bluuebunny/crossref_metadata_2025 huggingface dataset:
Requirement of c-kit for development of intestinal pacemaker system
Authors: Maeda Hitomi , Yamagata Akihiro , Nishikawa Satomi , Yoshinaga Kazuya , Kobayashi Shigeru , Nishi Katsuhide , Nishikawa Shin-Ichi
Published: April 2021

A discovery that the protooncogene encoding the receptor tyrosine kinase, c-kit, is allelic with the Dominant white spotting (W) locus establishes that c-kit plays a functional role in the development of three cell lineages, melanocyte, germ cell, and hematopoietic cell which are defective in W mutant mice. Recent analyses of c-kit expression in various tissues of mouse, however, have demonstrated that c-kit is expressed in more diverse tissues which are phenotypically normal in W mutant mice. Thus, whether or not c-kit expressed outside the three known cell lineages plays a functional role is one of the important questions needing answering in order to fully elucidate the role of c-kit in the development of the mouse. Here, we report that some of the cells in smooth muscle layers of developing intestine express c-kit. Blockade of its function for a few days postnatally by an antagonistic anti-c-kit monoclonal antibody (mAb) results in a severe anomaly of gut movement, which in BALB/c mice produces a lethal paralytic ileus. Physiological analysis indicates that the mechanisms required for the autonomic pacing of contraction in an isolated gut segment are defective in the anti-c-kit mAb-treated mice, W/Wvmice and even W/+ mice. These findings suggest that c-kit plays a crucial role in the development of a component of the pacemaker system that is required for the generation of autonomic gut motility.
Membrane-bound molecules in rat cerebral cortex regulate thalamic innervation
Authors: Götz Magdalena , Novak Nino , Bastmeyer Martin , Bolz Jürgen
Published: April 2021

During development of the thalamocortical projection, afferent fibers from the thalamus reach the cortex at a time when their target cells have just been generated but have not yet migrated to their final position. Thalamic axons begin to invade the cortex only shortly before their target layer 4 is formed. The mechanisms responsible for the innervation and termination of thalamic fibers in the cortex are not known. Here we show that the growth of thalamic axons in vitro is influenced by the age of cortical explants. Cortical explants of early embryonic stages were not invaded by thalamic explants, whereas thalamic fibers entered explants from postnatal cortices and terminated properly in their target layer 4 in vitro. Outgrowth assays on cortical cell membranes prepared at different developmental stages revealed that the growth of thalamic axons is selectively influenced by growth-promoting molecules that are upregulated during development. Moreover, experiments with postnatal cortical membranes isolated from distinct layers revealed that the growth of thalamic axons is selectively reduced on membranes prepared from layer 4. These results provide evidence that membrane-bound molecules in the cortex are involved in both the regulation of thalamic innervation into the cortical layers and their termination in the correct target layer.
Regionality of egg cytoplasm that promotes muscle differentiation in embryo of the ascidian, Halocynthia roretzi
Authors: Nishida Hiroki
Published: April 2021

Development of ascidians occurs in typical mosaic fashion: blastomeres isolated from early embryos differentiate into tissues according to their normal fates, an indication that cytoplasmic determinants exist in early blastomeres. To provide direct evidence for such cytoplasmic determinants, we have devised methods for fusing blastomeres and cytoplasmic fragments from various regions. (1) Presumptive-epidermis blastomeres were fused to cytoplasmic fragments from various regions of blastomeres of 8-cell embryos of Halocynthia roretzi and development of muscle cells was monitored by an antibody to ascidian myosin. Muscle differentiation was observed only when presumptive-epidermis blastomeres were fused with fragments from the posterior region of B4.1 (posterior-vegetal) blastomeres, the normal progenitor of muscle cells. The results indicate that muscle determinants are present and localized in the cytoplasm that enters muscle-lineage cells. (2) To investigate the presence and localization of muscle determinants in the egg, cytoplasmic fragments from various regions of unfertilized and fertilized eggs were fused with the presumptive-epidermis blastomeres, and formation of muscle cells was assessed by monitoring myosin, actin and acetylcholinesterase expression. These proteins were expressed only when cytoplasm from a restricted region of the eggs, i.e. the vegetal region, after the first phase of ooplasmic segregation, and posterior region, after the second phase of segregation, were fused. Based on these experiments, it is suggested that muscle determinants are segregated by ooplasmic movements after fertilization. They move initially to the vegetal pole of the egg and, prior to first cleavage, to the posterior region from whence future muscle-lineage blastomeres are formed. The inferred movements of muscle determinants correspond to those of the myoplasm, a microscopically visible portion of the egg cytoplasm.
Cranial and trunk neural crest cells use different mechanisms for attachment to extracellular matrices
Authors: Lallier Thomas , Leblanc Gabrielle , Artinger Kristin B. , Bronner-Fraser Marianne
Published: April 2021

We have used a quantitative cell attachment assay to compare the interactions of cranial and trunk neural crest cells with the extracellular matrix (ECM) molecules fibronectin, laminin and collagen types I and IV. Antibodies to the 1 subunit of integrin inhibited attachment under all conditions tested, suggesting that integrins mediate neural crest cell interactions with these ECM molecules. The HNK-1 antibody against a surface carbohydrate epitope under certain conditions inhibited both cranial and trunk neural crest cell attachment to laminin, but not to fibronectin. An antiserum to α1 integrin inhibited attachment of trunk, but not cranial, neural crest cells to laminin and collagen type I, though interactions with fibronectin or collagen type IV were unaffected.

The surface properties of trunk and cranial neural crest cells differed in several ways. First, trunk neural crest cells attached to collagen types I and IV, but cranial neural crest cells did not. Second, their divalent cation requirements for attachment to ECM molecules differed. For fibronectin substrata, trunk neural crest cells required divalent cations for attachment, whereas cranial neural crest cells bound in the absence of divalent cations. However, cranial neural crest cells lost this cation-independent attachment after a few days of culture. For laminin substrata, trunk cells used two integrins, one divalent cation-dependent and the other divalent cation-independent (Lallier, T. E. and Bronner-Fraser, M. (1991) Development 113, 1069-1081). In contrast, cranial neural crest cells attached to laminin using a single, divalent cation-dependent receptor system. Immunoprecipitations and immunoblots of surface labelled neural crest cells with HNK-1, α1 integrin and β1 integrin antibodies suggest that cranial and trunk neural crest cells possess biochemically distinct integrins. Our results demonstrate that cranial and trunk cells differ in their mechanisms of adhesion to selected ECM components, suggesting that they are non-over-lapping populations of cells with regard to their adhesive properties.
An amphioxus homeobox gene: sequence conservation, spatial expression during development and insights into vertebrate evolution
Authors: Holland Peter W. H. , Holland Linda Z. , Williams Nicola A. , Holland Nicholas D.
Published: April 2021

The embryology of amphioxus has much in common with vertebrate embryology, reflecting a close phylogenetic relationship between the two groups. Amphioxus embryology is simpler in several key respects, however, including a lack of pronounced craniofacial morphogenesis. To gain an insight into the molecular changes that accompanied the evolution of vertebrate embryology, and into the relationship between the amphioxus and vertebrate body plans, we have undertaken the first molecular level investigation of amphioxus embryonic development. We report the cloning, complete DNA sequence determination, sequence analysis and expression analysis of an amphioxus homeobox gene, AmphiHox3, evolutionarily homologous to the thirdmost 3′ paralogous group of mammalian Hox genes. Sequence comparison to a mammalian homologue, mouse Hox-2.7 (HoxB3), reveals several stretches of amino acid conservation within the deduced protein sequences. Whole mount in situ hybridization reveals localized expression of AmphiHox3 in the posterior mesoderm (but not in the somites), and region-specific expression in the dorsal nerve cord, of amphioxus neurulae, later embryos and larvae. The anterior limit to expression in the nerve cord is at the level of the four/five somite boundary at the neurula stage, and stabilises to just anterior to the first nerve cord pigment spot to form. Comparison to the anterior expression boundary of mouse Hox-2.7 (HoxB3) and related genes suggests that the vertebrate brain is homologous to an extensive region of the amphioxus nerve cord that contains the cerebral vesicle (a region at the extreme rostral tip) and extends posterior to somite four. This proposed homology implies that the vertebrate brain probably did not evolve solely from the cerebral vesicle of an amphioxus-like ancestor, nor did it arise entirely de novo anterior to the cerebral vesicle.
The consequences of ubiquitous expression of the wingless gene in the Drosophila embryo
Authors: Noordermeer Jasprien , Johnston Paul , Rijsewijk Frans , Nusse Roel , Lawrence Peter A.
Published: April 2021

The segment polarity gene wingless has an essential function in cell-to-cell communication during various stages of Drosophila development. The wingless gene encodes a secreted protein that affects gene expression in surrounding cells but does not spread far from the cells where it is made. In larvae, wingless is necessary to generate naked cuticle in a restricted part of each segment.

To test whether the local accumulation of wingless is essential for its function, we made transgenic flies that express wingless under the control of a hsp70 promoter (HS-wg flies). Uniform wingless expression results in a complete naked cuticle, uniform armadillo accumulation and broadening of the engrailed domain. The expression patterns of patched, cubitus interruptus Dominant and Ultrabithorax follow the change in engrailed.

The phenotype of heatshocked HS-wg embryos resembles the segment polarity mutant naked, suggesting that embryos that overexpress wingless or lack the naked gene enter similar developmental pathways.

The ubiquitous effects of ectopic wingless expression may indicate that most cells in the embryo can receive and interpret the wingless signal. For the development of the wild-type pattern, it is required that wingless is expressed in a subset of these cells.
Maintenance of the engrailed expression pattern by Polycomb group genes in Drosophila
Authors: Moazed Danesh , O’Farrell Patrick H.
Published: April 2021

The stable maintenance of expression patterns of homeotic genes depends on the function of a number of negative trans-regulators, termed the Polycomb (Pc) group of genes. We have examined the pattern of expression of the Drosophila segment polarity gene, engrailed (en), in embryos mutant for several different members of the Pc group. Here we report that embryos mutant for two or more Pc group genes show strong ectopic en expression, while only weak derepression of en occurs in embryos mutant for a single Pc group gene. This derepression is independent of two known activators of en expression: en itself and wingless. Additionally, in contrast to the strong ectopic expression of homeotic genes observed in extra sex combs− (esc−) mutant embryos, the en expression pattern is nearly normal in esc embryos. This suggests that the esc gene product functions in a pathway independent of the other genes in the group. The data indicate that the same group of genes is required for stable restriction of en expression to a striped pattern and for the restriction of expression of homeotic genes along the anterior-posterior axis, and support a global role for the Pc group genes in stable repression of activity of developmental selector genes.
Coordinate embryonic expression of three zebrafish engrailed genes
Authors: Ekker Marc , Wegner Jeremy , Andrée Akimenko Marie , Westerfield Monte
Published: April 2021

We have identified three genes, expressed in zebrafish embryos, that are members of the engrailed gene family. On the basis of sequence comparisons and analyses of their expression patterns, we suggest that two of these genes, eng2 and eng3, are closely related to the En-2 gene of other vertebrates. The third gene, eng1, is probably the zebrafish homolog of En-1. Sub-sets of cells at the developing junction between the midbrain and hindbrain express three different combinations of these genes, revealing a previously unknown complexity of this region of the CNS. Other cells, for example, jaw and myotomal muscle precursors, express two of the three genes in combinations which, in the myotomal muscles, change during development. Cells in the developing hindbrain and fins express only a single engrailed gene. We propose that the fates and patterning of these cells may be regulated by the coordinate expression of particular combinations of these closely related homeoproteins.
Reduced junctional permeability at interrhombomeric boundaries
Authors: Martinez S. , Geijo E. , Sánchez-Vives M. V. , Puelles L. , Gallego R.
Published: April 2021

Intercellular communication is considered to have a role during pattern specification processes in early embry onic development. This report analyzes the changing gap junctional communication properties of chick neu roepithelial cells depending on their position relative to the segmental partitions of the rhombencephalon. Inter cellular electrical coupling and dye transfer were studied with microelectrode techniques. Neuroepithelial cells were electrically coupled irrespective of their location relative to interneuromeric boundaries. lontophoretic injection of biocytin or Lucifer Yellow into single cells inside the rhombomeres was followed by transjunctional diffusion to the surrounding cells. In contrast, dye trans fer was strictly limited when the diffusion zone con tacted the cells forming the interneuromeric limits. Label injected into the boundary cells did not spread to other cells at all. Avian interrhombomeric boundaries are thus sites of reduced junctional permeability during early morphogenesis.
Mox-1 and Mox-2 define a novel homeobox gene subfamily and are differentially expressed during early mesodermal patterning in mouse embryos
Authors: Candia Albert F. , Hu Jinxin , Crosby Jeffrey , Lalley Peter A. , Noden Drew , Nadeau Joseph H. , Wright Christopher V. E.
Published: April 2021

We have isolated two mouse genes, Mox-1 and Mox-2 that, by sequence, genomic structure and expression pattern, define a novel homeobox gene family probably involved in mesodermal regionalization and somitic differentiation. Mox-1 is genetically linked to the keratin and Hox-2 genes of chromosome 11, while Mox-2 maps to chromosome 12.

At primitive streak stages (approximately 7.0 days post coitum), Mox-1 is expressed in mesoderm lying posterior of the future primordial head and heart. It is not expressed in neural tissue, ectoderm, or endoderm. Mox-1 expression may therefore define an extensive ‘posterior’ domain of embryonic mesoderm before, or at the earliest stages of, patterning of the mesoderm and neuroectoderm by the Hox cluster genes.

Between 7.5 and 9.5 days post coitum, Mox-1 is expressed in presomitic mesoderm, epithelial and differentiating somites (dermatome, myotome and sclerotome) and in lateral plate mesoderm. In the body of mid-gestation embryos, Mox-1 signal is restricted to loose undifferentiated mesenchyme. Mox-1 signal is also prominent over the mesenchyme of the heart cushions and truncus arteriosus, which arises from epithelialmesenchymal transformation and over a limited number of craniofacial foci of neural crest-derived mesenchyme that are associated with muscle attachment sites.

The expression profile of Mox-2 is similar to, but different from, that of Mox-1. For example, Mox-2 is apparently not expressed before somites form, is then expressed over the entire epithelial somite, but during somitic differentiation, Mox-2 signal rapidly becomes restricted to sclerotomal derivatives.

The expression patterns of these genes suggest regulatory roles for Mox-1 and Mox-2 in the initial anteriorposterior regionalization of vertebrate embryonic mesoderm and, in addition, in somite specification and differentiation.
Reorganisation of the microtubular cytoskeleton by embryonic microtubule-associated protein 2 (MAP2c)
Authors: Weisshaar Brigitte , Doll Thierry , Matus Andrew
Published: April 2021

Microtubule-associated protein 2c (MAP2c) is one of a set of embryonic MAP forms that are expressed during neuronal differentiation in the developing nervous system. We have investigated its mode of action by expressing recombinant protein in non-neuronal cell lines using cell cDNA transfection techniques. At every level of expression, all the MAP2c was bound to cellular microtubules. At low MAP2c levels, the microtubules retained their normal arrangement, radiating from the centrosomal microtubule-organising centre (MTOC) but at higher levels an increasing proportion of microtubules occurred independently of the MTOC. In most cells, radially oriented microtubules still attached to the MTOC co-existed with detached microtubules, suggesting that the primary effect of MAP2 is to increase the probability that tubulin polymerisation will occur independently of the MTOC. The MTOC-independent microtubules formed bundles whose distribution depended on their length in relation to the diameter of the transfected cell. Short bundles were attached to the cell cortex at one end and followed a straight course through the cytoplasm, whereas longer bundles followed a curved path around the periphery of the cell. By comparing these patterns to those produced by two chemical agents that stabilise microtubules, taxol and dimethyl sulphoxide, we conclude that effects of MAP2c arise from two sources. It stabilises microtubules without providing assembly initiation sites and as a result produces relatively few, long microtubule bundles. These bend only when they encounter the restraining influence of the cortical cytoskeleton of the cell, indicating that MAP2c also imparts stiffness to them. By conferring these properties of stability and stiffness to neuronal microtubules MAP2c contributes to supporting the structure of developing neurites.
Epithelial differentiation and intercellular junction formation in the mouse early embryo
Authors: Fleming Tom P. , Javed Qamar , Hay Mark
Published: April 2021

Trophectoderm differentiation during blastocyst formation provides a model for investigating how an epithelium develops in vivo. This paper briefly reviews our current understanding of the stages of differentiation and possible control mechanisms. The maturation of structural intercellular junctions is considered in more detail. Tight junction formation, essential for blastocoele cavitation and vectorial transport activity, begins at compaction (8-cell stage) and appears complete before fluid accumulation begins a day later (approx 32-cell stage). During this period, initial focal junction sites gradually extend laterally to become zonular and acquire the peripheral tight junction proteins ZO-1 and cingulin. Our studies indicate that junction components assemble in a temporal sequence with ZO-1 assembly preceding that of cingulin, suggesting that the junction forms progressively and in the ‘membrane to cytoplasm’ direction. The protein expression characteristics of ZO-1 and cingulin support this model. In contrast to ZO-1, cingulin expression is also detectable during oogenesis where the protein is localised in the cytocortex and in adjacent cumulus cells. However, maternal cingulin is metabolically unstable and does not appear to contribute to later tight junction formation in trophectoderm. Cell-cell interactions are important regulators of the level of synthesis and state of assembly of tight junction proteins, and also control the tissue-specificity of expression. In contrast to the progressive nature of tight junction formation, nascent desmosomes (formed from cavitation) appear mature in terms of their substructure and composition. The rapidity of desmosome assembly appears to be controlled by the time of expression of their transmembrane glycoprotein constituents; this occurs later than the expression of more cytoplasmic desmosome components and intermediate filaments which would therefore be available for assembly to occur to completion.
The role of intermediate filaments in early Xenopus development studied by antisense depletion of maternal mRNA
Authors: Heasman Janet , Torpey Nicholas , Wylie Chris
Published: April 2021

The effects of depleting a maternal cytokeratin mRNA on the developing embryo are described. Cytokeratins are members of the intermediate filament family of cytoskeletal proteins, and are expressed in a cortical network of the superficial cytoplasm of the oocyte. After fertilisation, a new cortical network is built up, which comes to occupy only the most superficial cells of the blastula. The maternal cytokeratin mRNA is abundantly translated, both during oogenesis, and during oocyte maturation and after fertilisation. Depletion of the mRNA results in depletion of the cortical filaments at the blastula stage and leads to gastrulation abnormalities. We discuss the various possible control experiments required for antisense oligo depletion studies and the implications of these results for cytokeratin function.
Specification of the body plan during Xenopus gastrulation: dorsoventral and anteroposterior patterning of the mesoderm
Authors: Slack J. M. W. , Isaacs H. V. , Johnson G. E. , Lettice L. A. , Tannahill D. , Thompson J.
Published: April 2021

Although the mesoderm itself is induced at the blastula stage, its subdivision mainly occurs in response to further inductive signals during gastrulation. In the late blastula, most of the mesoderm has a ventral-type commitment except for the small organizer region which extends about 30° on each side of the dorsal midline. During gastrulation, dorsal convergence movements bring the cells of the lateroventral marginal zone up near the dorsal midline and into the range of the dorsalizing signal emitted by the organizer. This dorsalizing signal operates throughout gastrulation, can cross a Nuclepore membrane, and is not mimicked by lithium, FGFs or activin.

Anteroposterior specification also takes place during gastrulation and is probably controlled by a dominant region at the posterior end of the forming axis.

We have studied the expression patterns in Xenopus of three members of the FGF family: bFGF, int-2 and a newly discovered species, eFGF. These all have mesoderm inducing activity on isolated animal caps, but are likely also to be involved with the later interactions. RNAase protections and in situ hybridizations show that the int-2 and eFGF mRNAs are concentrated at the posterior end, while bFGF is expressed as a posterior to anterior gradient from tailbud to head.

Studies of embryos in which bFGF is overexpressed from synthetic mRNA show that biological activity is far greater when a functional signal sequence is provided. This suggests that int-2 and eFGF, which possess signal sequences, are better candidates for inducing factors in vivo than is bFGF.
Induction of anteroposterior neural pattern in Xenopus by planar signals
Authors: Doniach Tabitha
Published: April 2021

Neural pattern in vertebrates has been thought to be induced in dorsal ectoderm by ‘vertical’ signals from underlying, patterned dorsal mesoderm. In the frog Xenopus laevis, it has recently been found that general neural differentiation and some pattern can be induced by ‘planar’ signals, i.e. those passing through the single plane formed by dorsal mesoderm and ectoderm, without the need for vertical interactions. Results in this paper, using the frog Xenopus laevis, indicate that four position-specific neural markers (the homeobox genes engrailed-2(en-2), XlHboxl and XlHboxó and the zinc-finger gene Krox-20) are expressed in planar explants of dorsal mesoderm and ectoderm (‘Keller explants’), in the same anteroposterior order as that in intact embryos. These genes are expressed regardless of convergent extension of the neurectoderm, and in the absence of head mesoderm. In addition, en-2 and XlHbox1 are not expressed in ectoderm when mesoderm is absent, but they and XlHbox6 are expressed in naïve, ventral ectoderm which has had only planar contact with dorsal mesoderm, en-2 expression can be induced ectopically, in ectoderm far anterior to the region normally fated to express it, suggesting that a prepattern is not required to determine where it is expressed. Finally, the mesoderm in planar explants expresses en-2 and XlHbox1 in an appropriate regional manner, indicating that A-P pattern in the mesoderm does not require vertical contact with ectoderm. Overall, these results indicate that anteroposterior neural pattern can be induced in ectoderm soley by planar signals from the mesoderm. Models for the induction of anteroposterior neural pattern by planar and vertical signals are discussed.
Pattern formation during gastrulation in the sea urchin embryo
Authors: McClay David R. , Armstrong Norris A. , Hardin Jeff
Published: April 2021

The sea urchin embryo follows a relatively simple cell behavioral sequence in its gastrulation movements. To form the mesoderm, primary mesenchyme cells ingress from the vegetal plate and then migrate along the basal lamina lining the blastocoel. The presumptive secondary mesenchyme and endoderm then invaginate from the vegetal pole of the embryo. The archenteron elongates and extends across the blastocoel until the tip of the archenteron touches and attaches to the opposite side of the blastocoel. Secondary mesenchyme cells, originally at the tip of the archenteron, differentiate to form a variety of structures including coelomic pouches, esophageal muscles, pigment cells and other cell types. After migration of the secondary mesenchyme cells from their original position at the tip of the archenteron, the endoderm fuses with an invagination of the ventral ectoderm (the stomodaem), to form the mouth and complete the process of gastrulation. A larval skeleton is made by primary mesenchyme cells during the time of archenteron and mouth formation.

A number of experiments have established that these morphogenetic movements involve a number of cell autonomous behaviors plus a series of cell interactions that provide spatial, temporal and scalar information to cells of the mesoderm and endoderm. The cell autonomous behaviors can be demonstrated by the ability of micromeres or endoderm to perform their morphogenetic functions if either is isolated and grown in culture. The requirement for cell interactions has been demonstrated by manipulative experiments where it has been shown that axial information, temporal information, spatial information and scalar information is obtained by mesoderm and endoderm from other embryonic cells. This information governs the cell autonomous behavior and places the cells in the correct embryonic context
The midblastula transition, the YSL transition and the onset of gastrulation in Fundulus
Authors: Trinkaus J. P.
Published: April 2021

The first signs of cell motility appear in Fundulus toward the end of cleavage, after cleavages 11 and 12. When blastomeres cease cleaving, their surfaces undulate and form blebs. At first, these blebbing cells remain in place. Gradually thereafter they begin movement, with blebs and filolamellipodia serving as organs of locomotion. Non-motile cleaving blastomeres have thus differentiated into motile blastula cells. This transformation corresponds to the midblastula transition of amphibian embryos.

Gastrulation in Fundulus begins with vegetalward contraction of the external yolk syncytial layer. This causes narrowing of the E-YSL and initiates the epibolic expansion of the blastoderm. Convergent movements of deep cells within the blastoderm begin toward the end of this contraction. The YSL forms as a result of invasion of the yolk cell cytoplasm by nuclei from open marginal blastomeres during cleavage. These YSL nuclei then undergo five metachronous divisions. After this, they divide no more. YSL contraction begins approximately 1.5 hours after cessation of these divisions (21-22°C). This cessation of nuclear divisions is preceded by a gradual decrease in rate. (1) The duration of each succeeding mitosis increases steadily and often some nuclei do not divide at mitosis V. (2) The duration of interphases between succeeding mitoses also increases, but to a much greater degree, and the longest interphase by far is the last one, I-IV, between mitoses IV and V. (3) The mitotic waves responsible for mitosis V move much more slowly than those for the first four mitoses and invariably decelerate. This gradual cessation of YSL nuclear divisions clearly sets the stage for the contraction of YSL cytoplasm and thus the beginning of gastrulation. We call this the YSL transition. It is not to be confused with the midblastula transition, which occurs 3-4 hours earlier. The MBT commences cytodifferentiation; the YSL transition commences morphogenesis.
NCAM and its polysialic acid moiety: a mechanism for pull/push regulation of cell interactions during development?
Authors: Rutishauser U.
Published: April 2021

Many cell adhesion molecules have a distinct pattern of expression and well-defined role in cell-cell recognition. In contrast, NCAM is broadly expressed and perturbations of its function affect many diverse aspects of embryonic development Evidence has been obtained suggesting that the molecule and its polysialic acid moiety serve not only to contribute to specific interactions, but also to regulate overall cell-cell apposition. In this latter mode, the molecule can have both a positive and a negative effect on a wide variety of contact-dependent cellular events.
Rhombomere transplantation repatterns the segmental organization of cranial nerves and reveals cell-autonomous expression of a homeodomain protein
Authors: Kuratani Shigeru C. , Eichele Gregor
Published: April 2021

The developing vertebrate hindbrain consists of segmental units known as rhombomeres. Hindbrain neuroectoderm expresses 3 Hox 1 and 2 cluster genes in characteristic patterns whose anterior limit of expression coincides with rhombomere boundaries. One particular Hox gene, referred to as Ghox 2.9, is initially expressed throughout the hindbrain up to the anterior border of rhombomere 4 (r4). Later, Ghox 2.9 is strongly upregulated in r4 and Ghox 2.9 protein is found in all neuroectodermal cells of r4 and in the hyoid crest cell population derived from this rhombomere. Using a polyclonal antibody, Ghox 2.9 was immunolocalized after transplanting r4 within the hindbrain. Wherever r4 was transplanted, Ghox 2.9 expression was cell-autonomous, both in the neuroectoderm of the graft and in the hyoid crest cell population originating from the graft.

In all vertebrates, rhombomeres and cranial nerves (nerves V, VII+VIII, IX, X) exhibit a stereotypic relationship: nerve V arises at the level of r2, nerve VII+VIII at r4 and nerves IX-X extend caudal to r6. To examine how rhombomere transplantation affects this pattern, operated embryos were stained with monoclonal antibodies E/C8 (for visualization of the PNS and of even-numbered rhombomeres) and HNK-1 (to detect crest cells and odd-numbered rhombomeres). Upon transplantation, rhombomeres did not change E/C8 or HNK-1 expression or their ability to produce crest cells. For example, transplanted r4 generated a lateral stream of crest cells irrespective of the site into which it was grafted. Moreover, later in development, ectopic r4 formed an additional cranial nerve root. In contrast, transplantation of r3 (lacks crest cells) into the region of r7 led to inhibition of nerve root formation in the host.

These findings emphasize that in contrast to spinal nerve segmentation, which entirely depends on the pattern of somites, cranial nerve patterning is brought about by factors intrinsic to rhombomeres and to the attached neural crest cell populations. The patterns of the neuroectoderm and of the PNS are specified early in hindbrain development and cannot be influenced by tissue transplantation. The observed cell-autonomous expression of Ghox 2.9 (and possibly also of other Hox genes) provides further evidence for the view that Hox gene expression underlies, at least in part, the segmental specification within the hindbrain neuroectoderm.
Mouse Cdx-1 expression during gastrulation
Authors: Meyer Barbara I. , Gruss Peter
Published: April 2021

We describe the expression pattern of the mouse Cdx-1 gene during early development, examined by both RNA and protein analyses. Cdx-1 expression began with the onset of the head process formation (day 7.5) in ectodermal and mesodermal cells of the primitive streak. Expression extended initially to the middle of the prospective hindbrain and subsequently regressed caudad to the spinal cord level by day 9.5. The mesoderm-specific expression was detected in the first somites and could be followed during their differentiation to the myotome of the dorsal somitic edge by day 12. The developing limb buds and the mesonephros exhibited expression up to day 12. No signal could be detected in notochordal cells and cells of the definitive endoderm. Thus, Cdx-1 is expressed during gastrulation when anterior-posterior positional values are established along the embryonic axes. Furthermore, the expression correlates with the formation of segmented tissue in the posterior hindbrain, the spinal cord and structures like the mesonephros.
Induction of floor plate differentiation by contact-dependent, homeogenetic signals
Authors: Placzek Marysia , Jessell Thomas M. , Dodd Jane
Published: April 2021

The floor plate is located at the ventral midline of the neural tube and has been implicated in neural cell patterning and axon guidance. To address the cellular mechanisms involved in floor plate differentiation, we have used an assay that monitors the expression of floorplate-specific antigens in neural plate explants cultured in the presence of inducing tissues. Contact-mediated signals from both the notochord and the floor plate act directly on neural plate cells to induce floor plate differentiation. Floor plate induction is initiated medially by a signal from the notochord, but appears to be propagated to more lateral cells by homeogenetic signals that derive from medial floor plate cells. The response of neural plate cells to inductive signals declines with embryonic age, suggesting that the mediolateral extent of the floor plate is limited by a loss of competence of neural cells. The rostral boundary of the floor plate at the midbrain-forebrain junction appears to result from the lack of inducing activity in prechordal mesoderm and the inability of rostral neural plate cells to respond to inductive signals.
Genetic analysis of the Drosophila cdc2 homolog
Authors: Stern Bodo , Ried Gabriele , Clegg Nigel J. , Grigliatti Thomas A. , Lehner Christian F.
Published: April 2021

We have identified mutations in the Drosophila cdc2 gene. The recessive lethality of these mutant alleles was rescued after P-element-mediated transformation with a genomic cdc2 fragment. Sequence analysis of amorphic alleles revealed non-conservative exchanges in evolutionary conserved positions. These alleles caused lethality at the larval-pupal interphase due to the absence of imaginal tissues. Embryonic lethality resulted when the maternal Dm cdc2 contribution was reduced through the use of a temperature-sensitive allele. Dm cdc2 function, therefore, is essential for cell proliferation throughout development. Dm cdc2 function is clearly required for mitosis, but no evidence for a requirement in S-phase was obtained. The reversible block of the mitotic proliferation which was observed in the PNS of mutant embryos occurred exclusively in the G2-phase. Moreover, while the mitotic proliferation of imaginal cells was blocked in the amorphic mutant larvae, non-imaginal larval cells continued to grow and endoreplicate their DNA. The Dm cdc2 mutant phenotype could neither be rescued with Dm cdc2c (encoding a cdc2-like kinase) nor enhanced by a reduction of the Dm cdc2c gene dose. These results indicate that the Dm cdc2- and Dm cdc2c-kinases control different processes.
Properties of the dorsalizing signal in gastrulae of Xenopus laevis
Authors: Lettice Laura A. , Slack Jonathan M. W.
Published: April 2021

According to the ‘three signal model’, the regional specification of tissue type within the mesoderm of Xenopus laevis occurs in a process called ‘dorsalization’. We have studied the timing and transmission characteristics of this signal, and assessed the dorsalizing activity of the lithium ion and a panel of cytokines.

The marginal zone has been fate mapped during gastrulation by colloidal gold labelling and it is shown that the ventral tissue undergoes substantial circumferential expansion. The fate map information is used to provide tissues of constant cellular composition for experiments conducted at different stages.

The stage at which dorsalization can occur has been investigated by means of heterochronic dorsal-ventral combinations. The results indicate that the interaction occurs during gastrulation, with a decline in both signal strength and competence of the ventral marginal zone to respond as gastrulation proceeds.

The signal is capable of passing through arrangements of membranes that exclude the possibility of cytoplasmic contact, implying that it can be carried by a diffusible morphogen.

The effect on the ventral marginal zone of lithium and a number of cytokines has also been studied. While none appears to function as a dorsalizing signal, lithium acts during blastula stages to alter the response to the mesoderm-inducing signal such that the inductions are of a more dorsal character.

These data confirm that the dorsalizing signal is independent of and operates later than the signal(s) from the vegetal hemisphere that induce mesoderm during the blastula stages.
Mesonephric contribution to testis differentiation in the fetal mouse
Authors: Buehr Mia , Gu Subin , McLaren Anne
Published: April 2021

Testes from 11.5-day-old mouse embryos, with and without attached mesonephroi, were cultured for 7 days. Isolated testes failed to develop well-differentiated testis cords: however, when cultured attached to a mesonephros from either a male or a female donor embryo, testes developed cords that were normal in appearance. Testes cultured next to a mesonephric region but separated from it by a permeable filter, did not develop normal cords, nor did testes grafted to fragments of embryonic limb or heart. When testes were grafted to mesonephric regions from mice carrying a transgenic marker, the marker was found in some of the peritubular myoid cells and other interstitial cells of the testis, but not in the Sertoli cells or the germ cells. We conclude that after 11.5 days post coitum, cells can migrate from the mesonephric region into the differentiating testis and can contribute to the interstitial cell population, and that this contribution is necessary for the establishment of normal cord structure. The germ cells in all cultured testes, whether or not differentiated cords were present, were T1 prospermatogonia: no meiotic germ cells were seen.
Programmed cell death during Drosophila embryogenesis
Authors: Abrams John M. , White Kristin , Fessler Liselotte I. , Steller Hermann
Published: April 2021

The deliberate and orderly removal of cells by programmed cell death is a common phenomenon during the development of metazoan animals. We have examined the distribution and ultrastructural appearance of cell deaths that occur during embryogenesis in Drosophila melanogaster. A large number of cells die during embryonic development in Drosophila. These cells display ultrastructural features that resemble apoptosis observed in vertebrate systems, including nuclear condensation, fragmentation and engulfment by macrophages. Programmed cell deaths can be rapidly and reliably visualized in living wild-type and mutant Drosophila embryos using the vital dyes acridine orange or nile blue. Acridine orange appears to selectively stain apoptotic forms of death in these preparations, since cells undergoing necrotic deaths were not significantly labelled. Likewise, toluidine blue staining of fixed tissues resulted in highly specific labelling of apoptotic cells, indicating that apoptosis leads to specific biochemical changes responsible for the selective affinity to these dyes. Cell death begins at stage 11 (∼7 hours) of embryogenesis and thereafter becomes widespread, affecting many different tissues and regions of the embryo. Although the distribution of dying cells changes drastically over time, the overall pattern of cell death is highly reproducible for any given developmental stage. Detailed analysis of cell death in the central nervous system of stage 16 embryos (13-16 hours) revealed asymmetries in the exact number and position of dying cells on either side of the midline, suggesting that the decision to die may not be strictly predetermined at this stage. This work provides the basis for further molecular genetic studies on the control and execution of programmed cell death in Drosophila.
Cellular patterning of fast and slow fibres in the intermandibularis muscle of chick embryos
Authors: Robson L. G.
Published: April 2021

The way in which the pattern of cell types arises during development of individual muscles was explored. The pattern of cellular differentiation resulting from the synthesis of particular fast and slow myosin heavy chains (MyHC) was investigated in the intermandibularis muscle in the lower jaw of chick embryos. The intermandibularis muscle has a proximodistal pattern of fibre type distribution. The distal region of the muscle contains a ratio of 1.5:1 fast to slow muscle fibres, which increases to >2.5:1 in the proximal region. The intermandibularis muscle is assembled in a proximodistal sequence, with both fast and slow muscle cells differentiating within the earliest muscle and then establishing the specific pattern of cell types. This pattern is not dependent on a specific innervation source, as normal lower jaw muscles develop and the intermandibularis has the same graded cellular pattern when the mandibular primordium is grafted to the limb bud stump. Micromass cultures were used to explore the pool of potentially myogenic cells that are available to construct the muscles. Even before the muscle differentiates in vivo, both fast and slow cells are present in the primordia. These potentially myogenic cells are already distributed within the primordium in a proximodistal fashion that mimics the cellular pattern found in the muscle that develops.
Repression of Drosophila pair-rule segmentation genes by ectopic expression of tramtrack
Authors: Brown J. Lesley , Wu Carl
Published: April 2021

The tramtrack (ttk) protein has been proposed as a maternally provided repressor of the fushi tarazu (ftz) gene in Drosophila embryos at the preblastoderm stage. Consistent with this hypothesis, we have detected by immunohistochemistry the presence of ttk protein in preblastoderm embryos. This is followed by a complete decay upon formation of the cellular blastoderm when ftz striped expression is at its peak. In addition, the highly complex pattern of zygotic ttk expression suggests specific functions for ttk late in development that are separate from the regulation of ftz. We have produced ttk protein ectopically in blastoderm-stage embryos transformed with a heat shock-ttk construct. Ectopic ttk caused complete or near-complete repression of the endogenous ftz gene, as well as significant repression of the pair-rule genes even skipped, odd skipped, hairy and runt. These findings suggest that specific repression by ttk (or by undiscovered repressors) may be more than an isolated phenomenon during the rapid cleavage divi-sions, a period when the need for genetic repression has not been generally anticipated.
Allocation of the thoracic imaginal primordia in the Drosophila embryo
Authors: Cohen Barbara , Simcox Amanda A. , Cohen Stephen M.
Published: April 2021

The primordia of the thoracic imaginal discs of the Drosophila embryo originate as groups of cells spanning the parasegment boundary. We present evidence that the thoracic imaginal primordia are allocated in response to signals from the wingless (wg) and decapen - taplegic (dpp) gene products. Rows of cells that express wg intersect rows of cells that express dpp to form a ladder-like pattern in the ectoderm of the germ band extended embryo. The imaginal primordia originate as groups of cells which lie near these intersection points. We have used a molecular probe derived from the Distal-less (Dll) gene to show that this population contains progenitor cells for both the dorsal (i.e. wing) and ventral (i.e. leg) discs. Although we show that Dll function is not required for allocation of imaginal cells, acti-vation of an early Dll enhancer may serve as a molecular marker for allocation. A group of cells, which includes the imaginal progenitors, activate this enhancer in response to intercellular signals from wg and perhaps from dpp. We have used a conditional allele of wg to show that wg function is transiently required for both allocation of the imaginal primordia and for initiation of Dll expression in these cells during the brief interval when wg and dpp form the ladder-like pattern. Allocation of the imaginal primordium and activation of Dll expression appear to be parallel responses to a single set of positional cues.
Germline chimeric chickens from dispersed donor blastodermal cells and compromised recipient embryos
Authors: Carsience R. Stuart , Clark Mary Ellen , Verrinder Gibbins Ann M. , Etches Robert J.
Published: April 2021

Stage-X blastoderms, within intact eggs from White Leghorn hens, were exposed to 500-700 rads of radiation from a 60Co source prior to injection, into the subgerminal cavity, of approximately 100 or 200-400 dispersed cells from stage-X blastoderms isolated from eggs laid by Barred Plymouth Rock hens. Embryos developing past day 14 of incubation and hatched chicks were assessed for donor and recipient cell contribution to the melanocyte population through examination of black and yellow down pigmentation, respectively (Barred Plymouth Rocks have a recessive allele at the I locus while the White Leghorns have a dominant allele at the I locus). Of the 809 embryos injected with approximately 100 cells, 192 developed past day 14 and black pigmentation, indicating somatic chimerism, was observed on 118 of the 192 (58%) embryos and chicks. Of the 296 embryos injected with 200-400 donor cells, 86 developed past day 14 of incubation. Somatic chimerism was observed on 55 of the 86 (64%) embryos and chicks. To test for germline chimerism, birds surviving to maturity were mated to Barred Plymouth Rocks. Five somatically chimeric females were produced when approximately 100 cells were injected, and one was a germline chimera. Six somatic female chimeras were produced following the injection of 200-400 cells, three of which proved to be germline chimeras by the presence of Barred Rock chicks among their offspring. Two of the nine males produced by injecting approximately 100 cells were germline chimeras. Five of 6 somatic male chimeras, produced by injection of 200-400 cells, have sired black chicks; furthermore, one of the roosters produced by the injection of 200-400 cells has sired only donor-derived chicks. These data indicate that irradiation of the recipient embryo, prior to injection of the donor cells, consistently yields somatic and germline chimeric chickens. The ability to insert donor cells into the germline provides a powerful new tool that will facilitate molecular and cellular manipulation of the developing chick embryo.