bluuebunny/crossref_metadata_2025 huggingface dataset:
Root development in Arabidopsis: four mutants with dramatically altered root morphogenesis
Authors: Benfey Philip N. , Linstead Paul J. , Roberts Keith , Schiefelbein John W. , Hauser Marie-Theres , Aeschbacher Roger A.
Published: April 2021

A genetic analysis of root development in Arabidopsis thaliana has identified mutants that have abnormal morphogenesis. Four of these root morphogenesis mutants show dramatic alterations in post-embryonic root development. The short-root mutation results in a change from indeterminate to determinate root growth and the loss of internal root cell layers. The cobra and lion’s tail mutations cause abnormal root cell expansion which is conditional upon the rate of root growth. Expansion is greatest in the epidermal cells in cobra and in the stele cells in lion’s tail . The sabre mutation causes abnormal cell expansion that is greatest in the root cortex cell layer and is independent of the root growth rate. The tissuespecific effects of these mutations were characterized with monoclona antibodies and a transgenic marker line. Genetic combinations of the four mutants have provided insight into the regulation of growth and cell shape during Arabidopsis root development.
Cellular organisation of the Arabidopsis thaliana root
Authors: Dolan Liam , Janmaat Kees , Willemsen Viola , Linstead Paul , Poethig Scott , Roberts Keith , Scheres Ben
Published: April 2021

The anatomy of the developing root of Arabidopsis is described using conventional histological techniques, scanning and transmission electron microscopy. The root meristem is derived from cells of the hypophysis and adjacent cells of the embryo proper. The postembryonic organization of the root is apparent in the mature embryo and is maintained in the growing primary root after germination. Cell number and location is relatively invariant in the primary root, with 8 cortical and endo-dermal cell files but more variable numbers of pericycle and epidermal cells. The organisation of cells in lateral roots is similar to that of the primary root but with more variability in the numbers of cell files in each layer. [3H]thymidine labeling of actively growing roots indicates that a quiescent centre of four central cells (derived from the hypophysis) is located between the root cap columella and the stele. This plate of four cells is surrounded by three groups of cells in, proximal, distal and lateral positions. The labeling patterns of these cells suggest that they are the initials for the files of cells that comprise the root. They give rise to four sets of cell files: the stele, the cortex and endodermis, the epidermis and lateral root-cap and the columella. A model of meristem activity is proposed based on these data. This description of Arabidopsis root structure underpins future work on the developmental genetics of root morphogenesis.
Activity and expression pattern of cyclin-dependent kinase 5 in the embryonic mouse nervous system
Authors: Tsai Li-Huei , Takahashi Takao , Caviness Verne S. , Harlow Ed
Published: April 2021

Cyclin-dependent kinase 5 (cdk5) was originally isolated on the basis of its close primary sequence homology to the human cdc2 serine/threonine kinase, the prototype of the cyclin-dependent kinases. While kinase activities of both cdc2 and cdk2 are detected in proliferating cells and are essential for cells to progress through the key transition points of the cell cycle, cdk5 kinase activity has been observed only in lysates of adult brain. In this study, we compared the activity and expression of cdk5 with that of cdc2 and cdk2 in the embryonic mouse forebrain. The expression and activity of cdk5 increased progressively as increasing numbers of cells exited the proliferative cycle. In contrast, the expression and activity of cdc2 and cdk2 were maximum at gestational day 11 (E11) when the majority of cells were proliferating and fell to barely detectable levels at E17 at the end of the cytogenetic period. Immunohistochemical studies showed that cdk5 is expressed in postmitotic neurons but not in glial cells or mitotically active cells. Expression of cdk5 was concentrated in fasciculated axons of postmitotic neurons. In contrast to other cell division cycle kinases to which it is closely related, cdk5 appears not to be expressed in dividing cells in the developing brain. These observations suggest that cdk5 may have a role in neuronal differentiation but not in the cell division cycle in the embryonic nervous system.
Xwnt-11: a maternally expressed Xenopus wnt gene
Authors: Ku Min , Melton Douglas A.
Published: April 2021

We have isolated and characterized a novel Xenopus wnt gene, Xwnt-11, whose expression pattern and overexpression phenotype suggest that it may be important for dorsal-ventral axis formation. Xwnt-11 mRNA is present during oogenesis and embryonic development through swimming tadpole stages. Xwnt-11 mRNA is ubiquitous in early oocytes and is localized during mid-oogenesis. By late oocyte stages, Xwnt-11 mRNA is localized to the vegetal cortex, with some mRNA in the vegetal cytoplasm. After egg maturation, Xwnt-11 mRNA is released from the vegetal cortex and is found in the vegetal cytoplasm. This early pattern of Xwnt-11 mRNA localization is similar to another vegetally localized maternal mRNA, Vg1 (D. A. Melton (1987)Nature 328, 80–82). In the late blastula, Xwnt-11 mRNA is found at high levels in the dorsal marginal zone. As gastrulation proceeds, Xwnt-11 mRNA appears in the lateral and ventral marginal zone and, during tadpole stages, it is found in the somites and first branchial arch. Injection of Xwnt-11 mRNA into UV-ventralized embryos can substantially rescue the UV defect by inducing the formation of dorsal tissues. The rescued embryos develop somitic muscle and neural tube; however, they lack notochord and anterior head structures.
In vitro manipulation of early mouse embryos induces HIV1-LTRlacZ transgene expression
Authors: Vernet Muriel , Cavard Catherine , Zider Alain , Fergelot Patricia , Grimber Gisèle , Briand Pascale
Published: April 2021

We report here that the transcriptional activity of early mouse embryos is affected by their manipulation and culture in vitro, using transgenic embryos that express the reporter gene lacZ. We examined the pattern of expression of the lacZ gene fused to the human immunodeficiency virus type 1 long terminal repeat during the preimplantation stages. Transgene expression is induced as early as the two-cell stage in embryos developed in vitro, while there is no constitutive expression at the same stage in embryos developed in vivo. We have established a relation between this inducible expression occurring in vitro and an oxidative stress phenomenon. Indeed, when the culture medium is supplemented with antioxidants such N-acetyl-cysteine or CuZn-superoxide dismutase the transgene expression is markedly reduced. We also present evidence that the transgene expression in vitro coincides with the onset of the embryonic genome activation as attested by the synthesis of the 70 ×103Mr protein complex. Therefore, this transgene expression could prove to be a useful tool in our understanding of the molecular mechanisms involved in this crucial developmental event.
Drosophila segmentation: after the first three hours
Authors: Lawrence Peter A. , Sampedro Javier
Published: April 2021

“The instruments of darkness tell us truths, Win us with honest trifles, to betray’s In deepest consequence”
From growth cone to synapse: the life history of the RP3 motor neuron
Authors: Broadie Kendal , Sink Helen , Vactor David Van , Fambrough Douglas , Whitington Paul M. , Bate Michael , Goodman Corey S.
Published: April 2021

In Drosophila, the ability to analyze the development of individually identified neurons with a variety of imaging and biophysical techniques can be complemented by sophisticated genetics and molecular biology. This powerful combination is allowing the development and function of single neurons and their synaptic connections to be unraveled at an unparalleled level of resolution. In this article, we focus on a single, identified motoneuron – RP3 – arguably the best understood neuron in the fruitfly. Many events in the life history of RP3 are well characterized, including cell migration, axon outgrowth and pathfinding within the central nervous system, pathfinding in the periphery to its appropriate muscle target domain, the specific recognition of its muscle targets, the events of synapse formation and maturation, and its mature function in the locomotion of the fly larva. Genetic analysis has revealed mutations in a number of different genes which affect specific aspects of RP3 development from axon outgrowth to synapse formation.
Genetic analysis of the sevenless signal transduction pathway of Drosophila
Authors: Hafen E. , Dickson B. , Raabe T. , Brunner D. , Oellers N. , van der Straten A.
Published: April 2021

The specification of tlie R7 photoreceptor cell fate in the devloping eye of Drosophila depends on the local activation of the sevenless (sev) receptor tyrosine kinase (RTK) by boss, a protein expressed on the membrane of the neighboring RS cell. Constitutive activation of the sevreceptor results in a dosage dependent increase in the number tuber of R7 cells per ommatidium. Genetic screens have been used to identify mutations that alter the efficiency of signal transduction. Subsequent molecular characterization of the corresponding genes has led to the identification of a number of proteins involved in transducing the signal from the receptor to the nucleus. In contrast to the receptor and its ligand, these components are shared between different signal transduction pathways not only in Drosophila but homologous components are also involved in signal transduction in other organisms.
The torso pathway in Drosophila: a model system to study receptor tyrosine kinase signal transduction
Authors: Lu Xiangyi , Perkins Lizabeth A. , Perrimon Norbert
Published: April 2021

In the Drosophila embryo, specification of terminal cell fates that result in the formation of both the head (acron) and tail (telson) regions is under the control of the torso (tor) receptor tyrosine kinase. The current knowledge suggests that activation of tor at the egg pole initiates a signal transduction pathway that is mediated sequentially by the guanine nucleotide releasing factor son of sevenless (Sos), the p21Rasl GTPase, the serine/threonine kinase D-raf and the tyrosine/threonine kinase MAPKK (Dsorl). Subsequently, it is postulated that activation, possibly by phosphorylation, of a transcription factor at the egg poles activates the transcription of the terminal gap genes tatlless and huckehein. These gap genes, which encode putative transcription factors, then control the expression of more downstream factors that ultimately result in head and tail differentiation. Also involved in tor signaling is the non-receptor protein tyrosine phosphatase corkscrew (csw). Here, we review the current model and discuss future research directions in this field.
Wingless, the Drosophila homolog of the proto-oncogene Wnt-1, can transform mouse mammary epithelial cells
Authors: Ramakrishna Naren R. , Brown Anthony M. C.
Published: April 2021

The segment polarity gene wingless is the Drosophila ortholog of mouse Wni-1, a proto-oncogene capable of causing transformation of mammary epithelial cells. These two genes presently represent the best studied members of the Wnt gene family. To evaluate the functional significance of the sequence conservation between wingless and Wnt-1, we have examined the effects of expressing the Drosophila gene in mouse mammary epithelial cell lines, wingless induced morphological transformation, focus formation, and mitogenesis in confluent cultures of these cells, with resulting phenotypes comparable to those obtained with mouse Bnt-1. In addition, RAC311c mammary cells expressing wingless were tumorigenic, indicating that the Drosophila gene is capable of inducing full neoplastic transformation. In cell co-culture experiments, wingless caused transformation via a paracrine mechanism, consistent with the extracellular location of its product and its proposed mechanism of action in Drosophila embryos. Our results indicate that wingless is functionally analogous to Wnt-1 in these mammary cell transformation assays and imply a striking conservation in the properties of the two gene products and their mechanisms of action.
The mechanism of cleavage in amphibian and sturgeon eggs
Authors: Zotin A. I.
Published: April 2021

The Cleavage plane in animal cells and eggs is known to be determined by the position of the mitotic apparatus (cf. Wilson, 1925). A displacement of the mitotic apparatus by centrifugation (Harvey, 1935), mechanical deformation of eggs (Rappaport, 1960, 1961) or by microsurgical operation (Carlson, 1952; Waddington, 1952; Kawamura, 1960; Chambers & Chambers, 1961; Dan & Kuno Kojima, 1963) results in a corresponding change in the location of the cleavage furrow. This shows that the position of the cleavage furrow is determined by the mitotic apparatus of the cell.
Polyembryony inDendrocoelum lacteumO. F. Müller
Authors: Kościelski Bogusław
Published: April 2021

The development of freshwater Turbellaria has been investigated by few scientists, but the embryonic development of Dendrocoelum lacteum was described in detail by Fuliński (1914, 1916). An experimental analysis of the embryonic development of Dendrocoelum lacteum and Planaria torva has been conducted by Seilern-Aspang (1957, 1958). By placing the contents of a cocoon of Planaria torva on a cover-slide one can observe the course of early development. Certain conditions of culture give rise to deformations of the embryonic rudiments, which in turn leads to polyembryony (Seilern-Aspang, 1951b). The phenomenon of polyembryony also occurs in Procerodes lobata (Seilern-Aspang, (1957a). In our investigations normal polyembryony in Dendrocoelum lacteum has been studied in embryonic development not disturbed by experimental factors.
La compétence neurogène du feuillet externe du blastoderme de Poulet en fonction du facteur‘temps’
Authors: Gallera par J. , Ivanov I.
Published: April 2021

The neural competence of the epiblast of the chick blastoderm as a function of time

All the experiments were done on White Leghorn chick blastoderms, cultured in vitro by a slightly modified version of New’s technique. The anterior region of the full primitive streak (Hensen’s node and the material slightly posterior to it) was excised and implanted on blastoderms of different stages of development. The grafts were always of uniform size (0 · 6 × 0 · 4 mm.). They were implanted either in the antero-lateral region of the area pellucida or in the area opaca. Generally, the grafts gave rise to material which exceeded their presumptive fate. These grafts would normally have formed the notochord, some somites, part of the mid-brain and the hind-brain and, lastly, the roof of the foregut. Our grafts also gave rise to prechordal mesoblast, and some even gave rise to lateral plates. The differentiation of these grafts does not depend on their localization and is not influenced by the age of the host. All the grafts transplanted on blastoderms which had not yet passed the full primitive streak stage, induced the formation of brain which in many cases develops into a more or less reduced medullary anlage. During the whole period of the formation of the primitive streak, the capacity of the ectoblast to form cerebral structures decreases very slowly. However, the frequency of the medullary inductions falls off rapidly from the mid-primitive streak stage on-wards. None of the grafts, made on the blastoderms at the beginning of the regression of the Hensen’s node or later, induced neural structures. At the most, the host ectoblast in contact with the grafts sometimes presents a placode-like appearance. Even these secondary types of inductions are no longer obtained when the implantations are made in the blastoderms at early head-fold or later stages.
The wingless signalling pathway and the patterning of the wing margin in Drosophila
Authors: Couso Juan Pablo , Bishop Sarah A. , Arias Alfonso Martinez
Published: April 2021

The margin of the wing of Drosophila is defined and patterned from a stripe of cells expressing the wingless (wg) gene that is established during the third larval instar in the developing wing blade. The expression of the genes cut and achaete in a small domain in the prospective wing margin region reflects the activity of wg and probably mediate its function. Our results indicate that, in the wing margin, the wingless signal requires the activity of at least three genes: armadillo (arm), dishevelled (dsh) and shaggy (sgg) and that the functional relationship between these genes and wg is the same as that which exist during the patterning of the larval epidermis. These observations indicate that arm, dsh and sgg encode elements of a unique ‘wingless signalling pathway’ that is used several times throughout development.
Distinct developmental expression of a new avian fibroblast growth factor receptor
Authors: Marcelle Christophe , Eichmann Anne , Halevy Orna , Bréant Christiane , Douarin Nicole M. Le
Published: April 2021

We have cloned a new member of the fibroblast growth factor receptor family from avian embryonic RNA. The FREK (for fibroblast growth factor receptor-like embryonic kinase) primary transcript can be alternatively spliced in a tissue- and stage-specific manner to give rise to molecules containing either two or three Ig-like domains. During elongating primitive streak stages, FREK is expressed in the rostral and lateral epiblast and in the Hensen’s node. From 2.5 days of development (E 2.5) on, it is expressed in various ectoderm- and mesoderm-derived structures. Most striking is FREK expression in the skeletal muscle lineage. It is highly expressed in the early myotome and, at later stages, in all skeletal muscles of the embryo. From E9 to hatching, FREK expression in the muscles decreases dramatically but is maintained in satellite cells of adult muscles. FREK transcript is elevated upon addition of basic fibroblast growth factor to serum-starved satellite cells. From this sudy, we conclude: (1) that the structure and pattern of expression of FREK set it apart from other cloned fibroblast growth factor receptors (FGFR) and suggest that FREK is a new member of that family; (2) that FREK may play multiple roles in early avian development, including a specialized role in the early differentiation of skeletal muscle.
Ingrowth by photoreceptor axons induces transcription of a retrotransposon in the developing Drosophila brain
Authors: Mozer Brian A. , Benzer Seymour
Published: April 2021

The development of the lamina, the first optic ganglion of the fly visual system, depends on inductive cues from the innervating photoreceptor axons. lacZ expression from a Pelement insertion, A72, occurs in the anlage of the lamina coincident with axon ingrowth from the eye imaginal disc. In eyeless mutants lacking photoreceptor axons, lacZ expression did not occur.

The P-element was found to have inserted within the 3 long terminal repeat (LTR) of a ‘17.6’ type retrotransposon. The expression pattern of 17.6 transcripts in the brain in wild-type and eyeless mutants paralleled the expression of the lacZ reporter. Analysis of 17.6 cis-regulatory sequences indicates that the lamina-specific expression is due to the combined action of an enhancer element in the LTR and a repressor element within the internal body of the retrotransposon. The regulation of the 17.6 retrotransposon provides a model for the study of innervationdependent gene expression in postsynaptic cells during neurogenesis.
The role of Pax-1 in axial skeleton development
Authors: Wallin Johan , Wilting Jörg , Koseki Haruhiko , Fritsch Rüdiger , Christ Bodo , Balling Rudi
Published: April 2021

Previous studies have identified a single amino-acid substitution in the transcriptional regulator Pax-1 as the cause of the mouse skeletal mutant undulated (un). To evaluate the role of Pax-1 in the formation of the axial skeleton we have studied Pax-1 protein expression in early sclerotome cells and during subsequent embryonic development, and we have characterized the phenotype of three different Pax-1 mouse mutants, un, undulated-extensive (unex ) and Undulated short-tail (Uns ). In the Uns mutation the whole Pax-1 locus is deleted, resulting in the complete absence of Pax-1 protein in these mice. The other two genotypes are interpreted as hypomorphs. We conclude that Pax-1 is necessary for normal vertebral column formation along the entire axis, although the severity of the phenotype is strongest in the lumbar region and the tail. Pax-1-deficient mice lack vertebral bodies and intervertebral discs. The proximal part of the ribs and the rib homologues are also missing or severely malformed, whereas neural arches are nearly normal. Pax-1 is thus required for the development of the ventral parts of vertebrae. Embryonic analyses reveal that although sclerotomes are formed in mutant embryos, abnormalities can be detected from day 10.5 p.c onwards. The phenotypic analyses also suggest that the notochord still influences vertebral body formation some days after the sclerotomes are formed. Furthermore, the notochord diameter is larger in mutant embryos from day 12 p.c., due to increased cell proliferation. In the strongly affected genotypes the notochord persists as a rod-like structure and the nucleus pulposus is never properly formed. Since the notochord is Pax-1-negative these findings suggest a bidirectional interaction between notochord and paraxial mesoderm. The availability of these Pax-1 mutant alleles permitted us to define an early role for Pax-1 in sclerotome patterning as well as a late role in intervertebral disc development. Our observations suggest that Pax-1 function is required for essential steps in ventral sclerotome differentiation, i.e. for the transition from the mesenchymal stage to the onset of chondrogenesis.
Ectopic expression of either the Drosophila gooseberry-distal or proximal gene causes alterations of cell fate in the epidermis and central nervous system
Authors: Zhang Yu , Ungar Anne , Fresquez Catalina , Holmgren Robert
Published: April 2021

Previous studies have shown that the segment polarity locus gooseberry, which contains two closely related transcripts gooseberry-proximal and gooseberry-distal, is required for proper development in both the epidermis and the central nervous system of Drosophila. In this study, the roles of the gooseberry proteins in the process of cell fate specification have been examined by generating two fly lines in which either gooseberry-distal or gooseberryproximal expression is under the control of an hsp70 promoter. We have found that ectopic expression of either gooseberry protein causes cell fate transformations that are reciprocal to those of a gooseberry deletion mutant. Our results suggest that the gooseberry-distal protein is required for the specification of naked cuticle in the epidermis and specific neuroblasts in the central nervous system. These roles may reflect independent functions in neuroblasts and epidermal cells or a single function in the common ectodermal precursor cells. The gooseberryproximal protein is also found in the same neuroblasts as gooseberry-distal and in the descendants of these cells.
Pseudodirected variation in the requirement of cultured plant cells for celldivision factors
Authors: Meins Frederick , Seldran Monique
Published: April 2021

Cells cultured from explants of tobacco leaf require exogenous cell-division factors such as the cytokinin kinetin for sustained proliferation. When cytokinin-requiring (C− ) cells are cultured on medium containing 1/100 the optimum cytokinin concentration they rapidly give rise to cytokinin-autotrophic (C+ ) variants. Some of these variants result from a meiotically transmitted change at the Habituated leaf-2 locus. We measured the rate of phenotypic variation by a simple, quantitative method and found that cultured tobacco cells alternate between the C− and C+ states at extremely high rates of approx. 10−2 per cell generation, which is 102 to 103 -fold more rapid than most somatic mutations in tobacco. These changes are so rapid that the classical distinction between random and induced events is blurred. Selection of alternate phenotypes arising by rapid, reversible cellular variation results in changes that appear to be directed at the tissue level. This phenomenon, called pseudodirected variation, is of particular interest because it suggests novel stochastic mechanisms of cytokinin action and a plausible explanation for the directed, but plastic nature of development in plants.
Microtubule translocation and polymerisation during cortical rotation in Xenopus eggs
Authors: Houliston Evelyn
Published: April 2021

The development of dorsal axial structures in frogs depends on a process of cortical rotation in which the cortex of the fertilised egg becomes displaced with respect to the cytoplasm. An array of aligned microtubules that develops between the vegetal cortex and cytoplasm is implicated in generating movement. Rhodamine-tubulin was injected into eggs to allow patterns of microtubule movement and polymerisation in the vegetal array to be examined. Time-lapse video microscopy of living eggs showed that most of these microtubules move with the vegetal cytoplasm relative to the cortex, at the same speed as cytoplasmic pigment granules. This implies that movement is generated between the microtubules of the vegetal array and the cortex. A few microtubules were also detected that appeared immobile with respect to the cortex. Rhodamine-tubulin became incorporated into vegetal microtubules when injected at any time during the period of cortical rotation. The newly formed microtubules connected the vegetal array and internal cytoplasm. This indicates that local outward-directed polymerisation continues in this region as the cortex translocates. Experiments with low doses of nocodazole showed that this continuing polymerisation does not contribute to the rotation. Concentrations of the drug that prevented tubulin polymerisation had no effect on the speed of rotation if applied after the vegetal array had formed. The same doses prevented movement if applied early enough to prevent the formation of the array. These observations support the idea that mechanochemical enzymes associated with the vegetal microtubules translocate the cortex along microtubules anchored in the subcortical cytoplasm.
Maintenance of Wnt-3 expression in Purkinje cells of the mouse cerebellum depends on interactions with granule cells
Authors: Salinas Patricia C. , Fletcher Colin , Copeland Neal G. , Jenkins Nancy A. , Nusse Roel
Published: April 2021

Wnt genes encode secreted proteins implicated in cell fate changes during development. To define specific cell populations in which Wnt genes act, we have examined Wnt expression in the cerebellum. This part of the brain has a relatively simple structure and contains well-characterized cell populations. We found that Wnt-3 is expressed during development of the cerebellum and that expression is restricted to the Purkinje cell layer in the adult. Wnt-3 expression in Purkinje cells increases postnatally as granule cells start to make contacts with Purkinje cells. To investigate whether interactions with granule cells influence Wnt-3 expression in Purkinje cells, we examined gene expression in several mouse mutants, using the expression of En-2 to follow the fate of granule cells. In the weaver mutant, in which granule cells fail to migrate and subsequently die in the external granular layer, Wnt-3 expression was normal at postnatal day 15 (P15). At that time, some granule cells are still present in the external granular layer. At P28, however, when granule cells could no longer be detected, Wnt-3 expression was almost absent. In the meander tail mutant, in which the anterior cerebellar lobes lack granule cells, Wnt-3 expression was only detected in the normal posterior lobes. Since En genes are implicated in cell-cell interactions mediated by Wnt genes, we examined En-2/En-2 mutant mice, finding normal Wnt3 expression, indicating that the effect of granule cells on the maintenance of Wnt-3 is not mediated by En-2. Our results show that Wnt-3 expression in Purkinje cells is modulated by their presynaptic granule cells at the time of neuronal maturation.
Development of specific muscle and cutaneous sensory projections in cultured segments of spinal cord
Authors: Sharma K. , Korade Z. , Frank E.
Published: April 2021

Development of sensory projections was studied in cultured spinal segments with attached dorsal root ganglia. In spinal segments from stage 30 (E6.5) and older chicken embryos, prelabeled muscle and cutaneous afferents established appropriate projections. Cutaneous afferents terminated solely within the dorsolateral laminae, whereas some muscle afferents (presumably Ia afferents) projected ventrally towards motoneurons. Development of appropriate projections suggests that sufficient cues are preserved in spinal segments to support the formation of modality-specific sensory projections. Further, because these projections developed in the absence of muscle or skin, these results show that the continued presence of peripheral targets is not required for the formation of specific central projections after stage 29 (E6.0).

Development of the dorsal horn in cultured spinal segments was assessed using the dorsal midline as a marker. In ovo, this midline structure appears at stage 29. Lack of midline formation in stage 28 and 29 cultured spinal segments suggests that the development of the dorsal horn is arrested in this preparation. This is consistent with earlier reports suggesting that dorsal horn development may be dependent on factors outside the spinal cord.

Because dorsal horn development is blocked in cultured spinal segments, this preparation makes it possible to study the consequences of premature ingrowth of sensory axons into the spinal cord. In chicken embryos sensory afferents reach the spinal cord at stage 25 (E4.5) but do not arborize within the gray matter until stage 30. During this period dorsal horn cells are still being generated. In spinal segments, only those segments that have developed a midline at the time of culture support the formation of specific sensory projections. The end of the waiting period therefore coincides with the formation of a dorsal midline and, interestingly, also with the development of cues in the dorsal horn that are required for the formation of specific sensory projections. Based on these results, we propose that one important function of the waiting period is to delay the ingrowth of sensory fibers until appropriate guidance cues have developed within the dorsal horn.
Proneural genes influence gliogenesis in Drosophila
Authors: Giangrande Angela
Published: April 2021

Fly glial cells in the wing peripheral nervous system of Drosophila melanogaster originate from underlying epithelial cells. Two findings indicate that gliogenesis is closely associated with neurogenesis. First, it only occurs in regions that also give rise to sensory organs. Second, in mutants that induce the development of ectopic sensory organs glial cells develop at new positions. These findings prompted a genetic analysis to establish whether glial and sensory organ differentiation depend on the same genes. Loss of function mutations of the achaete-scute complex lead to a significant reduction of sensory bristles and glial cells. Genes within the complex affect gliogenesis with different strength and display some functional redundancy. Thus, neurogenesis and gliogenesis share the same genetic pathway. Despite these similarities, however, the mechanism of action of the achaete-scute complex seems to be different in the two processes. Neural precursors express products of the complex, therefore the role of these genes on neurogenesis is direct. However, markers specific to glial cells do not colocalize with products of the achaete-scute complex, showing that the complex affects gliogenesis indirectly. These observations lead to the hypothesis that glio-genesis is induced by the presence of sensory organ cells, either the precursor or its progeny.
The mouse Fgf8 gene encodes a family of polypeptides and is expressed in regions that direct outgrowth and patterning in the developing embryo
Authors: Crossley Philip H. , Martin Gail R.
Published: April 2021

Evidence is accumulating that members of the FGF gene family provide signals that act locally to regulate growth and patterning in vertebrate embryos. In this report, we provide a detailed analysis of the mouse Fgf8 gene. We have mapped the Fgf8 locus to the distal region of mouse chromosome 19, and sequenced the 5′coding region of the gene. Our data identify a new coding exon, and locate multiple splice donor and splice acceptor sites that can be used to produce at least seven transcripts encoding a family of secreted FGF8 proteins with different N termini. From these results, it appears that Fgf8 is structurally the most complex member of the FGF family described to date. In the embryo, many of the regions in which Fgf8 RNA is localized are known to direct outgrowth and patterning, including the apical ectodermal ridge of the limb bud, the primitive streak and tail bud, the surface ectoderm overlying the facial primorida and the midbrain-hindbrain junction, suggesting that FGF8 may be a component of the regulatory signals that emanate from these regions.
An inductive role for the endoderm in Xenopus cardiogenesis
Authors: Nascone Nanette , Mercola Mark
Published: April 2021

Heart induction in Xenopus has been thought to be dependent primarily on the interaction of the heart primordia with the Spemann organizer. We demonstrate, however, that signals derived from the deep dorsoanterior endoderm during early gastrulation are also essential for heart formation. The presence of deep endoderm dramatically enhances heart formation in explants of heart primordia, both in the presence and absence of organizer. Likewise, extirpation of the entire endoderm can decrease the frequency of heart formation in embryos that retain organizer activity. Finally, we show that the combined presence of both endoderm and organizer is necessary and sufficient to induce heart in ventral mesoderm explants that would not otherwise form heart tissue. Xenopus heart induction, therefore, may be a multistep process requiring separate dorsalization and cardiogenic signalling events. This is the first demonstration of a heart-inducing role for the endoderm in Xenopus, indicating that the mechanism of heart formation may be similar in most vertebrates.
Development of the mammalian ear: coordinate regulation of formation of the tympanic ring and the external acoustic meatus
Authors: Mallo Moisés , Gridley Thomas
Published: April 2021

The tympanic membrane in mammals is a trilaminar structure formed by the apposition of two epithelial cell layers, along with an intervening layer of cells derived from pharyngeal arch mesenchyme. One epithelial layer is contributed by the external acoustic meatus, a derivative of the first pharyngeal cleft. The other epithelial layer is contributed by the tubotympanic recess, a derivative of the first pharyngeal pouch. We demonstrate here an absolute correlation between formation of the external acoustic meatus and formation of the tympanic ring, a first archderived membrane bone that anchors the tympanic membrane. Experimental loss of the tympanic ring by retinoic acid treatment, or duplication of the ring in Hoxa-2 null mutant embryos, resulted in corresponding alterations in formation of the external acoustic meatus. We suggest that the tympanic ring primordium induces formation and morphogenesis of the external acoustic meatus, and that expression of the Hoxa-2 and goosecoid genes may be involved in regulating the formation and morphogenesis of these structures.
Activation of different myogenic pathways: myf-5 is induced by the neural tube and MyoD by the dorsal ectoderm in mouse paraxial mesoderm
Authors: Cossu G. , Kelly R. , Tajbakhsh S. , Donna S. Di , Vivarelli E. , Buckingham M.
Published: April 2021

Newly formed somites or unsegmented paraxial mesoderm (UPM) have been cultured either in isolation or with adjacent structures to investigate the influence of these tissues on myogenic differentiation in mammals. The extent of differentiation was easily and accurately quantified by counting the number of β-galactosidase-positive cells, since mesodermal tissues had been isolated from transgenic mice that carry the n-lacZ gene under the transcriptional control of a myosin light chain promoter, restricting expression to striated muscle. The results obtained showed that axial structures are necessary to promote differentiation of paraxial mesoderm, in agreement with previous observations. However, it also appeared that the influence of axial structures could be replaced by dorsolateral tissues, adjacent to the paraxial mesoderm. To elucidate which of these tissues exerts this positive effect, we cultured the paraxial mesoderm with a variety of adjacent structures, either adherent to the mesoderm or recombined in vitro. The results of these experiments indicated that the dorsal ectoderm exerts a positive influence on myogenesis but only if left in physical proximity to it. In contrast, lateral mesoderm delays the positive effect of the ectoderm (and has no effect on its own) suggesting that this tissue produces an inhibitory signal.

To investigate whether axial structures and dorsal ectoderm induce myogenesis through common or separate pathways, we dissected the medial half of the unsegmented paraxial mesoderm and cultured it with the adjacent neural tube. We also cultured the lateral half of the unsegmented paraxial mesoderm with adjacent ectoderm. The induction of the myogenic regulatory factors myf-5 and MyoD was monitored by double staining of cultured cells with antibodies against MyoD and β-galactosidase since the tissues were isolated from mouse embryos that carry n-lacZ targeted to the myf-5 gene, so that myf-5-expressing cells could be easily identified by either histochemical or immunocytochemical staining for β-galactosidase. After 1 day in culture myogenic cells from the medial half expressed myf-5 but not MyoD, while myogenic cells from the lateral half expressed MyoD but not myf-5. By the next day in vitro, however, most myogenic cells expressed both gene products. These data suggest that the neural tube activates myogenesis in the medial half of paraxial mesoderm through a myf-5-dependent pathway, while the dorsal ectoderm activates myogenesis through a MyoDdependent pathway. The possible developmental significance of these observations is discussed and a model of myogenic determination in mammals is proposed.
eagle, a member of the steroid receptor gene superfamily, is expressed in a subset of neuroblasts and regulates the fate of their putative progeny in the Drosophila CNS
Authors: Higashijima Shin-ichi , Shishido Emiko , Matsuzaki Masanori , Saigo Kaoru
Published: April 2021

We isolated and characterized the eagle gene, encoding a member of the steroid receptor superfamily in Drosophila. In the central nervous system, eagle RNA was expressed in a limited number of cells. During stages 10 and 11, eagle RNA expression was observed in four neuroblasts, NB2-4, NB3–3, NB6–4 and NB7–3. Except for NB6–4, eagle RNA expression reached a maximum at the very beginning of expression or in the period of neuroblast delamination. Weak eagle RNA expression was also observed in a few putative progeny of NB7–3 during stages, late 11 and 12. All eagle RNA in abdominal segments disappeared at stage 13. Using an eagle-kinesin-lacZ fusion gene as a reporter, the division, migration, and axonogenesis in eagle-positive cells and their derivatives were examined. At stage 14, several types of neural or glial cells were detected which include EG and EW interneurons joining to the anterior and posterior commissures, respectively. Lack of eagle expression caused altered axonogenesis in an appreciable fraction of eagle-Kinesin-LacZ-positive neurons. Some EG cells failed to acquire the neural fate or underwent an extremely delayed differentiation, while EW neurons produced neurites in abnormal directions, suggesting that eagle may play a critical role in development of the progeny of eagle-positive neuroblasts.
Granule cell specification in the developing mouse brain as defined by expression of the zinc finger transcription factor RU49
Authors: Yang Xiangdong W. , Zhong Ru , Heintz Nathaniel
Published: April 2021

The creation of specific neuronal cell types within the developing brain is a critical and unsolved biological problem. Precedent from invertebrate development, and from vertebrate myogenesis and lymphogenesis, has established that cell specification often involves transcription factors that are expressed throughout the differentiation of a given cell type. In this study, we have identified the Zn2+ finger transcription factor RU49 as a definitive marker for the cerebellar granule neuron lineage. Thus, RU49 is expressed in the earliest granule cell progenitors at the rhombic lip as they separate from the ventricular zone of the neural tube to generate a secondary proliferative matrix, and it continues to be expressed in differentiating and mature granule neurons. Proliferating granule cell progenitors isolated from the rhombic lip at E14 or from the external germinal layer at P6 continue to express RU49 in vitro. Both the olfactory bulb and dentate gyrus granule cell lineages also express this factor as they are generated with the developing brain. RU49 binds a novel bipartite DNA-binding element in a manner consistent with chemical rules governing the DNA-binding specificity of this class of transcription factor. The novel biochemical properties of RU49 and its restricted expression within the three lineages of CNS granule neurons suggest that RU49 may play a critical role in their specification. Furthermore, these results raise the interesting possibility that the generation of these three neuronal populations to form displaced germinative zones within the developing brain may reflect their use of a common developmental mechanism involving RU49.
decapentaplegic, a target gene of the wingless signalling pathway in the Drosophila midgut
Authors: Yu Xiang , Hoppler Stefan , Eresh Salih , Bienz Mariann
Published: April 2021

dishevelled, shaggy/zeste-white 3 and armadillo are required for transmission of the wingless signal in the Drosophila epidermis. We show that these genes act in the same epistatic order in the embryonic midgut to transmit the wingless signal. In addition to mediating transcriptional stimulation of the homeotic genes Ultrabithorax and labial, they are also required for transcriptional repression of labial by high wingless levels. Efficient labial expression thus only occurs within a window of intermediate wingless pathway activity. Finally, the shaggy/zeste-white 3 mutants revealed that wingless signalling can stimulate decapentaplegic transcription in the absence of Ultrabithorax, identifying decapentaplegic as a target gene of wingless. As decapentaplegic itself is required for wingless expression in the midgut, this represents a positive feed-back loop between two cell groups signalling to each other to stimulate each other’s signal production.
IGF-I, insulin and FGFs induce outgrowth of the limb buds of amelic mutant chick embryos
Authors: Dealy Caroline N. , Kosher Robert A.
Published: April 2021

IGF-I, insulin, FGF-2 and FGF-4 have been implicated in the reciprocal interactions between the apical ectodermal ridge (AER) and underlying mesoderm required for outgrowth and patterning of the developing limb. To study further the roles of these growth factors in limb outgrowth, we have examined their effects on the in vitro morphogen-esis of limb buds of the amelic mutant chick embryos wingless (wl) and limbless (ll). Limb buds of wl and ll mutant embryos form at the proper time in development, but fail to undergo further outgrowth and subsequently degenerate. Wl and ll limb buds lack thickened AERs capable of promoting limb outgrowth, and their thin apical ectoderms fail to express the homeobox-containing gene Msx-2, which is highly expressed by normal AERs and has been implicated in regulating AER activity. Here we report that exogenous IGF-I and insulin, and, to a lesser extent, FGF-2 and FGF-4 induce the proliferation and directed outgrowth of explanted wl and ll mutant limb buds, which in vitro, like in vivo, normally fail to undergo outgrowth and degenerate. IGF-I and insulin, but not FGFs, also cause the thin apical ectoderms of wl and ll limb buds to thicken and form structures that grossly resemble normal AERs and, moreover, induce high level expression of Msx-2 in these thickened AER-like structures. Neither IGF-I, insulin nor FGFs induce expression of the homeobox-containing gene Msx-1 in the subapical mesoderm of wl or ll limb buds, although FGFs, but not IGF-I or insulin, maintain Msx-1 expression in normal (non-mutant) limb bud explants lacking an AER. The implications of these results to the relationships among the wl and ll genes, IGF-I/insulin, FGFs, Msx-2 and Msx-1 in the regulation of limb outgrowth is discussed.
The morphogenesis of the pancreatic mesenchyme is uncoupled from that of the pancreatic epithelium in IPF1/PDX1-deficient mice
Authors: Ahlgren U. , Jonsson J. , Edlund H.
Published: April 2021

We have previously shown that mice carrying a null mutation in the homeobox gene ipf1, now renamed to pdx1, selectively lack a pancreas. To elucidate the level at which PDX1 is required during the development of the pancreas, we have in this study analyzed the early stages of pancreas ontogeny in PDX−/− mice. These analyses have revealed that the early inductive events leading to the formation of the pancreatic buds and the appearance of the early insulin and glucagon cells occur in the PDX1-deficient embryos. However, the sub-sequent morphogenesis of the pancreatic epithelium and the progression of differentiation of the endocrine cells are arrested in the pdx1−/− embryos. In contrast, the pancreatic mesenchyme grows and develops, both morphologically and functionally, independently of the epithelium. We also show that the pancreatic epithelium in the pdx1 mutants is unable to respond to the mesenchymal-derived signal(s) which normally promote pancreatic morphogenesis. Together these data provide evidence that PDX-1 acts cell autonomously and that the lack of a pancreas in the pdx1−/− mice is due to a defect in the pancreatic epithelium.
Inhibition of testicular germ cell apoptosis and differentiation in mice misexpressing Bcl-2 in spermatogonia
Authors: Furuchi Takemitsu , Masuko Kazue , Nishimune Yoshitake , Obinata Masuo , Matsui Yasuhisa
Published: April 2021

During normal spermatogenesis, more than half of the germ cells undergo apoptosis, but the physiological significance and molecular mechanisms of this programmed cell death are largely unknown. Because Bcl-2 functions as a death repressor, we have investigated the effect of misexpressing Bcl-2 in spermatogonia in transgenic mice using the human bcl-2 cDNA under the control of the human polypeptide chain elongation factor 1α (EF-1α) promoter. In the 2-week-old transgenic testes, exogenous Bcl-2 was expressed in spermatogonia and massive accumulation of spermatogonia was observed in seminiferous tubules by 4 weeks. At this time, only a few spermatocytes were apparent, and the accumulated cells degenerated, leading to vacuolization in some seminiferous tubules by 7 weeks. In older transgenic mice, abnormal accumulation of spermatogonia and degeneration of these germ cells was still observed, but some seminiferous tubules in which the level of Bcl-2 expression was reduced recovered normal spermatogenesis. These observations indicate that spermatogonial apoptosis is part of the normal program of mammalian spermatogenesis and is regulated by a pathway affected by Bcl-2.