary Lou
Published: April 2021

mRNAs concentrated in specific regions of the oocyte have been found to encode determinants that specify cell fate. We show that an intermediate filament fraction isolated from Xenopus stage VI oocytes specifically contains, in addition to Vg1 RNA, a new localized mRNA, Xcat-2. Like Vg1, Xcat-2 is found in the vegetal cortical region, is inherited by the vegetal blastomeres during development, and is degraded very early in development. Sequence analysis suggests that Xcat-2 encodes a protein that belongs to the CCHC RNA-binding family of zinc finger proteins. Interestingly, the closest known relative to Xcat-2 in this family is nanos, an RNA localized to the posterior pole of the Drosophila oocyte whose protein product suppresses the translation of the transcription factor hunchback. The localized and maternally restricted expression of Xcat-2 RNA suggests a role for its protein in setting up regional differences in gene expression that occur early in development.
Negative autoregulation by Ultrabithorax controls the level and pattern of its expression
Authors: Irvine Kenneth D. , Botas Juan , Jha Sanjaya , Mann Richard S. , Hogness David S.
Published: April 2021

The Drosophila homeotic gene Ultrabithorax (Ubx) encodes transcriptional regulatory proteins (UBX) that specify thoracic and abdominal segmental identities. Ubx autoregulation was examined by manipulating UBX levels, both genetically and with an inducible transgene, and monitoring the effect of these manipulations on the expression of Ubx and Ubx-lacZ reporter genes. Positive autoregulation by Ubx is restricted to the visceral meso-derm, while in other tissues Ubx negatively autoregu-lates. In some cases, negative autoregulation stabilizes UBX levels, while in others it modulates the spatial and temporal patterns of UBX expression. This modulation of UBX expression may enable Ubx to specify distinct identities in different segments. The upstream control region of Ubx contains multiple autoregulatory elements for both positive and negative autoregulation.
Perpendicular contact guidance of CNS neuroblasts on artificial microstructures
Authors: Nagata Isao , Kawana Akio , Nakatsuji Norio
Published: April 2021

Rodent CNS neuroblasts show parallel and perpendic-ular contact guidance behaviors on aligned neurite bun-dles in microexplant cultures (Nakatsuji, N. and Nagata, I. (1989) Development, 106, 441-447; N. I. and N. N. (1991) ibid., 112, 581-590). To test the hypothesis that the physical surface structure of the neurite bundle causes the perpendicular contact guidance, we cultured dissociated neuroblasts on quartz plates on which grat-ing-like microstructures were fabricated by lithographic techniques. Various types of CNS neuroblasts, but not PNS neurons, oriented their processes and migrated both perpendicular and parallel to the axis of the microstructure. Perpendicular orientation was fre-quently observed when the microstructured grooves had depths between 0.3 m and 0.8 m and a width of 1 m, which roughly mimics a tightly aligned neurite bundle. Thus, CNS neuroblasts have the ability to extend their processes and migrate perpendicular to aligned surface microstructures.
Origins, migration and differentiation of glial cells in the insect enteric nervous system from a discrete set of glial precursors
Authors: Copenhaver Philip F.
Published: April 2021

The enteric nervous system (ENS) of the moth, Mand - uca sexta, consists of two primary cellular domains and their associated nerves. The neurons of the anterior domain occupy two small peripheral ganglia (the frontal and hypocerebral ganglia), while a second population of neurons occupies a branching nerve plexus (the enteric plexus) that spans the foregut-midgut boundary. Previ-ously, we have shown these two regions arise by sepa-rate programs of neurogenesis: cells that form the ante-rior enteric ganglia are generated from three discrete proliferative zones that differentiate within the foregut epithelium. In contrast, the cells of the enteric plexus (the EP cells) emerge from a neurogenic placode within the posterior lip of the foregut. Both sets of neurons sub-sequently undergo an extended period of migration and reorganization to achieve their mature distributions. We now show that prior to the completion of neurogenesis, an additional class of precursor cells is generated from the three proliferative zones of the foregut. Coincident with the onset of neuronal migration, this precursor class enters a phase of enhanced mitotic activity, giving rise to a population of cells that continue to divide as the ENS matures. Using clonal analyses of individual precursors, we demonstrate that the progeny of these cells become distributed along the same pathways taken by the migratory neurons; subsequently, they contribute to an ensheathing layer around the branches of the enteric plexus and the enteric ganglia. We conclude that this additional precursor class, which shares a common developmental origin with the enteric neurons, gives rise to a distinct population of peripheral glial cells. More-over, the distribution of enteric glial cells is achieved by their migration and differentiation along the same path-ways that are formed during the preceding phases of neuronal migration.
Axis establishment and microtubule-mediated waves prior to first cleavage in Beroe ovata
Authors: Houliston Evelyn , Carré Danièle , Johnston Jennifer A. , Sardet Christian
Published: April 2021

The single axis (oral-aboral) and two planes of symme-try of the ctenophore Beroe ovata become established with respect to the position of zygote nucleus formation and the orientation of first cleavage. Bisection of Beroe eggs at different times revealed that differences in egg organisation are established in relation to the pre-sumptive oral-aboral axis before first cleavage. Lateral fragments produced after but not before the time of first mitosis developed into larvae lacking comb-plates on one side. Time-lapse video demonstrated that waves of cytoplasmic reorganisation spread through the layer of peripheral cytoplasm (ectoplasm) of the egg during the 80 minute period between pronuclear fusion and first cleavage, along the future oral-aboral axis. These waves are manifest as the progressive displacement and dis-persal of plaques of accumulated organelles around supernumerary sperm nuclei, and a series of surface movements. Their timing and direction of propagation suggest they may be involved in establishing cytoplas-mic differences with respect to the embryonic axis.

Inhibitor experiments suggested that the observed cytoplasmic reorganisation involves microtubules. Noco-dazole and taxol, which prevent microtubule turnover, blocked plaque dispersal and reduced surface move-ments. The microfilament-disrupting drug cytochalasin B did not prevent plaque dispersal but induced abnor-mal surface contractions. We examined changes in microtubule organisation using immunofluorescence on eggs fixed at different times and in live eggs following injection of rhodamine-tubulin. Giant microtubule asters become associated with each male pronucleus after the end of meiosis. Following pronuclear fusion they disappear successively, those nearest the zygote nucleus shrinking first, to establish gradients of aster size within single eggs. Regional differences in micro-tubule behaviour around the time of mitosis were revealed by brief taxol treatment, which induced the for-mation of small microtubule asters in the region of the nucleus or spindle during both first and second cell cycles. The observed wave of change may thus reflect the local appearance and spreading of mitotic activity as the zygote nucleus approaches mitosis.
Specifying the path of the intersegmental nerve of the Drosophila embryo: a role for Delta and Notch
Authors: Giniger Edward , Jan Lily Yeh , Jan Yuh Nung
Published: April 2021

The intersegmental nerve (ISN) of the Drosophila embryo follows a reproducible course near the anterior border of each segment. Based on the experiments reported here, we suggest that growth of the axons constituting the nerve is guided, in part, by the transmembrane proteins Delta and Notch. In particular, we suggest that expression of Delta protein on a branch of the trachea provides a path for the nerve through the lateral part of the embryo, and that the growing axons use the Notch protein on their surfaces to recognize this path. Consistent with this idea, we show that disruption of the trachea abolishes the ability of the ISN to extend through this part of the embryonic periphery. Finally, we argue that the same regulatory network that directs these peripheral axons also specifies the trajectory of part of the axonal scaffold of the central nervous system.
Antisense inhibition of AMEL translation demonstrates supramolecular controls for enamel HAP crystal growth during embryonic mouse molar development
Authors: Diekwisch Thomas , David Sasson , Bringas Pablo , Santos Valentino , Slavkin Harold C.
Published: April 2021

During tooth development, enamel organ epithelial cells express a tissue-specific gene product (amelogenin) which presumably functions to control calcium hydroxyapatite crystal growth patterns during enamel biomineralization. The present studies were designed to test the hypothesis that amelogenin as a supramolecular aggregate regulates crystal growth during enamel biomineralization. Antisense oligodeoxynucleotide strategy was used in a simple organ culture system to inhibit amelogenin translation. Under these experimental conditions, antisense treatment prior to and during amelogenin expression resulted in inhibition of amelogenin translation products within immunoprecipitated [35S]methionine metabolically labeled proteins. To determine the efficiency of antisense treatment in this model system, digoxigenin-labeled oligodeoxynucleotides were observed to diffuse throughout the tooth explants including the target ameloblast cells within 24 hours. Ultrastructural analyses of amelogenin supramolecular assembly as electron-dense stippled materials in antisense treated cultures demonstrated dysmorphology of the extracelular enamel matrix with a significant reduction in crystal length and width. We conclude that secreted extracellular proteins form a supramolecular aggregate, which controls both the orientation and dimensions of enamel crystal formation during tooth development.
A singularity of PDGF alpha-receptor expression in the dorsoventral axis of the neural tube may define the origin of the oligodendrocyte lineage
Authors: Pringle Nigel P. , Richardson William D.
Published: April 2021

During rat embryogenesis, PDGF alpha receptor (PDGF-R) mRNA is expressed in the ventral half of the spinal cord in two longitudinal columns, one each side of the central canal. Initially, these columns are only two cells wide but the cells subsequently appear to proliferate and disseminate throughout the spinal cord. Our previous studies of PDGF-R expression in the developing CNS suggested that PDGF-R may be a useful marker of the oligodendrocyte lineage in situ. The data presented here complement those studies and lead us to propose that the earliest oligodendrocyte precursors in the spinal cord originate in a very restricted region of the ventricular zone during a brief window of time around embryonic day 14 (E14). In the embryonic brain, migrating PDGF-R+ cells appear to originate in a localized germinal zone in the ventral diencephalon (beneath the foramen of Monro). Our data demonstrate that gene expression and cell fate can be regulated with exquisite spatial resolution along the dorsoventral axis of the mammalian neural tube.
Postnatal maturation of the dendritic fields of motoneuron pools supplying flexor and extensor muscles of the distal forelimb in the rat
Authors: Curfs M. H. J. M. , Gribnau A. A. M. , Dederen P. J. W. C.
Published: April 2021

In the rat cervical spinal cord the corticospinal projection on motoneurons either direct or indirect (via interneurons) comes about postnatally making it accessible for experimental research. Therefore, the postnatal developmental changes of motoneurons and in particular their dendritic fields were examined. Motoneurons innervating the two antagonistic muscles in the distal forepaw, the m. flexor digitorum profundus and the m. extensor digitorum communis, were retrogradely labelled by intramuscular injections of cholera toxin subunit B conjugated with horseradish peroxidase in rats of various postnatal ages. Following a 48-72 hour survival period the motoneurons and their dendritic fields were studied in the seventh and eighth cervical spinal cord segments.

Both the number and the position of motoneurons were found to remain constant throughout postnatal development. Extensor motoneurons were positioned dorsolaterally in the ventral horn at the border of grey and white matter, flexor motoneurons were in general medial to extensor motoneurons. The results on the dendritic field demonstrate firstly, that during postnatal development the extension of the dendrites of both flexor and extensor motoneurons changes from spreading out in all directions at postnatal day 2 to spreading in only a few, specific directions from postnatal day 21 onwards, with the restriction that both motoneuron pools follow a different time scale to achieve this. Secondly, both pools have a temporal dendritic component extending into the white matter of the lateral funiculus. Thirdly, the dendritic extension pattern of flexor motoneurons differs from that of extensor motoneurons: the former has a permanent component in the medial part of lamina VI while the latter only has a transient component (from postnatal day 2 to 10) in the lateral part of lamina VI. The functional implications of the different dendritic extension patterns are discussed.
Complex regulation of early paired expression: initial activation by gap genes and pattern modulation by pair-rule genes
Authors: Gutjahr Thomas , Frei Erich , Noll Markus
Published: April 2021

The paired gene is one of approximately 30 zygotic segmentation genes responsible for establishing the segmented body plan of Drosophila melanogaster. To gain insight into the mechanism by which the paired gene is expressed in a complex temporal and spatial pattern, we have examined paired protein expression in wild-type and mutant embryos. In wild-type embryos, paired protein is expressed in several phases. Initial expression in broad domains evolves into a pair-rule pattern of eight stripes during cellularization. Subsequently, a segmentpolarity-like pattern of fourteen stripes emerges. Later, at mid-embryogenesis, paired is expressed in specific regions of the head and in specific cells of the central nervous system. Analysis of the initial paired expression in the primary pair-rule mutants even-skipped, runt and hairy, and in all gap mutants suggests that the products of the gap genes hunchback, Krüppel, knirps and giant activate paired expression in stripes. With the exception of stripe 1, which is activated by even-skipped, and stripe 8, which depends upon runt, the primary pair-rule proteins are required for subsequent modulation rather than activation of the paired stripes. The factors activating paired expression in the pair-rule mode appear to interact with those activating it along the dorsoventral axis.
A role for wingless in the segmental gradient of Drosophila?
Authors: Sampedro Javier , Johnston Paul , Lawrence Peter A.
Published: April 2021

The wild-type functions of the Wnt family of genes are still little understood (for review see Nusse and Varmus, Cell 69, 1073-1087, 1992). In Drosophila, the wingless (D- Wnt-1) protein is expressed in segmental stripes: its absence leads to a complete failure of segmentation, loss of engrailed expression and lack of pattern in the cuticle. A predominating hypothesis is that the spatial distribution of wingless is crucial to pattern; it might carry an instructive signal from cells that secrete the protein to cells nearby, or it might form a concentration gradient which acts as a morphogen.

We tested these hypotheses by expressing wingless ubiquitously in wingless embryos. The distribution of wingless protein in these embryos is uniform. Despite this, engrailed expression persists, is confined to the most anterior third of the parasegment, and delineates the parasegment border. The cuticle shows a segmentally reiterated pattern and, dorsally, the denticles are normally distributed and oriented. Because all these position-specific features cannot have been placed by a local source or a differential distribution of wingless protein, we conclude that, in the early embryo, the role of wing - less is neither to act as a local instructive signal, nor as a morphogen. We propose an alternative hypothesis that the wild-type function of the wingless protein is to maintain and ‘seal’ the parasegment borders; in its absence the borders fail to isolate abutting segmental gradients.
achaete-scute feminizing activities and Drosophila sex determination
Authors: Parkhurst Susan M. , Lipshitz Howard D. , Ish-Horowicz David
Published: April 2021

Sex determination in Drosophila depends on X-linked ‘numerator’ genes activating early Sex-lethal (Sxl) transcription in females. One numerator gene, sisterless-b (sis-b), corresponds to the achaete-scute (AS-C) T4 basichelix-loop-helix (bHLH) gene. Two other closely related AS-C bHLH genes, T3 and T5, appear not to function as numerator elements. We analyzed endogenous AS-C expression and show that T4 is the major AS-C numerator gene because it is expressed earlier and more strongly than are T3 and T5. Only T4 expression is detectable during the early syncytial stages when Sxl state is being determined. Nevertheless, the effects of ectopic AS-C gene expression show that T3 and T5 proteins display weak but significant feminizing activities, enhancing male-lethality, and rescuing the femalelethality of sis mutations. Detailed examination of Sxl expression in rescued embryos suggests that female cells may be viable in the absence of detectable Sxl protein expression.