bluuebunny/crossref_metadata_2025 huggingface dataset:
Two new roles for theDrosophilaAP patterning system in early morphogenesis
Authors: Blankenship J. Todd , Wieschaus Eric
Published: April 2021

Although gastrulation is regarded as the stage during Drosophila development when the AP patterning system first influences morphological processes, transcription is regulated in complex patterns already at cycle 10. How soon this transcriptional complexity produces spatial differences in morphology has been unclear. We report on two new processes that establish visible morphological inhomogeneities before the onset of gastrulation. The first of these is the regulation of syncytial nuclear densities in the anterior end of the egg and represents the first zygotically driven AP asymmetry in the embryo. The second process is the generation of a fine-scale pattern in the actin/myosin array during cellularization. We find three domains of different yolk stalk diameters as well as depths of cellularization along the AP axis. These domains are established under the control of the AP patterning system and require bicoid activity. The anterior-most domain is a region of large yolk stalk diameters and corresponds to the region of decreased nuclear densities observed during syncytial stages. The middle domain shows smaller yolk stalk diameters and more rapid cellularization. Its establishment requires wild-type paired activity and thus indirectly requires bicoid. It occurs in a region of the embryo that ultimately gives rise to the cephalic furrow and may account for the effect of paired on that structure during gastrulation. Our results therefore suggest a link between cytoskeletal organization during cellularization and subsequent morphogenetic processes of gastrulation.
Function of basonuclin in increasing transcription of the ribosomal RNA genes during mouse oogenesis
Authors: Tian Qinjie , Kopf Gregory S. , Brown Raymond S. , Tseng Hung
Published: April 2021

Active protein synthesis during early oogenesis requires accelerated transcription of ribosomal RNA genes (rDNAs). In response to this demand, rDNAs are amplified more than 1000-fold early in Xenopus oogenesis. Here, we report evidence that rDNA is not amplified in mouse oocytes, but these cells may instead employ the zinc-finger protein basonuclin, a putative rDNA transcription factor, to enhance rRNA synthesis. This conclusion is based on observations that basonuclin is localized in the nucleolus in the mouse oocyte early in its growth phase, when Rrna transcription is highly active; and that the binding sites of basonuclin zinc fingers on the human and mouse rDNA promoters are homologous. In a co-transfection assay, basonuclin can elevate transcription from an rDNA promoter, and its zinc-finger domain can inhibit RNA polymerase I transcription, as detected by a run-on assay, in growing mouse oocytes.
The Wnt-activated Xiro1 gene encodes a repressor that is essential for neural development and downregulates Bmp4
Authors: Gómez-Skarmeta José Luis , Calle-Mustienes Elisa de la , Modolell Juan
Published: April 2021

In the early Xenopus embryo, the Xiro homeodomain proteins of the Iroquois (Iro) family control the expression of proneural genes and the size of the neural plate. We report that Xiro1 functions as a repressor that is strictly required for neural differentiation, even when the BMP4 pathway is impaired. We also show that Xiro1 and Bmp4 repress each other. Consistently, Xiro1 and Bmp4 have complementary patterns of expression during gastrulation. The expression of Xiro1 requires Wnt signaling. Thus, Xiro1 is probably a mediator of the known downregulation of Bmp4 by Wnt signaling.
C. elegans POP-1/TCF functions in a canonical Wnt pathway that controls cell migration and in a noncanonical Wnt pathway that controls cell polarity
Authors: Herman Michael A.
Published: April 2021

In Caenorhabditis elegans, Wnt signaling pathways are important in controlling cell polarity and cell migrations. In the embryo, a novel Wnt pathway functions through a β-catenin homolog, WRM-1, to downregulate the levels of POP-1/Tcf in the posterior daughter of the EMS blastomere. The level of POP-1 is also lower in the posterior daughters of many anteroposterior asymmetric cell divisions during development. I have found that this is the case for of a pair of postembryonic blast cells in the tail. In wild-type animals, the level of POP-1 is lower in the posterior daughters of the two T cells, TL and TR. Furthermore, in lin-44/Wnt mutants, in which the polarities of the T cell divisions are frequently reversed, the level of POP-1 is frequently lower in the anterior daughters of the T cells. I have used a novel RNA-mediated interference technique to interfere specifically with pop-1 zygotic function and have determined that pop-1 is required for wild-type T cell polarity. Surprisingly, none of the three C. elegans β-catenin homologs appeared to function with POP-1 to control T cell polarity. Wnt signaling by EGL-20/Wnt controls the migration of the descendants of the QL neuroblast by regulating the expression the Hox gene mab-5. Interfering with pop-1 zygotic function caused defects in the migration of the QL descendants that mimicked the defects in egl-20/Wnt mutants and blocked the expression of mab-5. This suggests that POP-1 functions in the canonical Wnt pathway to control QL descendant migration and in novel Wnt pathways to control EMS and T cell polarities.
Lineage, cell polarity and inscuteable function in the peripheral nervous system of the Drosophila embryo
Authors: Orgogozo Virginie , Schweisguth François , Bellaïche Yohanns
Published: April 2021

The stereotyped pattern of the Drosophila embryonic peripheral nervous system (PNS) makes it an ideal system to use to identify mutations affecting cell polarity during asymmetric cell division. However, the characterisation of such mutations requires a detailed description of the polarity of the asymmetric divisions in the sensory organ lineages. We describe the pattern of cell divisions generating the vp1-vp4a mono-innervated external sense (es) organs. Each sensory organ precursor (SOP) cell follows a series of four asymmetric cell divisions that generate the four es organs cells (the socket, shaft, sheath cells and the es neurone) together with one multidendritic (md) neurone. This lineage is distinct from any of the previously proposed es lineages. Strikingly, the stereotyped pattern of cell divisions in this lineage is identical to those described for the embryonic chordotonal organ lineage and for the adult thoracic bristle lineage. Our analysis reveals that the vp2-vp4a SOP cells divide with a planar polarity to generate a dorsal pIIa cell and a ventral pIIb cell. The pIIb cell next divides with an apical-basal polarity to generate a basal daughter cell that differentiates as an md neurone. We found that Inscuteable specifically accumulated at the apical pole of the dividing pIIb cell and regulated the polarity of the pIIb division. This study establishes for the first time the function of Inscuteable in the PNS, and provides the basis for studying the mechanisms controlling planar and apical-basal cell polarities in the embryonic sensory organ lineages.
Spatiotemporal selectivity of response to Notch1 signals in mammalian forebrain precursors
Authors: Chambers Christopher B. , Peng Ying , Nguyen Hoang , Gaiano Nicholas , Fishell Gord , Nye Jeffrey S.
Published: April 2021

The olfactory bulb, neocortex and archicortex arise from a common pool of progenitors in the dorsal telencephalon. We studied the consequences of supplying excess Notch1 signal in vivo on the cellular and regional destinies of telencephalic precursors using bicistronic replication defective retroviruses. After ventricular injections midneurogenesis (E14.5), activated Notch1 retrovirus markedly inhibited the generation of neurons from telencephalic precursors, delayed the emergence of cells from the subventricular zone (SVZ), and produced an augmentation of glial progeny in the neo-and archicortex. However, activated Notch1 had a distinct effect on the progenitors of the olfactory bulb, markedly reducing the numbers of cells of any type that migrated there. To elucidate the mechanism of the cell fate changes elicited by Notch1 signals in the cortical regions, short-and long-term cultures of E14.5 telencephalic progenitors were examined. These studies reveal that activated Notch1 elicits a cessation of proliferation that coincides with an inhibition of the generation of neurons. Later, during gliogenesis, activated Notch1 triggers a rapid cellular proliferation with a significant increase in the generation of cells expressing GFAP. To examine the generation of cells destined for the olfactory bulb, we used stereotaxic injections into the early postnatal anterior subventricular zone (SVZa). We observed that precursors of the olfactory bulb responded to Notch signals by remaining quiescent and failing to give rise to differentiated progeny of any type, unlike cortical precursor cells, which generated glia instead of neurons. These data show that forebrain precursors vary in their response to Notch signals according to spatial and temporal cues, and that Notch signals influence the composition of forebrain regions by modulating the rate of proliferation of neural precursor cells.
Drosophila wing development in the absence of dorsal identity
Authors: O’Keefe David D. , Thomas John B.
Published: April 2021

The developing wing disc of Drosophila is divided into distinct lineage-restricted compartments along both the anterior/posterior (A/P) and dorsal/ventral (D/V) axes. At compartment boundaries, morphogenic signals pattern the disc epithelium and direct appropriate outgrowth and differentiation of adult wing structures. The mechanisms by which affinity boundaries are established and maintained, however, are not completely understood. Compartment-specific adhesive differences and inter-compartment signaling have both been implicated in this process. The selector gene apterous (ap) is expressed in dorsal cells of the wing disc and is essential for D/V compartmentalization, wing margin formation, wing outgrowth and dorsal-specific wing structures. To better understand the mechanisms of Ap function and compartment formation, we have rescued aspects of the ap mutant phenotype with genes known to be downstream of Ap. We show that Fringe (Fng), a secreted protein involved in modulation of Notch signaling, is sufficient to rescue D/V compartmentalization, margin formation and wing outgrowth when appropriately expressed in an ap mutant background. When Fng and αPS1, a dorsally expressed integrin subunit, are co-expressed, a nearly normal-looking wing is generated. However, these wings are entirely of ventral identity. Our results demonstrate that a number of wing development features, including D/V compartmentalization and wing vein formation, can occur independently of dorsal identity and that inter-compartmental signaling, refined by Fng, plays the crucial role in maintaining the D/V affinity boundary. In addition, it is clear that key functions of the ap selector gene are mediated by only a small number of downstream effectors.
A new visualization approach for identifying mutations that affect differentiation and organization of the Drosophila ommatidia
Authors: Pichaud Franck , Desplan Claude
Published: April 2021

The Drosophila eye is widely used as a model system to study neuronal differentiation, survival and axon projection. Photoreceptor differentiation starts with the specification of a founder cell R8, which sequentially recruits other photoreceptor neurons to the ommatidium. The eight photoreceptors that compose each ommatidium exist in two chiral forms organized along two axes of symmetry and this pattern represents a paradigm to study tissue polarity. We have developed a method of fluoroscopy to visualize the different types of photoreceptors and the organization of the ommatidia in living animals. This allowed us to perform an F1 genetic screen to isolate mutants affecting photoreceptor differentiation, survival or planar polarity. We illustrate the power of this detection system using known genetic backgrounds and new mutations that affect ommatidial differentiation, morphology or chirality.
Developmental origin of the rat adenohypophysis prior to the formation of Rathke’s pouch
Authors: Kouki Tom , Imai Hajime , Aoto Kazushi , Eto Kazuhiro , Shioda Seiji , Kawamura Kosuke , Kikuyama Sakae
Published: April 2021

In amphibians, it has already been shown that the adenohypophysis originates from the anterior neural ridge. During the migration and morphogenesis of this organ, the anterior neural ridge transiently forms a Rathke’s pouch- like structure by attaching itself to the rostral tip of the foregut, and finally gives rise to the adenohypophysis by detaching from the foregut and becoming connected to the infundibulum of the hypothalamus. In order to identify the origin of the adenohypophyseal cells in mammalian embryos prior to the formation of Rathke’s pouch (RP), we labeled the rostral end of the neural plate and the adjacent area focally with DiI at the open neurula stage (9.5 dpc).

After a 48-hours culture of the whole embryos, strongly labeled cells were detected in the RP only when DiI was applied to a small area situated just anterior to the rostral end of the neural plate. By explanting the labeled RP for a further 7 days, we confirmed immunohistochemically that the labeled cells developed into the secretory cells of the adenohypophysis. The developmental origin of the adenohypophysis is identified for the first time in the early mammalian embryo before the formation of RP.
SPATULA, a gene that controls development of carpel margin tissues inArabidopsis, encodes a bHLH protein
Authors: Heisler Marcus G. B. , Atkinson Angela , Bylstra Yasmin H. , Walsh Rebecca , Smyth David R.
Published: April 2021

Studies involving mutants of the gene SPATULA indicate that it promotes the growth of carpel margins and of pollen tract tissues derived from them. We show that it encodes a new member of the basic-helix-loop-helix family of transcription factors. SPATULA is expressed in marginal and pollen tract tissues throughout their development confirming its role in regulating their growth. It is also expressed in many other tissues where it may act redundantly to control growth, including the peripheral zone of the shoot apical meristem, and specific tissues within leaves, petals, stamens and roots. Expression in the stomium, funiculus and valve dehiscence zone indicates an additional role in abscission. SPATULA expression does not require the function of the other carpel development genes CRABS CLAW and AGAMOUS, although its expression is repressed in first whorl organs by the A function gene APETALA2. Further, we have shown that disruptions to gynoecial pattern formation seen in ettin mutants can largely be attributed to ectopic SPATULA action. ETTIN’s role seems to be to negatively regulate SPATULA expression in abaxial regions of the developing gynoecium. SPATULA is the first basic-helix-loop-helix gene in plants known to play a role in floral organogenesis.
The CUP-SHAPED COTYLEDON1 gene of Arabidopsis regulates shoot apical meristem formation
Authors: Takada Shinobu , Hibara Ken-ichiro , Ishida Tetsuya , Tasaka Masao
Published: April 2021

In higher plants, molecular mechanisms regulating shoot apical meristem (SAM) formation and organ separation are largely unknown. The CUC1 (CUP-SHAPED COTYLEDON1) and CUC2 are functionally redundant genes that are involved in these processes. We cloned the CUC1 gene by a map-based approach, and found that it encodes a NAC-domain protein highly homologous to CUC2. CUC1 mRNA was detected in the presumptive SAM during embryogenesis, and at the boundaries between floral organ primordia. Surprisingly, overexpression of CUC1 was sufficient to induce adventitious shoots on the adaxial surface of cotyledons. Expression analyses in the overexpressor and in loss-of-function mutants suggest that CUC1 acts upstream of the SHOOT MERISTEMLESS gene.
Mice doubly deficient for the Polycomb Group genes Mel18 and Bmi1 reveal synergy and requirement for maintenance but not initiation of Hox gene expression
Authors: Akasaka Takeshi , Lohuizen Maarten van , Lugt Nathalie van der , Mizutani-Koseki Yoko , Kanno Masamoto , Taniguchi Masaru , Vidal Miguel , Alkema Mark , Berns Anton , Koseki Haruhiko
Published: April 2021

Polycomb group genes were identified as a conserved group of genes whose products are required in multimeric complexes to maintain spatially restricted expression of Hox cluster genes. Unlike in Drosophila, in mammals Polycomb group (PcG) genes are represented as highly related gene pairs, indicative of duplication during metazoan evolution. Mel18 and Bmi1 are mammalian homologs of Drosophila Posterior sex combs. Mice deficient for Mel18 or Bmi1 exhibit similar posterior transformations of the axial skeleton and display severe immune deficiency, suggesting that their gene products act on overlapping pathways/target genes. However unique phenotypes upon loss of either Mel18 or Bmi1 are also observed. We show using embryos doubly deficient for Mel18 and Bmi1 that Mel18 and Bmi1 act in synergy and in a dose-dependent and cell type-specific manner to repress Hox cluster genes and mediate cell survival of embryos during development. In addition, we demonstrate that Mel18 and Bmi1, although essential for maintenance of the appropriate expression domains of Hox cluster genes, are not required for the initial establishment of Hox gene expression. Furthermore, we show an unexpected requirement for Mel18 and Bmi1 gene products to maintain stable expression of Hox cluster genes in regions caudal to the prospective anterior expression boundaries during subsequent development.
A novel disintegrin domain protein affects early cell type specification and pattern formation inDictyostelium
Authors: Varney Timothy R. , Ho Hoa , Petty Chere’ , Blumberg Daphne D.
Published: April 2021

The cellular slime mold, Dictyostelium discoideum is a non-metazoan organism, yet we now demonstrate that a disintegrin domain-containing protein, the product of the ampA gene, plays a role in cell type specification. Disintegrin domain-containing proteins are involved in Notch signaling in Drosophila and C. elegans via an ectodomain shedding mechanism that depends on a metalloprotease domain. The Dictyostelium protein lacks a metalloprotease domain. Nonetheless, analysis of cell type specific reporter gene expression during development of the ampA null strain identifies patterning defects that define two distinct roles for the AmpA protein in specifying cell fate. In the absence of a functional ampA gene, cells prematurely specify as prespore cells. Prestalk cell differentiation and migration are delayed. Both of these defects can be rescued by the inclusion of 10% wild-type cells in the developing null mutant aggregates, indicating that the defect is non-cell autonomous. The ampA gene is also demonstrated to be necessary in a cell-autonomous manner for the correct localization of anterior-like cells to the upper cup of the fruiting body. When derived from ampA null cells, the anterior-like cells are unable to localize to positions in the interior of the developing mounds. Wild-type cells can rescue defects in morphogenesis by substituting for null cells when they differentiate as anterior-like cells, but they cannot rescue the ability of ampA null cells to fill this role. Thus, in spite of its simpler structure, the Dictyostelium ampA protein carries out the same diversity of functions that have been observed for the ADAM and ADAMTS families in metazoans.
A germline-specific gap junction protein required for survival of differentiating early germ cells
Authors: Tazuke Salli I. , Schulz Cordula , Gilboa Lilach , Fogarty Mignon , Mahowald Anthony P. , Guichet Antoine , Ephrussi Anne , Wood Cricket G. , Lehmann Ruth , Fuller Margaret T.
Published: April 2021

Germ cells require intimate associations and signals from the surrounding somatic cells throughout gametogenesis. The zero population growth (zpg) locus of Drosophila encodes a germline-specific gap junction protein, Innexin 4, that is required for survival of differentiating early germ cells during gametogenesis in both sexes. Animals with a null mutation in zpg are viable but sterile and have tiny gonads. Adult zpg-null gonads contain small numbers of early germ cells, resembling stem cells or early spermatogonia or oogonia, but lack later stages of germ cell differentiation. In the male, Zpg protein localizes to the surface of spermatogonia, primarily on the sides adjacent to the somatic cyst cells. In the female, Zpg protein localizes to germ cell surfaces, both those adjacent to surrounding somatic cells and those adjacent to other germ cells. We propose that Zpg-containing gap junctional hemichannels in the germ cell plasma membrane may connect with hemichannels made of other innexin isoforms on adjacent somatic cells. Gap junctional intercellular communication via these channels may mediate passage of crucial small molecules or signals between germline and somatic support cells required for survival and differentiation of early germ cells in both sexes.
A proliferation gradient between proximal andmsxb-expressing distal blastema directs zebrafish fin regeneration
Authors: Nechiporuk Alex , Keating Mark T.
Published: April 2021

Previous studies of zebrafish fin regeneration led to the notion that the regeneration blastema is a homogeneous population of proliferating cells. Here, we show that the blastema consists of two components with markedly distinct proliferation properties. During early blastema formation, proliferating cells are evenly distributed. At the onset of regenerative outgrowth, however, blastemal cells are partitioned into two domains. Proximal blastemal cells proliferate at a high rate, shifting from a median G2 of more than 6 hours to approximately 1 hour. By contrast, the most distal blastemal cells do not proliferate. There is a gradient of proliferation between these extremes. Using bromodeoxyuridine incorporation and anti-phosphohistone H3 labeling, we find a 50-fold difference in proliferation across the gradient that extends approximately 50 μm, or ten cell diameters. We show that during early regeneration, proliferating blastemal cells express msxb, a homeodomain transcriptional repressor. While msxb is widely expressed among proliferating cells during blastema formation, its expression becomes restricted to a small number of non-proliferating, distal blastemal cells during regenerative outgrowth. Bromodeoxyuridine pulse-chase experiments show that distal and proximal blastemal cells are formed from proliferating, msxb-positive blastemal cells, not from preexisting slow-cycling cells. These data support the idea that blastema formation results from dedifferentiation of intraray mesenchymal cells. Based on these findings, we propose a new model of zebrafish fin regeneration in which the function of non-proliferating, msxb-expressing, distal blastemal cells is to specify the boundary of proliferation and provide direction for regenerative outgrowth.
β8 integrins are required for vascular morphogenesis in mouse embryos
Authors: Zhu Jiangwen , Motejlek Karin , Wang Denan , Zang Keling , Schmidt Andrea , Reichardt Louis F.
Published: April 2021

In order to assess the in vivo function of integrins containing the β8 subunit, we have generated integrin β8-deficient mice. Ablation of β8 results in embryonic or perinatal lethality with profound defects in vascular development. Sixty-five percent of integrin β8-deficient embryos die at midgestation, with evidence of insufficient vascularization of the placenta and yolk sac. The remaining 35% die shortly after birth with extensive intracerebral hemorrhage. Examination of brain tissue from integrin β8-deficient embryos reveals abnormal vascular morphogenesis resulting in distended and leaky capillary vessels, as well as aberrant brain capillary patterning. In addition, endothelial cell hyperplasia is found in these mutant brains. Expression studies show that integrin β8 transcripts are localized in endodermal cells surrounding endothelium in the yolk sac and in periventricular cells of the neuroepithelium in the brain. We propose that integrin β8 is required for vascular morphogenesis by providing proper cues for capillary growth in both yolk sac and embryonic brain. This study thus identifies a molecule crucial for vascular patterning in embryonic yolk sac and brain.
Misexpression of dHAND induces ectopic digits in the developing limb bud in the absence of direct DNA binding
Authors: McFadden David G. , McAnally John , Richardson James A. , Charité Jeroen , Olson Eric N.
Published: April 2021

Basic helix-loop-helix (bHLH) transcription factors control developmental decisions in a wide range of embryonic cell types. The HLH motif mediates homo- and heterodimerization, which juxtaposes the basic regions within the dimeric complex to form a bipartite DNA binding domain that recognizes a DNA consensus sequence known as an E-box. eHAND and dHAND (also known as HAND1 and HAND2) are closely related bHLH proteins that control cardiac, craniofacial and limb development. Within the developing limb, dHAND expression encompasses the zone of polarizing activity in the posterior region, where it has been shown to be necessary and sufficient to induce the expression of the morphogen sonic hedgehog. Misexpression of dHAND in the anterior compartment of the limb bud induces ectopic expression of sonic hedgehog, with resulting preaxial polydactyly and mirror image duplications of posterior digits. To investigate the potential transcriptional mechanisms involved in limb patterning by dHAND, we have performed a structure-function analysis of the protein in cultured cells and ectopically expressed dHAND mutant proteins in the developing limbs of transgenic mice. We show that an N-terminal transcriptional activation domain, and the bHLH region, are required for E-box-dependent transcription in vitro. Remarkably, however, digit duplication by dHAND requires neither the transcriptional activation domain nor the basic region, but only the HLH motif. eHAND has a similar limb patterning activity to dHAND in these misexpression experiments, indicating a conserved function of the HLH regions of these proteins. These findings suggest that dHAND may act via novel transcriptional mechanisms mediated by protein-protein interactions independent of direct DNA binding.
Rho1 interacts with p120ctn and α-catenin, and regulates cadherin-based adherens junction components inDrosophila
Authors: Magie Craig R. , Pinto-Santini Delia , Parkhurst Susan M.
Published: April 2021

Rho GTPases are important regulators of cellular behavior through their effects on processes such as cytoskeletal organization. Here we show interactions between Drosophila Rho1 and the adherens junction components α-catenin and p120ctn. We find that while Rho1 protein is present throughout the cell, it accumulates apically, particularly at sites of cadherin-based adherens junctions. Cadherin and catenin localization is disrupted in Rho1 mutants, implicating Rho1 in their regulation. p120ctn has recently been suggested to inhibit Rho activity through an unknown mechanism. We find that Rho1 accumulates in response to lowered p120ctn activity. Significantly, we find that Rho1 binds directly to α-catenin and p120ctn in vitro, and these interactions map to distinct surface-exposed regions of the protein not previously assigned functions. In addition, we find that both α-catenin and p120ctn co-immunoprecipitate with Rho1-containing complexes from embryo lysates. Our observations suggest that α-catenin and p120ctn are key players in a mechanism of recruiting Rho1 to its sites of action.
TheC. elegansPOU-domain transcription factor UNC-86 regulates thetph-1tryptophan hydroxylase gene and neurite outgrowth in specific serotonergic neurons
Authors: Sze Ji Ying , Zhang Shenyuan , Li Jie , Ruvkun Gary
Published: April 2021

A fundamental question in developmental neurobiology is how a common neurotransmitter is specified in different neuronal types?. We describe cell-specific regulation of the serotonergic phenotype by the C. elegans POU-transcription factor UNC-86. We show that unc-86 regulates particular aspects of the terminal neuronal identity in four classes of serotonergic neurons, but that the development of the ADF serotonergic neurons is regulated by an UNC-86-independent program. In the NSM neurons, the role of unc-86 is confined in late differentiation; the neurons are generated but do not express genes necessary for serotonergic neurotransmission. unc-86-null mutations affect the expression in NSM of tph-1, which encodes the serotonin synthetic enzyme tryptophan hydroxylase, and cat-1, which encodes a vesicular transporter that loads serotonin into synaptic vesicles, suggesting that unc-86 coordinately regulates serotonin synthesis and packaging. However, unc-86-null mutations do not impair the ability of NSM to reuptake serotonin released from the ADF serotonergic chemosensory neurons and this serotonin reuptake is sensitive to the serotonin reuptake block drugs imipramine and fluoxetine, demonstrating that serotonin synthesis and reuptake is regulated by distinct factors. The NSM neurons in unc-86-null mutants also display abnormal neurite outgrowth, suggesting a role of unc-86 in regulating this process as well.
Axial progenitors with extensive potency are localised to the mouse chordoneural hinge
Authors: Cambray Noemí , Wilson Valerie
Published: April 2021

Elongation of the mouse anteroposterior axis depends on a small population of progenitors initially located in the primitive streak and later in the tail bud. Gene expression and lineage tracing have shown that there are many features common to these progenitor tissues throughout axial elongation. However, the identity and location of the progenitors is unclear. We show by lineage tracing that the descendants of 8.5 d.p.c. node and anterior primitive streak which remain in the tail bud are located in distinct territories: (1) ventral node descendants are located in the widened posterior end of the notochord; and (2) descendants of anterior streak are located in both the tail bud mesoderm, and in the posterior end of the neurectoderm. We show that cells from the posterior neurectoderm are fated to give rise to mesoderm even after posterior neuropore closure. The posterior end of the notochord, together with the ventral neurectoderm above it, is thus topologically equivalent to the chordoneural hinge region defined in Xenopus and chick. A stem cell model has been proposed for progenitors of two of the axial tissues, the myotome and spinal cord. Because it was possible that labelled cells in the tail bud represented stem cells, tail bud mesoderm and chordoneural hinge were grafted to 8.5 d.p.c. primitive streak to compare their developmental potency. This revealed that cells from the bulk of the tail bud mesoderm are disadvantaged in such heterochronic grafts from incorporating into the axis and even when they do so, they tend to contribute to short stretches of somites suggesting that tail bud mesoderm is restricted in potency. By contrast, cells from the chordoneural hinge of up to 12.5 d.p.c. embryos contribute efficiently to regions of the axis formed after grafting to 8.5 d.p.c. embryos, and also repopulate the tail bud. These cells were additionally capable of serial passage through three successive generations of embryos in culture without apparent loss of potency. This potential for self-renewal in chordoneural hinge cells strongly suggests that stem cells are located in this region.
The role of Phox2b in synchronizing pan-neuronal and type-specific aspects of neurogenesis
Authors: Dubreuil Véronique , Hirsch Marie-Rose , Jouve Caroline , Brunet Jean-François , Goridis Christo
Published: April 2021

Within the developing vertebrate nervous system, specific subclasses of neurons are produced in vastly different numbers at defined times and locations. This implies the concomitant activation of a program that controls pan-neuronal differentiation and of a program that specifies neuronal subtype identity, but how these programs are coordinated in time and space is not well understood. Our previous loss- and gain-of-function studies have defined Phox2b as a homeodomain transcription factor that coordinately regulates generic and type-specific neuronal properties. It is necessary and sufficient to impose differentiation towards a branchio- and viscero-motoneuronal phenotype and at the same time promotes generic neuronal differentiation. We have examined the underlying genetic interactions. We show thatPhox2b has a dual action on pan-neuronal differentiation. It upregulates the expression of proneural genes (Ngn2) when expressed alone and upregulates the expression of Mash1 when expressed in combination with Nkx2.2. By a separate pathway, Phox2brepresses expression of the inhibitors of neurogenesis Hes5 andId2. The role of Phox2b in the specification of neuronal subtype identity appears to depend in part on its capacity to act as a patterning gene in the progenitor domain. Phox2b misexpression represses the Pax6 and Olig2 genes, which should inhibit a branchiomotor fate, and induces Nkx6.1 and Nkx6.2, which are expressed in branchiomotor progenitors. We further show that Phox2b behaves like a transcriptional activator in the promotion of both, generic neuronal differentiation and expression of the motoneuronal marker Islet1. These results provide insights into the mechanisms by which a homeodomain transcription factor through interaction with other factors controls both generic and type-specific features of neuronal differentiation.
The genetic network of prototypic planarian eye regeneration is Pax6 independent
Authors: Pineda David , Rossi Leonardo , Batistoni Renata , Salvetti Alessandra , Marsal Maria , Gremigni Vittorio , Falleni Alessandra , Gonzalez-Linares Javier , Deri Paolo , Saló Emili
Published: April 2021

We report the presence of two Pax6-related genes, Pax6A and Pax6B, which are highly conserved in two planarian species Dugesia japonica and Girardia tigrina (Platyhelminthes, Tricladida). Pax6A is more similar to other Pax6 proteins than Pax6B, which is the most divergent Pax6 described so far. The planarian Pax6 homologs do not show any clear orthology to the Drosophila duplicated Pax6 genes, eyeless and twin of eyeless, which suggests an independent Pax6 duplication in a triclad or platyhelminth ancestor. Pax6A is expressed in the central nervous system of intact planarians, labeling a subset of cells of both cephalic ganglia and nerve cords, and is activated during cephalic regeneration. Pax6B follows a similar pattern, but shows a lower level of expression. Pax6A and Pax6B transcripts are detected in visual cells only at the ultrastructural level, probably because a limited amount of transcripts is present in these cells. Inactivation of both Pax6A and Pax6B by RNA-mediated gene interference (RNAi) inhibits neither eye regeneration nor eye maintenance, suggesting that the genetic network that controls this process is not triggered by Pax6 in planarians.
Caenorhabditis elegansPlexinA, PLX-1, interacts with transmembrane semaphorins and regulates epidermal morphogenesis
Authors: Fujii Takashi , Nakao Fumi , Shibata Yukimasa , Shioi Go , Kodama Eiji , Fujisawa Hajime , Takagi Shin
Published: April 2021

The plexin family transmembrane proteins are putative receptors for semaphorins, which are implicated in the morphogenesis of animal embryos, including axonal guidance. We have generated and characterized putative null mutants of the C. elegans plexinA gene, plx-1. plx-1 mutants exhibited morphological defects: displacement of ray 1 and discontinuous alae. The epidermal precursors for the affected organs were aberrantly arranged in the mutants, and a plx-1::gfp transgene was expressed in these epidermal precursor cells as they underwent dynamic morphological changes. Suppression of C. elegans transmembrane semaphorins, Ce-Sema-1a and Ce-Sema-1b, by RNA interference caused a displacement of ray 1 similar to that of plx-1 mutants, whereas mutants for the Ce-Sema-2a/mab-20 gene, which encodes a secreted-type semaphorin, exhibited phenotypes distinct from those of plx-1 mutants. A heterologous expression system showed that Ce-Sema-1a, but not Ce-Sema-2a, physically bound to PLX-1. Our results indicate that PLX-1 functions as a receptor for transmembrane-type semaphorins, and, though Ce-Sema-2a and PLX-1 both play roles in the regulation of cellular morphology during epidermal morphogenesis, they function rather independently.
Fgf3 and Fgf8 are required together for formation of the otic placode and vesicle
Authors: Maroon Habib , Walshe Jennifer , Mahmood Radma , Kiefer Paul , Dickson Clive , Mason Ivor
Published: April 2021

Fgf3 has long been implicated in otic placode induction and early development of the otocyst; however, the results of experiments in mouse and chick embryos to determine its function have proved to be conflicting. In this study, we determined fgf3 expression in relation to otic development in the zebrafish and used antisense morpholino oligonucleotides to inhibit Fgf3 translation. Successful knockdown of Fgf3 protein was demonstrated and this resulted in a reduction of otocyst size together with reduction in expression of early markers of the otic placode.

fgf3 is co-expressed with fgf8 in the hindbrain prior to otic induction and, strikingly, when Fgf3 morpholinos were co-injected together with Fgf8 morpholinos, a significant number of embryos failed to form otocysts. These effects were made manifest at early stages of otic development by an absence of early placode markers (pax2.1 and dlx3) but were not accompanied by effects on cell division or death. The temporal requirement for Fgf signalling was established as being between 60% epiboly and tailbud stages using the Fgf receptor inhibitor SU5402. However, the earliest molecular event in induction of the otic territory, pax8 expression, did not require Fgf signalling, indicating an inductive event upstream of signalling by Fgf3 and Fgf8. We propose that Fgf3 and Fgf8 are required together for formation of the otic placode and act during the earliest stages of its induction.
Histopathologic basis of the teratogenic effects of H-1 virus on hamster embryos
Authors: Ferm Vergil H. , Kilham Lawrence
Published: April 2021

Rat virus (RV), an extremely small DNA virus with a diameter of 15 mμ (Dalton, Kilham & Zeigel, 1963), has exhibited a marked ability to proliferate in placental and fetal tissues following intravenous inoculations of pregnant hamsters, the maternal health remaining unaffected. Effects on the fetus, however, have varied with the strains of RV employed. In initial experiments, a strain which had been carried in forty-two passages through suckling hamsters was without apparent effect on hamster fetuses, although present in high titers in both placental and fetal tissues (Ferm & Kilham, 1963). These results encouraged similar studies on other strains of RV, attempting to find some which would proliferate in a similar manner and also induce teratogenic effects. These studies resulted in the finding of the teratogenicity of the H-1 strain of RV (Ferm & Kilham, 1964a). The purpose of the present report is to describe the histopathologic changes, including the numerous intranuclear inclusions, which accompanied fetal infections with H-1 virus and the spectrum of congenital malformations directly induced by viral action. The histopathologic processes found in these embryonic tissues may help to explain the morphogenetic basis of certain congenital malformations.
An immuno-fluorescent study of lens regeneration in larval Xenopus laevis
Authors: Campbell John C.
Published: April 2021

The appearance and localization of lens antigens in lens regenerating from the cornea of larval Xenopus laevis were investigated by the fluorescent antibody technique. The cornea of the normal eye was not stained by the antiserum used, but lens antigens were detected in the cytoplasm of cells of the inner layer of the corneal epithelium overlying the pupillary space within 24 hr. of lens removal. In the cells of the lens vesicle, before fibre formation, the labelling, which was over the cytoplasm, was weak, except for small areas of cells at the posterior margin of the vesicle from which the primary lens fibres are formed. The most intense fluorescence appears over the fibres and is stronger in older fibres, whether primary or secondary. The lens epithelium is always unlabelled. The limitations of the technique are considered and the results are discussed in the light of other immunological data on normally developing and regenerating lens.
Skeletal malformations in the Holtzman rat embryo following the administration of thalidomide
Authors: Dwornik J. J. , Moore K. L.
Published: April 2021

Pregnant Holtzman albino rats were given 25-, 50- or 500-mg. doses of thalidomide intraperitoneally, or by stomach tube, during the time their embryos’ limbs were developing. Except in rare instances, thalidomide had no effect on limb development. In one case, limb malformations closely resembling thalidomide deformities in man and rabbits were observed. The commonest effect of thalidomide on rat development was absent, delayed, or abnormal ossification of the 5th sternebra. The most serious abnormalities observed were maldevelopment, fusion or absence of vertebrae. Other skeletal defects observed occasionally were: ‘scrambled’ sternebrae, abnormal flexion of toes, ‘clubfoot ‘, absence of vertebrae, ribs and tail and supernumerary ribs. In general the 50-mg. dose of thalidomide induced the most serious defects. The method of administration (by stomach tube or by intraperitoneal injection) seemed to make no difference. The critical period during which thalidomide seems to act on the skeletal system in this strain of rat is from the 10th to 12th days of gestation. Some control fetuses had minor skeletal defects suggesting that sodium carboxymethylcellulose may also be a weak teratogen in this strain of rat. The thalidomide-produced vertebral and sternebral defects were not visible during macroscopic examination. Alizarin preparations were needed to demonstrate the defects; this suggests that the above technique should be a part of all investigations aimed at detecting teratogenicity of drugs.
Thalidomide injury to the myocardium of armadillo embryos
Authors: Marin-Padilla Miguel , Benirschke Kurt
Published: April 2021

An injury to the myocardium of armadillo embryos induced by thalidomide when administered to their mothers is reported. The injury consists of focal myocardial necrosis of both atria and ventricles. Myocardial necrosis is more severe and extensive in the youngest embryos (CR 3 mm.). The myocardial damage induced by thalidomide can cause fetal death. Cardiac rupture through an area of artrial necrosis and hemopericardium is reported in one embryo. Also, fetal death can occur from heart failure manifested by cardiomegaly, hepatomegaly and visceral congestion. The significance of myocardial necrosis as a possible cause of later congenital cardiac deformities is discussed.
The embryo-forming potencies of the young chick blastoderm
Authors: Eyal-Giladi Hefzibah , Spratt Nelson T.
Published: April 2021

Very young unincubated chick blastoderms (obtained from winter eggs) are almost equipotential (or totipotential) and under experimental conditions even central fragments can form an embryo in vitro. When a central cut is made in a blastoderm of this age an embryo tends to form along the cut edge and the original embryo-forming centre at the prospective posterior side is suppressed.

In slightly older unincubated chick blastoderms (obtained from summer eggs) and in 4−5 hr. incubated winter eggs the embryo-forming potency is lost from the central part and becomes confined first to the marginal zone and, within it, gradually to its posterior region.

In still older blastoderms, with an expanded hypoblast but prior to streak formation, central fragments of the epiblast with their underlying hypoblast re-acquire the capability to form an embryo.
Inhibition by α-methyl-norvaline of valine and leucine uptake into protein in the chick embryo
Authors: Deuchar E. M. , Dryland A. M. L.
Published: April 2021

α-Methyl-norvaline, an amino acid whose structure resembles both valine and leucine, has been found in certain circumstances to inhibit haemoglobin formation in the area vasculosa of the 48-hr. chick embryo. In the present experiments tracer methods have been used to show that it inhibits the uptake of both valine and leucine into protein of the embryos, and that analogue is itself taken up into protein. There is no evidence in autoradiographs of any differential effect of the analogue on amino acid uptake into blood cells. It had been suggested earlier, however, that the analogue may compete with valine (and possibly also with leucine) for uptake into terminal positions on the peptides ofhaemo-globin. Further experiments will aim at detecting whether, and at what sites, the analogue is taken up into the haemoglobin itself.
Effets du 5-fluoro-uracile et de la 6-m éthylpurine sur le développement de l’œuf de l’oursinParacentrotus lividus
Authors: Lallier par Roger
Published: April 2021

Le 5-fluoro-uracile, un analogue de base pyrimidique, interfère avec le métabolisme des acides nucléiques. Le 5-fluoro-uracile inhibe la méthylation au cours de la synthèse de la thymine (Eidinoff et al., 1957; Bosch étal., 1958; Cohen et al., 1958; Harbers et al., 1959). Le 5-fluoro-uracile est incorporé dans l’acide ribonucléique à la place de l’uracile (Bosch et al., 1958; Chaudhuri et al., 1958; Horowitz & Chargaff, 1959). Ce remplacement de base dans l’acide ribonucléique entraîne des modifications de la composition et des propriétés de certaines protéines synthétisées en présence de 5-fluoro-uracile (Naono & Gros, 1960; Hignett, 1964). La synthèse des ribosomes fonctionnels est inhibée par le 5-fluoro-uracile (Kempner & Miller, 1963).
Morphological and autoradiographic studies of cleft palate induced in rat embryos by maternal hypervitaminosis A
Authors: Kochhar Devendra M. , Johnson E. Marshall
Published: April 2021

Oral administration of 60,000 international units of vitamin A acetate to pregnant rats for 3 consecutive days beginning on either day 9 or 10 of pregnancy resulted in more than 80 per cent, of their embryos developing cleft palate. The initiation of palatine shelf movement in normal control embryos was associated with an aggregation of preosteoblastic tissue on the medial aspect of the vertically oriented shelves at the junction of their anterior two-thirds and posterior one-third. Delay in the movement of palatine shelves from the vertical to the horizontal position was not observed in a major percentage of the vitamin A treated embryos. In many vitamin A treated embryos a considerably lesser amount of mesenchymal tissue was contributed by the maxillary processes to form the palatine shelves. A process of heterotopic chondrogenesis was detected within the preosteoblastic tissues of maxillae and palatine shelves as early as day 15 and by day 17 had replaced a major portion of the maxillary bone. On the basis of the methods employed, more S35 was fixed by the tissues of treated embryos than by those of normal controls prior to the time of palatal closure.
Etude d’associations hétérologues de l’uretère et de différents mésenchymes de l’embryon de poulet, par la technique des greffes chorio-allantoïdiennes
Authors: Bishop-Calame par Simone
Published: April 2021

Des recherches récentes effectuées dans de nombreux laboratoires ont permis de mettre en évidence les interactions morphogènes qui s’exercent entre les deux composants, épithélial et mésenchymateux, d’une jeune ébauche en cours de différenciation. Ces travaux, utilisant la technique de digestion partielle par la trypsine qui permet de dissocier une jeune ébauche en ses deux composants (Moscona, 1952), confirment les résultats obtenus par Grobstein (1953,1955)sur les glandes salivaires et le métanephros de l’embryon de souris. Ils établissent les faits suivants :