bluuebunny/crossref_metadata_2025 huggingface dataset:
Establishment of segment polarity in the ectoderm of the leech Helobdella
Authors: Seaver Elaine C. , Shankland Marty
Published: April 2021

The segmented ectoderm and mesoderm of the leech arise via a stereotyped cell lineage from embryonic stem cells called teloblasts. Each teloblast gives rise to a column of primary blast cell daughters, and the blast cells generate descendant clones that serve as the segmental repeats of their particular teloblast lineage. We have examined the mechanism by which the leech primary blast cell clones acquire segment polarity – i.e. a fixed sequence of positional values ordered along the anteroposterior axis of the segmental repeat. In the O and P teloblast lineages, the earliest divisions of the primary blast cell segregate anterior and posterior cell fates along the anteroposterior axis. Using a laser microbeam, we ablated single cells from both o and p blast cell clones at stages when the clone was two to four cells in length. The developmental fate of the remaining cells was characterized with rhodamine-dextran lineage tracer. Twelve different progeny cells were ablated, and in every case the ablation eliminated the normal descendants of the ablated cell while having little or no detectable effect on the developmental fate of the remaining cells. This included experiments in which we specifically ablated those blast cell progeny that are known to express the engrailed gene, or their lineal precursors. These findings confirm and extend a previous study by showing that the establishment of segment polarity in the leech ectoderm is largely independent of cell interactions conveyed along the anteroposterior axis. Both intercellular signaling and engrailed expression play an important role in the segment polarity specification of the Drosophila embryo, and our findings suggest that there may be little or no conservation of this developmental mechanism between those two organisms.
HypomorphicMespallele distinguishes establishment of rostrocaudal polarity and segment border formation in somitogenesis
Authors: Nomura-Kitabayashi Aya , Takahashi Yu , Kitajima Satoshi , Inoue Tohru , Takeda Hiroyuki , Saga Yumiko
Published: April 2021

A bHLH-type transcription factor, Mesp2, plays an essential role in somite segmentation in mice. Zebrafish mespb (mesp-b), a putative homologue of mouse Mesp2, is transiently expressed in the rostral presomitic mesoderm similarly to Mesp2. To determine whether zebrafish mespb is a functional homologue of mouse Mesp2, zebrafish mespb was introduced into the mouse Mesp2 locus by homologous recombination. Introduced mespb almost rescued the Mesp2 deficiency in the homozygous mespb knockin mouse, indicating that mespb is a functional homologue of mouse Mesp2. Segmented somites were clearly observed although the partial fusion of the vertebral columns still occurred. Interestingly, however, the nature and dosage of the mespb gene affected the rescue event. A mouse line, which has a hypomorphic Mesp2 allele generated by the introduction of neo-mespb, gave rise to an epithelial somite without normal rostrocaudal (RC) polarity. RC polarity was also lacking in the presomitic mesoderm. The defects in RC polarity were determined by the altered expressions of Uncx4.1 and Dll1 in the segmented somites and presomitic mesoderm, respectively. In contrast, the expression of EphA4 (Epha4), lunatic fringe or protocadherin, thought to be involved in segment border formation, was fairly normal in hypomorphic mutant embryos. These results suggest that the Mesp family of transcription factors is involved in both segment border formation and establishment of RC polarity through different genetic cascades.
Modeling and computational analysis of EGF receptor-mediated cell communication inDrosophilaoogenesis
Authors: Shvartsman Stanislav Y. , Muratov Cyrill B. , Lauffenburger Douglas A.
Published: April 2021

Autocrine signaling through the Epidermal Growth Factor Receptor (EGFR) operates at various stages of development across species. A recent hypothesis suggested that a distributed network of EGFR autocrine loops was capable of spatially modulating a simple single-peaked input into a more complex two-peaked signaling pattern, specifying the formation of a pair organ in Drosophila oogenesis (two respiratory appendages on the eggshell). To test this hypothesis, we have integrated genetic and biochemical information about the EGFR network into a mechanistic model of transport and signaling. The model allows us to estimate the relative spatial ranges and time scales of the relevant feedback loops, to interpret the phenotypic transitions in eggshell morphology and to predict the effects of new genetic manipulations. We have found that the proposed mechanism with a single diffusing inhibitor is sufficient to convert a single-peaked extracellular input into a two-peaked pattern of intracellular signaling. Based on extensive computational analysis, we predict that the same mechanism is capable of generating more complex patterns. At least indirectly, this can be used to account for more complex eggshell morphologies observed in related fly species. We propose that versatility in signaling mediated by autocrine loops can be systematically explored using experiment-based mechanistic models and their analysis.
Towards a model of the organisation of planar polarity and pattern in theDrosophilaabdomen
Authors: Lawrence Peter A. , Casal José , Struhl Gary
Published: April 2021

The abdomen of adult Drosophila consists of a chain of alternating anterior (A) and posterior (P) compartments which are themselves subdivided into stripes of different types of cuticle. Most of the cuticle is decorated with hairs and bristles that point posteriorly, indicating the planar polarity of the cells. Here we research the link between pattern and polarity.Previously we showed that the pattern of the A compartment depends on the local concentration (the scalar) of a Hedgehog morphogen produced by cells in the P compartment. Here we present evidence that the P compartment is patterned by another morphogen, Wingless, which is induced by Hedgehog in A compartment cells and then spreads back into the P compartment. We also find that both Hedgehog and Wingless appear to specify pattern by activating the optomotor blind gene, which encodes a transcription factor.We re-examine our working model that planar polarity is determined by the cells reading the gradient in concentration (the vector) of a morphogen ‘X’ which is produced on receipt of Hedgehog. We present evidence that Hedgehog induces X production by driving optomotor blind expression. We tried but failed to identify X and present data that X is not likely to operate through the conventional Notch, Decapentaplegic, EGF or FGF transduction pathways, or to encode a Wnt. However, we argue that Wingless may act to enhance the production or organise the distribution of X. A simple model that accommodates our results is that X forms a monotonic gradient extending from the back of the A compartment to the front of the P compartment in the next segment, a unit constituting a parasegment.
In ovo transplantation of enteric nervous system precursors from vagal to sacral neural crest results in extensive hindgut colonisation
Authors: Burns Alan J. , Delalande Jean-Marie M. , Le Douarin Nicole M.
Published: April 2021

The enteric nervous system (ENS) is derived from vagal and sacral neural crest cells (NCC). Within the embryonic avian gut, vagal NCC migrate in a rostrocaudal direction to form the majority of neurons and glia along the entire length of the gastrointestinal tract, whereas sacral NCC migrate in an opposing caudorostral direction, initially forming the nerve of Remak, and contribute a smaller number of ENS cells primarily to the distal hindgut. In this study, we have investigated the ability of vagal NCC, transplanted to the sacral region of the neuraxis, to colonise the chick hindgut and form the ENS in an experimentally generated hypoganglionic hindgut in ovo model. Results showed that when the vagal NC was transplanted into the sacral region of the neuraxis, vagal-derived ENS precursors immediately migrated away from the neural tube along characteristic pathways, with numerous cells colonising the gut mesenchyme by embryonic day (E) 4. By E7, the colorectum was extensively colonised by transplanted vagal NCC and the migration front had advanced caudorostrally to the level of the umbilicus. By E10, the stage at which sacral NCC begin to colonise the hindgut in large numbers, myenteric and submucosal plexuses in the hindgut almost entirely composed of transplanted vagal NCC, while the migration front had progressed into the pre-umbilical intestine, midway between the stomach and umbilicus. Immunohistochemical staining with the pan-neuronal marker, ANNA-1, revealed that the transplanted vagal NCC differentiated into enteric neurons, and whole-mount staining with NADPH-diaphorase showed that myenteric and submucosal ganglia formed interconnecting plexuses, similar to control animals. Furthermore, using an anti-RET antibody, widespread immunostaining was observed throughout the ENS, within a subpopulation of sacral NC-derived ENS precursors, and in the majority of transplanted vagal-to-sacral NCC. Our results demonstrate that: (1) a cell autonomous difference exists between the migration/signalling mechanisms used by sacral and vagal NCC, as transplanted vagal cells migrated along pathways normally followed by sacral cells, but did so in much larger numbers, earlier in development; (2) vagal NCC transplanted into the sacral neuraxis extensively colonised the hindgut, migrated in a caudorostral direction, differentiated into neuronal phenotypes, and formed enteric plexuses; (3) RET immunostaining occurred in vagal crest-derived ENS cells, the nerve of Remak and a subpopulation of sacral NCC within hindgut enteric ganglia.
A study of mesoderm patterning through the analysis of the regulation ofXmyf-5expression
Authors: Polli Matthew , Amaya Enrique
Published: April 2021

Xenopus laevis has been a particularly useful model organism for identifying factors involved in the induction and patterning of the mesoderm, however, much remains to be learned about how these factors interact. The myogenic transcription factor Xmyf-5 is the earliest known gene to be expressed specifically in the dorsolateral mesoderm of the gastrula, a domain that is established by the interaction of dorsal and ventral signals. For this reason, we have begun to investigate how the expression of Xmyf-5 is regulated. We have identified a 7.28 kb Xenopus tropicalis Xmyf-5 (Xtmyf-5) genomic DNA fragment that accurately recapitulates the expression of the endogenous gene. Deletion and mutational analysis has identified HBX2, an essential element, approximately 1.2 kb upstream from the start of transcription, which is necessary for both activation and repression of Xtmyf-5 expression, implying that positional information is integrated at this site. Electrophoretic mobility shift assays demonstrate that HBX2 specifically interacts with gastrula stage embryonic extracts and that in vitro translated Xvent-1 protein binds to one of its functional motifs. Combined with gain- and loss-of-function experiments, the promoter analysis described here suggests that Xvent-1 functions to repress Xmyf-5 expression in the ventral domain of the marginal zone. Furthermore, the identification of HBX2 provides a tool with which to identify other molecules involved in the regulation of Xmyf-5 expression during gastrulation.
abdominal Aspecifies one cell type inDrosophilaby regulating one principal target gene
Authors: Brodu Véronique , Elstob Philip R. , Gould Alex P.
Published: April 2021

The Hox/homeotic genes encode transcription factors that generate segmental diversity during Drosophila development. At the level of the whole animal, they are believed to carry out this role by regulating a large number of downstream genes. Here we address the unresolved issue of how many Hox target genes are sufficient to define the identity of a single cell. We focus on the larval oenocyte, which is restricted to the abdomen and induced in response to a non-cell autonomous, transient and highly selective input from abdominal A (abdA). We use Hox mutant rescue assays to demonstrate that this function of abdA can be reconstituted by providing Rhomboid (Rho), a processing factor for the EGF receptor ligand, secreted Spitz. Thus, in order to make an oenocyte, abdA regulates just one principal target, rho, that acts at the top of a complex hierarchy of cell-differentiation genes. These studies strongly suggest that, in at least some contexts, Hox genes directly control only a few functional targets within each nucleus. This raises the possibility that much of the overall Hox downstream complexity results from cascades of indirect regulation and cell-to-cell heterogeneity.
Investigation of the role of cell-cell interactions in division plane determination during maize leaf development through mosaic analysis of thetangledmutation
Authors: Walker Keely L. , Smith Laurie G.
Published: April 2021

Most plant cells divide in planes that can be predicted from their shapes according to simple geometrical rules, but the division planes of some cells appear to be influenced by extracellular cues. In the maize leaf, some cells divide in orientations not predicted by their shapes, raising the possibility that cell-cell communication plays a role in division plane determination in this tissue. We investigated this possibility through mosaic analysis of the tangled (tan) mutation, which causes a high frequency of cells in all tissue layers to divide in abnormal orientations. Clonal sectors of tan mutant tissue marked by a closely linked albino mutation were examined to determine the phenotypes of cells near sector boundaries. We found that tan mutant cells always showed the mutant phenotype regardless of their proximity to wild-type cells, demonstrating that the wild-type Tan gene acts cell-autonomously in both lateral and transverse leaf dimensions to promote normally oriented divisions. However, if the normal division planes of wild-type cells depend on cell-cell communication involving the products of genes other than Tan, then aberrantly dividing tan mutant cells might send abnormal signals that alter the division planes of neighboring cells. The cell-autonomy of the tan mutation allowed us to investigate this possibility by examining wild-type cells near the boundaries of tan mutant sectors for evidence of aberrantly oriented divisions. We found that wild-type cells near tan mutant cells did not divide differently from other wild-type cells. These observations argue against the idea that the division planes of proliferatively dividing maize leaf epidermal cells are governed by short-range communication with their nearest neighbors.
The induced sectorArabidopsisapical embryonic fate map
Authors: Saulsberry Alexandria , Martin Paula R. , O’Brien Tim , Sieburth Leslie E. , Pickett F. Bryan
Published: April 2021

Creation of an embryonic fate map may provide insight into the patterns of cell division and specification contributing to the apical region of the early Arabidopsis embryo. A fate map has been constructed by inducing genetic chimerism during the two-apical-cell stage of embryogenesis to determine if the orientation of the first anticlinal cell division correlates with later developmental axes. Chimeras were also used to map the relative locations of precursors of the cotyledon and leaf primordia. Genetic chimeras were induced in embryos doubly heterozygous for a heat shock regulated Cre recombinase and a constitutively expressed β-glucuronidase (GUS) gene flanked by the loxP binding sites for Cre. Individual cells in the two-apical-cell stage embryo responding to heat shock produce GUS-negative daughter cells. Mature plants grown from seed derived from treated embryos were scored for GUS-negative sector extent in the cotyledons and leaves. The GUS-negative daughters of apical cells had a strong tendency to contribute primarily to one cotyledon or the other and to physically adjacent true leaf margins. This result indicated that patterns of early cell division correlate with later axes of symmetry in the embryo and that these patterns partially limit the fates available for adoption by daughter cells. However, GUS-negative sectors were shared between all regions of the mature plant, suggesting that there is no strict fate restriction imposed on the daughters of the first apical cells.
Cellular diversity in the developing nervous system: a temporal view fromDrosophila
Authors: Brody Thomas , Odenwald Ward F.
Published: April 2021

This article considers the evidence for temporal transitions in CNS neural precursor cell gene expression during development. In Drosophila, five prospective competence states have so far been identified, characterized by the successive expression of Hb→Kr→Pdm→Cas→Gh in many, but not all, neuroblasts. In each temporal window of transcription factor expression, the neuroblast generates sublineages whose temporal identity is determined by the competence state of the neuroblast at the time of birth of the sublineage. Although similar regulatory programs have not yet been identified in mammals, candidate regulatory genes have been identified. Further investigation of the genetic programs that guide both invertebrate and vertebrate neural precursor cell lineage development will ultimately lead to an understanding of the molecular events that control neuronal diversity.
Persistent myogenic capacity of the dermomyotome dorsomedial lip and restriction of myogenic competence
Authors: Venters Sara J. , Ordahl Charles P.
Published: April 2021

The dorsomedial lip (DML) of the somite dermomyotome is the source of cells for the early growth and morphogenesis of the epaxial primary myotome and the overlying dermomyotome epithelium. We have used quail-chick transplantation to investigate the mechanistic basis for DML activity. The ablated DML of chick wing-level somites was replaced with tissue fragments from various mesoderm regions of quail embryos and their capacity to form myotomal tissue assessed by confocal microscopy. Transplanted fragments from the epithelial sheet region of the dermomyotome exhibited full DML growth and morphogenetic capacity. Ventral somite fragments (sclerotome), head paraxial mesoderm or non-paraxial (lateral plate) mesoderm tested in this assay were each able to expand mitotically in concert with the surrounding paraxial mesoderm, although no myogenic potential was evident. When ablated DMLs were replaced with fragments of the dermomyotome ventrolateral lip of wing-level somites or pre-somitic mesoderm (segmental plate), myotome development was evident but was delayed or otherwise limited in some cases. Timed DML ablation-replacement experiments demonstrate that DML activity is progressive throughout the embryonic period (to at least E7) and its continued presence is necessary for the complete patterning of each myotome segment. The results of serial transplantation and BrdU pulse-chase experiments are most consistent with the conclusion that the DML consists of a self-renewing population of progenitor cells that are the primary source of cells driving the growth and morphogenesis of the myotome and dermomyotome in the epaxial domain of the body.
Abnormal lymphatic vessel development in neuropilin 2 mutant mice
Authors: Yuan Li , Moyon Delphine , Pardanaud Luc , Bréant Christiane , Karkkainen Marika J. , Alitalo Kari , Eichmann Anne
Published: April 2021

Neuropilin 2 is a receptor for class III semaphorins and for certain members of the vascular endothelial growth factor family. Targeted inactivation of the neuropilin 2 gene (Nrp2) has previously shown its role in neural development. We report that neuropilin 2 expression in the vascular system is restricted to veins and lymphatic vessels. Homozygous Nrp2 mutants show absence or severe reduction of small lymphatic vessels and capillaries during development. This correlated with a reduction of DNA synthesis in the lymphatic endothelial cells of the mutants. Arteries, veins and larger, collecting lymphatic vessels developed normally, suggesting that neuropilin 2 is selectively required for the formation of small lymphatic vessels and capillaries.
Asymmetric Numb distribution is critical for asymmetric cell division of mouse cerebral cortical stem cells and neuroblasts
Authors: Shen Qin , Zhong Weimin , Jan Yuh Nung , Temple Sally
Published: April 2021

Stem cells and neuroblasts derived from mouse embryos undergo repeated asymmetric cell divisions, generating neural lineage trees similar to those of invertebrates. In Drosophila, unequal distribution of Numb protein during mitosis produces asymmetric cell divisions and consequently diverse neural cell fates. We investigated whether a mouse homologue m-numb had a similar role during mouse cortical development.

Progenitor cells isolated from the embryonic mouse cortex were followed as they underwent their next cell division in vitro. Numb distribution was predominantly asymmetric during asymmetric cell divisions yielding a β-tubulin III− progenitor and a β-tubulin III+ neuronal cell (P/N divisions) and predominantly symmetric during divisions producing two neurons (N/N divisions). Cells from the numb knockout mouse underwent significantly fewer asymmetric P/N divisions compared to wild type, indicating a causal role for Numb.

When progenitor cells derived from early (E10) cortex undergo P/N divisions, both daughters express the progenitor marker Nestin, indicating their immature state, and Numb segregates into the P or N daughter with similar frequency. In contrast, when progenitor cells derived from later E13 cortex (during active neurogenesis in vivo) undergo P/N divisions they produce a Nestin+ progenitor and a Nestin– neuronal daughter, and Numb segregates preferentially into the neuronal daughter. Thus during mouse cortical neurogenesis, as in Drosophila neurogenesis, asymmetric segregation of Numb could inhibit Notch activity in one daughter to induce neuronal differentiation.

At terminal divisions generating two neurons, Numb was symmetrically distributed in approximately 80% of pairs and asymmetrically in 20%. We found a significant association between Numb distribution and morphology: most sisters of neuron pairs with symmetric Numb were similar and most with asymmetric Numb were different. Developing cortical neurons with Numb had longer processes than those without.

Numb is expressed by neuroblasts and stem cells and can be asymmetrically segregated by both. These data indicate Numb has an important role in generating asymmetric cell divisions and diverse cell fates during mouse cortical development.
Direct and indirect regulation ofderrière, aXenopusmesoderm-inducing factor, by VegT
Authors: White R. J. , Sun B. I. , Sive H. L. , Smith J. C.
Published: April 2021

One candidate for an endogenous mesoderm-inducing factor in Xenopus is derrière, a member of the TGFβ family closely related to Vg1. In this paper we first show that derrière is able to exert long-range effects in the early Xenopus embryo, reinforcing the view that it functions as a secreted factor required for proper formation of posterior structures. Analysis of the derrière promoter shows that expression of the gene is controlled through a complex inductive network involving VegT and TGFβ-related molecules and also, perhaps, FGF family members. The work confirms that derrière plays an important role in mesoderm formation and it illustrates the complex regulation to which inducing factors are subject.
Attractive and repulsive functions of Slit are mediated by different receptors in theDrosophilatrachea
Authors: Englund Camilla , Steneberg Pär , Falileeva Lyudmila , Xylourgidis Nikos , Samakovlis Christos
Published: April 2021

Oxygen delivery in many animals is enabled by the formation of unicellular capillary tubes that penetrate target tissues to facilitate gas exchange. We show that the tortuous outgrowth of tracheal unicellular branches towards their target tissues is controlled by complex local interactions with target cells. Slit, a phylogenetically conserved axonal guidance signal, is expressed in several tracheal targets and is required both for attraction and repulsion of tracheal branches. Robo and Robo2 are expressed in different branches, and are both necessary for the correct orientation of branch outgrowth. At the CNS midline, Slit functions as a repellent for tracheal branches and this function is mediated primarily by Robo. Robo2 is necessary for the tracheal response to the attractive Slit signal and its function is antagonized by Robo. We propose that the attractive and repulsive tracheal responses to Slit are mediated by different combinations of Robo and Robo2 receptors on the cell surface.
Requirement of signalling by receptor tyrosine kinase RET for the directed migration of enteric nervous system progenitor cells during mammalian embryogenesis
Authors: Natarajan Dipa , Marcos-Gutierrez Camelia , Pachnis Vassilis , de Graaff Esther
Published: April 2021

The majority of neurones and glia of the enteric nervous system (ENS) are derived from the vagal neural crest. Shortly after emigration from the neural tube, ENS progenitors invade the anterior foregut and, migrating in a rostrocaudal direction, colonise in an orderly fashion the rest of the foregut, the midgut and the hindgut. We provide evidence that activation of the receptor tyrosine kinase RET by glial cell line-derived neurotrophic factor (GDNF) is required for the directional migration of ENS progenitors towards and within the gut wall. We find that neural crest-derived cells present within foetal small intestine explants migrate towards an exogenous source of GDNF in a RET-dependent fashion. Consistent with an in vivo role of GDNF in the migration of ENS progenitors, we demonstrate that Gdnf is expressed at high levels in the gut of mouse embryos in a spatially and temporally regulated manner. Thus, during invasion of the foregut by vagal-derived neural crest cells, expression of Gdnf was restricted to the mesenchyme of the stomach, ahead of the invading NC cells. Twenty-four hours later and as the ENS progenitors were colonising the midgut,Gdnf expression was upregulated in a more posterior region —the caecum anlage. In further support of a role of endogenous GDNF in enteric neural crest cell migration, we find that in explant cultures GDNF produced by caecum is sufficient to attract NC cells residing in more anterior gut segments. In addition, two independently generated loss-of-function alleles of murine Ret, Ret.k— and miRet51, result in characteristic defects of neural crest cell migration within the developing gut. Finally, we identify phosphatidylinositol-3 kinase and the mitogen-activated protein kinase signalling pathways as playing crucial roles in the migratory response of enteric neural crest cells to GDNF.
Midline lineages in grasshopper produce neuronal siblings with asymmetric expression of Engrailed
Authors: Jia Xi Xi , Siegler Melody V. S.
Published: April 2021

The median neuroblast lineage of grasshopper has provided a model for the development of differing neuronal types within the insect central nervous system. According to the prevailing model, neurons of different types are produced in sequence. Contrary to this, we show that each ganglion mother cell from the median neuroblast produces two neurons of asymmetric type: one is Engrailed positive (of interneuronal fate); and one is Engrailed negative (of efferent fate). The mature neuronal population, however, results from differential neuronal death. This yields many interneurons and relatively few efferent neurons. Also contrary to previous reports, we find no evidence for glial production by the median neuroblast. We discuss evidence that neuronal lineages typically produce asymmetric progeny, an outcome that has important developmental and evolutionary implications.
Roles of the Homothorax/Meis/Prep homolog UNC-62 and the Exd/Pbx homologs CEH-20 and CEH-40 inC. elegansembryogenesis
Authors: Van Auken Kimberly , Weaver Daniel , Robertson Barbara , Sundaram Meera , Saldi Tassa , Edgar Lois , Elling Ulrich , Lee Monica , Boese Queta , Wood William B.
Published: April 2021

Co-factor homeodomain proteins such as Drosophila Homothorax (Hth)and Extradenticle (Exd) and their respective vertebrate homologs, the Meis/Prep and Pbx proteins, can increase the DNA-binding specificity of Hox protein transcription factors and appear to be required for many of their developmental functions. We show that the unc-62 gene encodes theC. elegans ortholog of Hth, and that maternal-effect unc-62mutations can cause severe posterior disorganization during embryogenesis (Nob phenotype), superficially similar to that seen in embryos lacking function of either the two posterior-group Hox genes nob-1 and php-3 or the caudal homolog pal-1. Other zygotically actingunc-62 alleles cause earlier embryonic arrest or incompletely penetrant larval lethality with variable morphogenetic defects among the survivors, suggesting that unc-62 functions are required at several stages of development. The differential accumulation of four unc-62transcripts is consistent with multiple functions. The C. elegans exdhomologs ceh-20 and ceh-40 interact genetically withunc-62 and may have overlapping roles in embryogenesis: neither CEH-20 nor CEH-40 appears to be required when the other is present, but loss of both functions causes incompletely penetrant embryonic lethality in the presence of unc-62(+) and complete embryonic lethality in the presence of an unc-62 hypomorphic allele.
Role of Lmx1b and Wnt1 in mesencephalon and metencephalon development
Authors: Matsunaga Eiji , Katahira Tatsuya , Nakamura Harukazu
Published: April 2021

The isthmus is the organizing center for the tectum and cerebellum. Fgf8 and Wnt1 are secreted molecules expressed around the isthmus. The function of Fgf8 has been well analyzed, and now accepted as the most important organizing signal. Involvement of Wnt1 in the isthmic organizing activity was suggested by analysis of Wnt1 knockout mice. But its role in isthmic organizing activity is still obscure. Recently, it has been shown that Lmx1b is expressed in the isthmic region and that it may occupy higher hierarchical position in the gene expression cascade in the isthmus. We have carried out misexpression experiment of Lmx1b and Wnt1, and considered their role in the isthmic organizing activity. Lmx1b or Wnt1misexpression caused expansion of the tectum and cerebellum. Fgf8 was repressed in a cells that misexpress Lmx1b, but Fgf8expression was induced around Lmx1b-misexpressing cells. As Lmx1b induced Wnt1 and Wnt1 induced Fgf8 expression in turn, Wnt1 may be involved in non cell-autonomous induction of Fgf8 expression by Lmx1b. Wnt1 could not induce Lmx1b expression so thatLmx1b may be put at the higher hierarchical position thanWnt1 in gene expression cascade in the isthmus. We have examined the relationship among isthmus related genes, and discuss the mechanism of the formation and maintenance of isthmic organizing activity.
Regulation of the chondrocyte phenotype by β-catenin
Authors: Ryu Je-Hwang , Kim Song-Ja , Kim Seon-Hee , Oh Chun-Do , Hwang Sang-Gu , Chun Churl-Hong , Oh Seung-Hyun , Seong Je-Kyung , Huh Tae-Lin , Chun Jang-Soo
Published: April 2021

β-Catenin regulates important biological processes, including embryonic development and tumorigenesis. We have investigated the role ofβ-catenin in the regulation of the chondrocyte phenotype. Expression ofβ-catenin was high in prechondrogenic mesenchymal cells, but significantly decreased in differentiated chondrocytes both in vivo and in vitro. Accumulation of β-catenin by the inhibition of glycogen synthase kinase-3β with LiCl inhibited chondrogenesis by stabilizing cell-cell adhesion. Conversely, the low level of β-catenin in differentiated articular chondrocytes was increased by post-translational stabilization during phenotypic loss caused by a serial monolayer culture or exposure to retinoic acid or interleukin-1β. Ectopic expression of β-catenin or inhibition of β-catenin degradation with LiCl or proteasome inhibitor caused de-differentiation of chondrocytes. Transcriptional activation ofβ-catenin by its nuclear translocation was sufficient to cause phenotypic loss of differentiated chondrocytes. Expression pattern of Jun, a known target gene of β-catenin, is essentially the same as that of β-catenin both in vivo and in vitro suggesting that Jun and possibly activator protein 1 is involved in the β-catenin regulation of the chondrocyte phenotype.
The role of RBF in developmentally regulated cell proliferation in the eye disc and in Cyclin D/Cdk4 induced cellular growth
Authors: Xin Shijie , Weng Li , Xu Jinhua , Du Wei
Published: April 2021

During Drosophila eye development, cell proliferation is coordinated with differentiation. Immediately posterior to the morphogenetic furrow, cells enter a synchronous round of S phase called second mitotic wave. We have examined the role of RBF, the Drosophila RB family homolog, in cell cycle progression in the second mitotic wave. RBF-280, a mutant form of RBF that has four putative cdk phosphorylation sites mutated, can no longer be regulated by Cyclin D or Cyclin E. Expression of RBF-280 in the developing eye revealed that RBF-280 does not inhibit G1/S transition in the second mitotic wave, rather it delays the completion of S phase and leads to abnormal eye development. These observations suggest that RB/E2F control the rate of S-phase progression instead of G1/S transition in the second mitotic wave. Characterization of the role of RBF in Cyclin D/Cdk4-mediated cellular growth showed that RBF-280 blocks Cyclin D/Cdk4 induced cellular growth in the proliferating wing disc cells but not in the non-dividing eye disc cells. By contrast, RBF-280 does not block activated Ras-induced cellular growth. These results suggest that the ability of Cyclin D/Cdk4 to drive growth in the proliferating wing cells is distinct from that in the none-dividing eye cells or the ability of activated Ras to induce growth, and that RBF may have a role in regulating growth in the proliferating wing discs.
Cardiovascular ephrinB2 function is essential for embryonic angiogenesis
Authors: Gerety Sebastian S. , Anderson David J.
Published: April 2021

EphrinB2, a transmembrane ligand of EphB receptor tyrosine kinases, is specifically expressed in arteries. In ephrinB2 mutant embryos, there is a complete arrest of angiogenesis. However, ephrinB2 expression is not restricted to vascular endothelial cells, and it has been proposed that its essential function may be exerted in adjacent mesenchymal cells. We have generated mice in which ephrinB2 is specifically deleted in the endothelium and endocardium of the developing vasculature and heart. We find that such a vascular-specific deletion of ephrinB2 results in angiogenic remodeling defects identical to those seen in the conventional ephrinB2 mutants. These data indicate that ephrinB2 is required specifically in endothelial and endocardial cells for angiogenesis, and that ephrinB2 expression in perivascular mesenchyme is not sufficient to compensate for the loss of ephrinB2 in these vascular cells.
Involvement of a matrix metalloproteinase in MIS-induced cell death during urogenital development
Authors: Roberts Lori M. , Visser Jenny A. , Ingraham Holly A.
Published: April 2021

Programmed cell death of the Müllerian duct eliminates the primitive female reproductive tract during normal male sexual differentiation. Müllerian inhibiting substance (MIS or AMH) triggers regression by propagating a BMP-like signaling pathway in the Müllerian mesenchyme that culminates in apoptosis of the Müllerian duct epithelium. Presently, the paracrine signal(s) used in this developmental event are undefined. We have identified a member of the matrix metalloproteinase gene family, Mmp2, as one of the first candidate target genes downstream of the MIS cascade to function as a paracrine death factor in Müllerian duct regression. Consistent with a role in regression, Mmp2 expression was significantly elevated in male but not female Müllerian duct mesenchyme. Furthermore, this sexually dimorphic expression of Mmp2 was extinguished in mice lacking the MIS ligand, suggesting strongly that Mmp2 expression is regulated by MIS signaling. Using rat organ genital ridge organ cultures, we found that inhibition of MMP2 activity prevented MIS-induced regression, whereas activation of MMP2 promoted ligand-independent Müllerian duct regression. Finally, MMP2 antisense experiments resulted in partial blockage of Müllerian duct regression. Based on our findings, we propose that similar to other developmental programs where selective elimination or remodeling of tissues occurs, localized induction of extracellular proteinases is critical for normal male urogenital development.
TheC. elegansLAR-like receptor tyrosine phosphatase PTP-3 and the VAB-1 Eph receptor tyrosine kinase have partly redundant functions in morphogenesis
Authors: Harrington Robert J. , Gutch Michael J. , Hengartner Michael O. , Tonks Nicholas K. , Chisholm Andrew D.
Published: April 2021

Receptor-like protein-tyrosine phosphatases (RPTPs) form a diverse family of cell surface molecules whose functions remain poorly understood. The LAR subfamily of RPTPs has been implicated in axon guidance and neural development. Here we report the molecular and genetic analysis of the C. elegans LAR subfamily member PTP-3. PTP-3 isoforms are expressed in many tissues in early embryogenesis, and later become localized to neuronal processes and to epithelial adherens junctions. Loss of function in ptp-3 causes low-penetrance defects in gastrulation and epidermal development similar to those of VAB-1 Eph receptor tyrosine kinase mutants. Loss of function in ptp-3 synergistically enhances phenotypes of mutations in the C. elegans Eph receptor VAB-1 and a subset of its ephrin ligands, but does not show specific interactions with several other RTKs or morphogenetic mutants. The genetic interaction of vab-1 and ptp-3 suggests that LAR-like RPTPs and Eph receptors have related and partly redundant functions in C. elegans morphogenesis.
Mosaic analyses reveal the function ofDrosophila Rasin embryonic dorsoventral patterning and dorsal follicle cell morphogenesis
Authors: James Karen E. , Dorman Jennie B. , Berg Celeste A.
Published: April 2021

In Drosophila melanogaster, the Ras signal transduction pathway is the primary effector of receptor tyrosine kinases, which govern diverse developmental programs. During oogenesis, epidermal growth factor receptor signaling through the Ras pathway patterns the somatic follicular epithelium, establishing the dorsoventral asymmetry of eggshell and embryo. Analysis of follicle cell clones homozygous for a null allele of Ras demonstrates that Ras is required cell-autonomously to repress pipe transcription, the critical first step in embryonic dorsoventral patterning. The effects of aberrant pipe expression in Ras mosaic egg chambers can be ameliorated, however, by post-pipe patterning events, which salvage normal dorsoventral polarity in most embryos derived from egg chambers with dorsal Ras clones. The patterned follicular epithelium also determines the final shape of the eggshell, including the dorsal respiratory appendages, which are formed by the migration of two dorsolateral follicle cell populations. Confocal analyses of mosaic egg chambers demonstrate that Ras is required both cell- and non cell-autonomously for morphogenetic behaviors characteristic of dorsal follicle cell migration, and reveal a novel, Ras-dependent pattern of basal E-cadherin localization in dorsal midline follicle cells.
The origin and continuous replacement of epidermal cells in the planarianPolycelis tenuis(lijima)
Authors: Skaer R. J.
Published: April 2021

RAY LANKESTER (1873) coined the term ‘Triploblastic’ and supposed that the gut, parenchyma and epidermis of Turbellaria corresponded to the germ layers of contemporary dogma. This idea is still current, though neither the origin, nor the maintenance of the epidermis of planarians has been investigated in detail. Most embryological studies have been restricted to early development, but Bardeen (1902) worked on embryos of a wide range of ages and claimed that their epidermal cells divide amitotically. Both Mattiesen (1904) and Fulinsky (1916), however, denied that cell division occurs there, and since this has been confirmed for the epidermis of the adult (Skaer, 1961), the cells must be recruited from elsewhere. I suggested that the entire epidermis might be continuously renewed by migration of cells from the parenchyma to the periphery.
Adult brain antigens demonstrated in chick embryos by fractionated antisera
Authors: Friedman Harvey P. , Wenger Byron S.
Published: April 2021

Antisera produced in rabbits against homogenates of adult chicken brains and absorbed with liver and yolk were used to detect, via complement fixation, the appearance of brain antigens during development of the embryonic chick brain. Reacting antigens are found at the earliest stages studied (stages 14–16, ca. 2 days) but remain at a low level (relative to protein) until after stage 28 ( days), when a rapid increase begins, reaching the adult level by stage 38 (12 days). Antisera were fractionated on DEAE cellulose and the anti-brain activity found in two fractions, identified as 7S and 19S respectively. The 19S antibody reacted with a chloroform-methanol extract of adult brain homogenate but not with the residue, while the 7S antibody reacted only with the chloroform-methanol insoluble residue. When tested against brains of embryos at different developmental stages, the 19S antibody reacted at a low and essentially unchanging level from the earliest stages studied to the adult. The 7S antibody gave no reaction with embryonic brain until after stage 28 at which time a reacting antigen was found which reached the adult level by stage 38. Based on the appearance in detectable quantity, and the rapid accumulation, of a brain-specific protein antigen, the period between stages 28 and 38 is considered to be critical for neural differentiation.
Epsilon granules in Drosophila pole cells and oöcytes
Authors: Ullmann Suzanne L.
Published: April 2021

The ultra-structure of the pole cells and oöcytes of Drosophila melanogaster, Drosophila willistoni and Drosophila virilis has been investigated. The eggs were dechorionated in 3 per cent, sodium hypochlorite, and the vitelline membrane removed in the fixative. Three fixatives were employed: (a) OsO4-veronal, at pH 7 · 4; (b) formol-phosphate with 7 · 5 per cent, sucrose, at pH 7 · 2; (c) gluteraldehyde, buffered at pH 7 · 4 with sodium cacodyalate. All three gave good results, but membranous structures remained rather indistinct after aldehyde fixation. Two kinds of granule occur within the pole cells, the polar and the epsilon granules. Both types may occur together within the same cell, but one or other type tends to predominate. The functions and interrelationships of these granules are obscure. Polar granules are spherical with a dense, granular cortex surrounding a less electron-opaque core, and lack a bounding membrane. Epsilon granules are ovoid and a delicate double membrane surrounds the coarse fibrous matrix, in which spaces occur. Their composition is unknown. Preliminary observations suggest that the epsilon granules arise from convolutions of the cell membranes which surround the mature oöcyte.
Cell movement within aggregates of the slime mouldDictyostelium discoideumrevealed by surface markers
Authors: Shaffer B. M.
Published: April 2021

Earlier workers examined the behaviour of foreign particles placed as markers on aggregates of D. discoideum that were migrating over the surface of the culture plate (Bonner, 1959; Francis, 1959, 1962). Comparable observations, made on aggregates in other conditions and at other stages, have now provided further information about the movement of individual cells within the aggregates. Before reporting them, the course of development must be described in some detail.
Action de la thalidomide sur l’embryon de poulet
Authors: Salzgeber par Berthe , Salaün Josselyne
Published: April 2021

The action of thalidomide on the fowl embryo The effects of thalidomide on fowl embryos of different developmental ages (30 hr. to 3 days, stages 8-21 of Hamburger & Hamilton) have been studied. The thalidomide was administered in different forms; as a suspension in distilled water, in dioxane diluted with distilled water, as a powder, and as a sodium salt soluble in water. Since other powders, e.g. animal charcoal or egg-shell débris, are teratogenic when applied at an early stage, the results with thalidomide powder were only considered for embryos older than 50 hr. (stage 16) at the time of treatment. In all four experimental series malformed embryos were found with similar frequencies, i.e. between 20 and 30 per cent. From 15 to 22 per cent, showed malformations of the limbs. The limb malformations occurred with the following order of incidence : micromelia, phocomelia, hemimelia, ectromelia. They were nearly always associated with coelosomy or anuria. Thalidomide, though not very toxic, does, therefore, have a teratogenic effect upon the limbs, but this effect is less specific and less consistent than the effect obtained with nitrogen mustard (Salzgeber, 1963).
Effets de la puromycine sur la mitose, la synthèse des protéines et celle du DNA au cours de la segmentation de l’oeuf de Batracien
Authors: Legros par F. , Bracket J.
Published: April 2021

The effects of puromycin on mitotis, protein synthesis and DNA synthesis during cleavage of the amphibian egg

A study has been made of the effects of puromycin injected into Pleurodeles eggs. Cleavage of the eggs is rapidly blocked by this substance, after only a few mitoses.

Autoradiography using radioactive precursors (leucine, thymidine) has revealed that puromycin has no effect on protein and DNA synthesis during the first 3 hr. following injection. Cytoplasmic protein synthesis is interrupted at the end of this delay during the interphase of the 64-blastomere stage. Occasionally this final division occurs, but even then the inhibition of protein synthesis prevents the formation of the mitotic apparatus and the reduplication of DNA. Mitotic inhibition follows, usually at the 128-blastomere stage. It may thus be concluded that the inhibition of protein synthesis in the cytoplasm precedes the blocking of DNA synthesis and nuclear protein synthesis.
Intraocular differentiation of rateggcylinders
Authors: Levak-Švajger B. , Škreb N.
Published: April 2021

The anterior chamber of the eye is a suitable environment for testing the capacities of embryonic shields of the mouse to differentiate, as has been shown by Grobstein (1951). The development proceeds at a slower rate than usually and no morphogenesis is observed, although many identifiable tissues are found. The differentiation progressively increases as later stages are used. On the other hand, if younger stages, e.g. tubal ova or morulae, are transplanted (Runner, 1947), no histogenesis can be obtained although the germ layers are formed.