bluuebunny/crossref_metadata_2025 huggingface dataset:
The transcription factor Sox9 is required for cranial neural crest development inXenopus
Authors: Spokony Rebecca F. , Aoki Yoichiro , Saint-Germain Natasha , Magner-Fink Emily , Saint-Jeannet Jean-Pierre
Published: April 2021

The SOX family of transcription factors has been implicated in cell fate specification during embryogenesis. One member of this family, Sox9, has been shown to regulate both chondrogenesis and sex determination in the mouse embryo. Heterozygous mutations in Sox9 result in Campomelic Dysplasia (CD), a lethal human disorder characterized by autosomal XY sex reversal, severe skeletal malformations and several craniofacial defects. Sox9 is also expressed in neural crest progenitors but very little is known about the function of Sox9 in the neural crest. We have cloned the Xenopus homolog of the Sox9 gene. It is expressed maternally and accumulates shortly after gastrulation at the lateral edges of the neural plate, in the neural crest-forming region. As development proceeds, Sox9 expression persists in migrating cranial crest cells as they populate the pharyngeal arches. Depletion of Sox9 protein in developing embryos, using morpholino antisense oligos, causes a dramatic loss of neural crest progenitors and an expansion of the neural plate. Later during embryogenesis, morpholino-treated embryos have a specific loss or reduction of neural crest-derived skeletal elements, mimicking one aspect of the craniofacial defects observed in CD patients. We propose that Sox9 is an essential component of the regulatory pathway that leads to cranial neural crest formation.
POP-1 controls axis formation during early gonadogenesis inC. elegans
Authors: Siegfried Kellee R. , Kimble Judith
Published: April 2021

The shape and polarity of the C. elegans gonad is defined during early gonadogenesis by two somatic gonadal precursor cells, Z1 and Z4, and their descendants. Z1 and Z4 divide asymmetrically to establish the proximal-distal axes of the gonad and to generate regulatory leader cells that control organ shape. In this paper, we report that pop-1, the C. elegans TCF/LEF-1 transcription factor, controls the first Z1/Z4 asymmetric division and hence controls proximal-distal axis formation. We have identified two pop-1(Sys) alleles (for symmetrical sisters) that render the Z1/Z4 divisions symmetrical. The pop-1(q645) allele is fully penetrant for the Sys gonadogenesis defect in hermaphrodites, but affects male gonads weakly; pop-1(q645) alters a conserved amino acid in the β-catenin binding domain. The pop-1(q624) allele is weakly penetrant for multiple defects and appears to be a partial loss-of-function mutation; pop-1(q624) alters a conserved amino acid in the HMG-box DNA binding domain. Zygotic pop-1(RNAi) confirms the role of pop-1 in Z1/Z4 asymmetry and reveals additional roles of pop-1, including one in leader cell migration. Two other Wnt pathway regulators, wrm-1 and lit-1, have the same effect as pop-1 on Z1/Z4 asymmetry. Therefore, wrm-1 and lit-1 are required for pop-1 function, rather than opposing it as observed in the early embryo. We conclude that POP-1 controls the Z1/Z4 asymmetric division and thereby establishes the proximal-distal axes of the gonad. This control over proximal-distal polarity extends our view of Wnt signaling in C. elegans, which had previously been known to control anterior-posterior polarities.
Brn3b/Brn3cdouble knockout mice reveal an unsuspected role for Brn3c in retinal ganglion cell axon outgrowth
Authors: Wang Steven W. , Mu Xiuqian , Bowers William J. , Kim Dong-Seob , Plas Daniel J. , Crair Michael C. , Federoff Howard J. , Gan Lin , Klein William H.
Published: April 2021

In mice, Brn3 POU domain transcription factors play essential roles in the differentiation and survival of projection neurons within the retina, inner ear, dorsal root and trigeminal ganglia. During retinal ganglion cell differentiation, Brn3b is expressed first, followed by Brn3a and Brn3c. Targeted deletion of Brn3b, but not Brn3a or Brn3c, leads to a loss of most retinal ganglion cells before birth. However, as a few retinal ganglion cells are still present in Brn3b–/– mice, Brn3a and Brn3c may partially compensate for the loss of Brn3b. To examine the role of Brn3c in retinal ganglion cell development, we generated Brn3b/Brn3c double knockout mice and analyzed their retinas and optic chiasms. Retinal ganglion cell axons from double knockout mice were more severely affected than were those from Brn3b-deficient mice, indicating that Brn3c was required for retinal ganglion cell differentiation and could partially compensate for the loss of Brn3b. Moreover, Brn3c had functions in retinal ganglion cell differentiation separate from those of Brn3b. Ipsilateral and misrouted projections at the optic chiasm were overproduced in Brn3b–/– mice but missing were entirely in optic chiasms of Brn3b/Brn3c double knockout mice, suggesting that Brn3c controlled ipsilateral axon production. Forced expression of Brn3c in Brn3b–/– retinal explants restored neurite outgrowth, demonstrating that Brn3c could promote axon outgrowth in the absence of Brn3b. Our results reveal a complex genetic relationship between Brn3b and Brn3c in regulating the retinal ganglion cell axon outgrowth.
The BMP/BMPR/Smad pathway directs expression of the erythroid-specific EKLF and GATA1 transcription factors during embryoid body differentiation in serum-free media
Authors: Adelman Carrie A. , Chattopadhyay Subrata , Bieker James J.
Published: April 2021

Erythroid cell-specific gene regulation during terminal differentiation is controlled by transcriptional regulators, such as EKLF and GATA1, that themselves exhibit tissue-restricted expression patterns. Their early expression, already in evidence within multipotential hematopoietic cell lines, has made it difficult to determine what extracellular effectors and transduction mechanisms might be directing the onset of their own transcription during embryogenesis. To circumvent this problem, we have taken the novel approach of investigating whether the ability of embryonic stem (ES) cells to mimic early developmental patterns of cellular expression during embryoid body (EB) differentiation can address this issue. We first established conditions whereby EBs could form efficiently in the absence of serum. Surprisingly, in addition to mesoderm, these cells expressed hemangioblast and hematopoietic markers. However, they did not express the committed erythroid markers EKLF and GATA1, nor the terminally differentiated β-like globin markers. Using this system, we determined that EB differentiation in BMP4 was necessary and sufficient to recover EKLF and GATA1 expression and could be further stimulated by the inclusion of VEGF, SCF, erythropoietin and thyroid hormone. EBs were competent to respond to BMP4 only until day 4 of differentiation, which coincides with the normal onset of EKLF expression. The direct involvement of the BMP/Smad pathway in this induction process was further verified by showing that erythroid expression of a dominant negative BMP1B receptor or of the inhibitory Smad6 protein prevented induction of EKLF or GATA1 even in the presence of serum. Although Smad1, Smad5 and Smad8 are all expressed in the EBs, BMP4 induction of EKLF and GATA1 transcription is not immediate. These data implicate the BMP/Smad induction system as being a crucial pathway to direct the onset of EKLF and GATA1 expression during hematopoietic differentiation and demonstrate that EB differentiation can be manipulated to study induction of specific genes that are expressed early within a lineage.
Transcription through theiab-7 cis-regulatory domain of the bithorax complex interferes with maintenance ofPolycomb-mediated silencing
Authors: Hogga Ilham , Karch François
Published: April 2021

The Fab-7 chromatin domain boundary insures functional autonomy of the iab-6 and iab-7 cis-regulatory domains in the bithorax complex (BX-C). We have previously shown that chromatin insulators such asgypsy or scsmin are potent insulators that cannot substitute for Fab-7 function within the BX-C. During the early stages of these swapping experiments, we initially used a fragment of scs that was slightly larger than a minimal scs element (scsmin). We report that this scs fragment, unlike scsmin, interferes in an orientation-dependent manner with the output of a regulatory region covering 80 kb of DNA (from iab-4 to iab-8). At the core of this orientation-dependent phenotype is a promoter located immediately adjacent to the scs insulator. In one orientation, the promoter traps the activity of theiab-3 through iab-5 cis-regulatory domains, diverting them from the abd-A gene. In the opposite orientation, the promoter is transcribing the iab-7 cis-regulatory domain, resulting in ectopic activation of the latter. Our data suggest that transcription through aPolycomb-Response Element (PRE) interferes with the maintenance of aPolycomb repression complex.
CSN5/Jab1mutations affect axis formation in theDrosophilaoocyte by activating a meiotic checkpoint
Authors: Doronkin Sergey , Djagaeva Inna , Beckendorf Steven K.
Published: April 2021

The COP9 signalosome (CSN) is linked to signaling pathways and ubiquitin-dependent protein degradation in yeast, plant and mammalian cells,but its roles in Drosophila development are just beginning to be understood. We show that during oogenesis CSN5/JAB1, one subunit of the CSN,is required for meiotic progression and for establishment of both the AP and DV axes of the Drosophila oocyte. The EGFR ligand Gurken is essential for both axes, and our results show that CSN5 mutations block the accumulation of Gurken protein in the oocyte. CSN5 mutations also cause the modification of Vasa, which is known to be required for Gurken translation. This CSN5 phenotype — defective axis formation, reduced Gurken accumulation and modification of Vasa — is very similar to the phenotype of the spindle-class genes that are required for the repair of meiotic recombination-induced, DNA double-strand breaks. When these breaks are not repaired, a DNA damage checkpoint mediated by mei-41 is activated. Accordingly, the CSN5 phenotype is suppressed by mutations inmei-41 or by mutations in mei-W68, which is required for double strand break formation. These results suggest that, like thespindle-class genes, CSN5 regulates axis formation by checkpoint-dependent, translational control of Gurken. They also reveal a link between DNA repair, axis formation and the COP9 signalosome, a protein complex that acts in multiple signaling pathways by regulating protein stability.
Mps1 defines a proximal blastemal proliferative compartment essential for zebrafish fin regeneration
Authors: Poss Kenneth D. , Nechiporuk Alex , Hillam Ann M. , Johnson Stephen L. , Keating Mark T.
Published: April 2021

One possible reason why regeneration remains enigmatic is that the dominant organisms used for studying regeneration are not amenable to genetic approaches. We mutagenized zebrafish and screened for temperature-sensitive defects in adult fin regeneration. The nightcap mutant showed a defect in fin regeneration that was first apparent at the onset of regenerative outgrowth. Positional cloning revealed that nightcapencodes the zebrafish orthologue of mps1, a kinase required for the mitotic checkpoint. mps1 expression was specifically induced in the proximal regeneration blastema, a group of cells that normally proliferate intensely during outgrowth. The nightcap mutation caused severe defects in these cells. However, msxb-expressing blastemal cells immediately distal to this proliferative region did not induce mps1and were retained in mutants. These results indicate that the proximal blastema comprises an essential subpopulation of the fin regenerate defined by the induction and function of Mps1. Furthermore, we show that molecular mechanisms of complex tissue regeneration can now be dissected using zebrafish genetics.
Cell fates and fusion in theC. elegansvulval primordium are regulated by the EGL-18 and ELT-6 GATA factors — apparent direct targets of the LIN-39 Hox protein
Authors: Koh Kyunghee , Peyrot Sara M. , Wood Cricket G. , Wagmaister Javier A. , Maduro Morris F. , Eisenmann David M. , Rothman Joel H.
Published: April 2021

Development of the vulva in C. elegans is mediated by the combinatorial action of several convergent regulatory inputs, three of which,the Ras, Wnt and Rb-related pathways, act by regulating expression of thelin-39 Hox gene. LIN-39 specifies cell fates and regulates cell fusion in the mid-body region, leading to formation of the vulva. In the lateral seam epidermis, differentiation and cell fusion have been shown to be regulated by two GATA-type transcription factors, ELT-5 and -6. We report that ELT-5 is encoded by the egl-18 gene, which was previously shown to promote formation of a functional vulva. Furthermore, we find that EGL-18(ELT-5), and its paralogue ELT-6, are redundantly required to regulate cell fates and fusion in the vulval primordium and are essential to form a vulva. Elimination of egl-18 and elt-6 activity results in arrest by the first larval stage; however, in animals rescued for this larval lethality by expression of ELT-6 in non-vulval cells, the post-embryonic cells(P3.p-P8.p) that normally become vulval precursor cells often fuse with the surrounding epidermal syncytium or undergo fewer than normal cell divisions,reminiscent of lin-39 mutants. Moreover, egl-18/elt-6reporter gene expression in the developing vulva is attenuated inlin-39(rf) mutants, and overexpression of egl-18 can partially rescue the vulval defects caused by reduced lin-39activity. LIN-39/CEH-20 heterodimers bind two consensus HOX/PBC sites in a vulval enhancer region of egl-18/elt-6, one of which is essential for vulval expression of egl-18/elt-6 reporter constructs. These findings demonstrate that the EGL-18 and ELT-6 GATA factors are essential, genetically redundant regulators of cell fates and fusion in the developing vulva and are apparent direct transcriptional targets of the LIN-39 Hox protein.
Patterning of the basal telencephalon and hypothalamus is essential for guidance of cortical projections
Authors: Marín Oscar , Baker Joshua , Puelles Luis , Rubenstein John L. R.
Published: April 2021

We have investigated the mechanisms that control the guidance of corticofugal projections as they extend along different subdivisions of the forebrain. To this aim, we analyzed the development of cortical projections in mice that lack Nkx2-1, a homeobox gene whose expression is restricted to two domains within the forebrain: the basal telencephalon and the hypothalamus. Molecular respecification of the basal telencephalon and hypothalamus in Nkx2-1-deficient mice causes a severe defect in the guidance of layer 5 cortical projections and ascending fibers of the cerebral peduncle. These axon tracts take an abnormal path when coursing through both the basal telencephalon and hypothalamus. By contrast, loss of Nkx2-1 function does not impair guidance of corticothalamic or thalamocortical axons. In vitro experiments demonstrate that the basal telencephalon and the hypothalamus contain an activity that repels the growth of cortical axons, suggesting that loss of this activity is the cause of the defects observed in Nkx2-1 mutants. Furthermore, analysis of the expression of candidate molecules in the basal telencephalon and hypothalamus of Nkx2-1 mutants suggests that Slit2 contributes to this activity.
Endoderm is required for vascular endothelial tube formation, but not for angioblast specification
Authors: Vokes Steven A. , Krieg Paul A.
Published: April 2021

Angioblasts, the precursor cells that comprise the endothelial layer of blood vessels, arise from a purely mesodermal population. Individual angioblasts coalesce to form the primary vascular plexus through a process called vasculogenesis. A number of reports in the literature suggest that signals from the adjacent endoderm are necessary to induce angioblast specification within the mesoderm. We present evidence, using both embryological and molecular techniques, indicating that endoderm is not necessary for the induction of angioblasts. Xenopus embryos that had endoderm physically removed at the onset of gastrulation still express vascular markers. Furthermore, animal caps stimulated with bFGF form angioblasts in the absence of any detectable endodermal markers. These results show that endoderm is not required for the initial formation of angioblasts. While Xenopus embryos lacking endoderm contain aggregates of angioblasts, these angioblasts fail to assemble into endothelial tubes. Endothelial tube formation can be rescued, however, by implantation of endodermal tissue from sibling embryos. Based on these studies in Xenopus, and corroborating experiments using the quail embryo, we conclude that endoderm is not required for angioblast specification, but does play an essential role in the formation of vascular tubes.
Development of pigment-cup eyes in the polychaetePlatynereis dumeriliiand evolutionary conservation of larval eyes in Bilateria
Authors: Arendt Detlev , Tessmar Kristin , de Campos-Baptista Maria-Ines Medeiros , Dorresteijn Adriaan , Wittbrodt Joachim
Published: April 2021

The role of Pax6 in eye development in insects and vertebrates supports the view that their eyes evolved from simple pigment-cup ocelli present in their last common ancestors (Urbilateria). The cerebral eyes in errant polychaetes represent prototype invertebrate pigment-cup ocelli and thus resemble the presumed ancestral eyes. We have analysed expression of conserved eye specification genes in the early development of larval and adult pigment-cup eyes in Platynereis dumerilii (Polychaeta, Annelida, Lophotrochozoa). Both larval and adult eyes form in close vicinity of the optic anlagen on both sides of the developing brain ganglia. While pax6 is expressed in the larval, but not in the developing, adult eyes, expression of six1/2 from trochophora stages onwards specifically outlines the optic anlagen and thus covers both the developing larval and adult eyes. Using Platynereis rhabdomeric opsin as differentiation marker, we show that the first pair of adult eye photoreceptor cells is detected within bilateral clusters that transitorily express ath, the Platynereis atonal orthologue, thus resembling proneural sensory clusters. Our data indicate that – similar to insects, but different from the vertebrates – polychaete six1/2 expression outlines the entire visual system from early developmental stages onwards and ath-positive clusters generate the first photoreceptor cells to appear. We propose that pax6-, six1/2- and ath-positive larval eyes, as found in today’s trochophora, were present already in Urbilateria.
Drosophilamyosin phosphatase and its role in dorsal closure
Authors: Mizuno Tomoaki , Tsutsui Kyoko , Nishida Yasuyoshi
Published: April 2021

Myosin phosphatase negatively regulates nonmuscle myosin II through dephosphorylation of the myosin regulatory light chain (MRLC). Its regulatory myosin-binding subunit, MBS, is responsible for regulating the catalytic subunit in response to upstream signals and for determining the substrate specificity. DMBS, the Drosophila homolog of MBS, was identified to study the roles of myosin phosphatase in morphogenesis. The embryos defective for both maternal and zygotic DMBS demonstrated a failure in dorsal closure. In the mutant embryos, the defects were mainly confined to the leading edge cells which failed to fully elongate. Ectopic accumulation of phosphorylated MRLC was detected in lateral region of the leading edge cells, suggesting that the role of DMBS is to repress the activation of nonmuscle myosin II at the subcellular location for coordinated cell shape change. Aberrant accumulation of F-actin within the leading edge cells may correspond to the morphological aberrations of such cells. Similar defects were seen in embryos overexpressing Rho-kinase, suggesting that myosin phosphatase and Rho-kinase function antagonistically. The genetic interaction of DMBS with mutations in the components of the Rho signaling cascade also indicates that DMBS functions antagonistically to the Rho signal transduction pathway. The results indicate an important role for myosin phosphatase in morphogenesis.
Control ofDrosophilaimaginal disc development byrotundandroughened eye: differentially expressed transcripts of the same gene encoding functionally distinct zinc finger proteins
Authors: St Pierre Susan E. , Galindo Maximo I. , Couso Juan P. , Thor Stefan
Published: April 2021

The Drosophila rotund gene is required in the wings, antenna, haltere, proboscis and legs. A member of the Rac family of GTPases, denoted the rotund racGAP gene, was previously identified in the rotund region. However, previous studies indicated that rotund racGAP was not responsible for the rotund phenotypes and that the rotund gene had yet to be identified. We have isolated the rotund gene and show that it is a member of the Krüppel family of zinc finger genes. The adjacent roughened eye locus specifically affects the eye and is genetically separable from rotund. However, roughened eye and rotund are tightly linked, and we have therefore also isolated the roughened eye transcript. Intriguingly, we show that roughened eye is part of the rotund gene but is represented by a different transcript. The rotund and roughened eye transcripts result from the utilization of two different promoters that direct expression in non-overlapping domains in the larval imaginal discs. The predicted Rotund and Roughened Eye proteins share the same C-terminal region, including the zinc finger domain, but differ in their N-terminal regions. Each cDNA can rescue only the corresponding mutation and show negative effects when expressed in each others domain of expression. These results indicate that in addition to the differential expression of rotund and roughened eye, their proteins have distinct activities. rotund and roughened eye act downstream of early patterning genes such as dachshund and appear to be involved in Notch signaling by regulating Delta, scabrous and Serrate.
The Kallmann syndrome gene homolog inC. elegansis involved in epidermal morphogenesis and neurite branching
Authors: Rugarli Elena I. , Di Schiavi Elia , Hilliard Massimo A. , Arbucci Salvatore , Ghezzi Cristina , Facciolli Anna , Coppola Giuseppe , Ballabio Andrea , Bazzicalupo Paolo
Published: April 2021

Kallmann syndrome is an inherited disorder defined by the association of anosmia and hypogonadism, owing to impaired targeting and migration of olfactory axons and gonadotropin-releasing hormone secreting neurons. The gene responsible for the X-linked form of Kallmann syndrome, KAL-1, encodes a secreted protein of still elusive function. It has been proposed that KAL-1 might be involved in some aspects of olfactory axon guidance. However, the unavailability of a mouse model, and the difficulties in studying cellular and axonal migration in vertebrates have hampered an understanding of its function. We have identified the C. elegans homolog, kal-1, and document its function in vivo. We show that kal-1 is part of a mechanism by which neurons influence migration and adhesion of epidermal cells undergoing morphogenesis during ventral enclosure and male tail formation. We also show that kal-1 affects neurite outgrowth in vivo by modulating branching. Finally, we find that human KAL-1 cDNA can compensate for the loss of worm kal-1 and that overexpression of worm or human KAL-1 cDNAs in the nematode results in the same phenotypes. These data indicate functional conservation between the human and nematode proteins and establish C. elegans as a powerful animal in which to investigate KAL function in vivo. Our findings add a new player to the set of molecules, which appear to underlie both morphogenesis and axonal/neuronal navigation in vertebrates and invertebrates.
The thyroid and the development of the nervous system in Eleutherodactylus martinicensis : an experimental study
Authors: Hughes Arthur
Published: April 2021

The effects of treatment with phenylthiourea and of thyroidectomy and hypophysectomy on the embryo of Eleutherodactylus are described, mainly with reference to changes within the nervous system. In Eleutherodactylus the events at organ level during metamorphosis which are controlled by the thyroid are reduced to loss of the tail and of the pronephros. The primary sensory neurones of the cord, the Rohon-Beard cells, are also lost at the same time. The normal loss of cells from the ventral horns during development is not affected by any treatment to which the embryo has been submitted. Development is retarded by treatment with phenylthiourea, and also by hypophysectomy even when the pituitary is not wholly removed. In embryos submitted to this operation there is a loss of affinity for silver in the neurones of the ventral horn, which is most severe when the pituitary is completely excised. The addition of thyroxine to thyroidectomized embryos has been shown to increase the number of fibres within a muscular and also a cutaneous nerve. The number of fibres in a muscular nerve may be reduced by thyroidectomy or by hypophysectomy. It is suggested that in muscular nerves the loss of fibres is unaffected by thyroid hormones but that the accession of new fibres is sensitive to their action. The movement of supernumerary grafted limbs is much enhanced by general retardation of development with phenylthiourea.
The role of the genome in the development of the haploid syndrome in Anura
Authors: Hamilton Louie
Published: April 2021

Homozygous diploid embryos were obtained by using Subtelny’s method of nuclear transplantation of haploid nuclei and selection of the eggs which were delayed in first cleavage. Genetically uniform clones of homozygous diploids were produced by serial transplantation and their uniformity assessed as tadpoles. Homozygous diploid nuclear transplant embryos are quite variable within clones, about 20% normal and 80% with the ‘haploid syndrome’. Hetero-zygous diploid transplant embryos are uniform and normal. It is concluded that lack of heterozygosity leads to developmental insta-bility of which the ‘haploid syndrome’ is an expression.
Temporal colinearity and the phylotypic progression: a basis for the stability of a vertebrate Bauplan and the evolution of morphologies through heterochrony
Authors: Duboule Denis
Published: April 2021

Vertebrate Hox genes are essential for the proper organiz ation of the body plan during development. Inactivation of these genes usually leads to important alterations, or transformations, in the identities of the affected develop ing structures. Hox genes are activated in a progressive temporal sequence which is colinear with the position of these genes on their respective complexes, so that ‘anterior’ genes are activated earlier than ‘posterior’ ones (temporal colinearity). Here, an hypothesis is considered in which the correct timing of activation of this gene family is necessary in order to properly establish the various expression domains. Slight modifications in the respective times of gene activation (heterochronies) may shift expression domains along the rostrocaudal axis and thus induce concurrent changes in morphologies. It is further argued that temporal colinearity only occurs in cells with high mitotic rates, which results in a strong linkale between patterning and growth control and makes the pat terning process unidirectional, from anterior, proximal and early, to posterior, distal and late, a model referred to as the ‘Einbahnstrasse’. While the nature of the mechanism(s) behind temporal and spatial colinearities is unknown, it is proposed that such a mechanism relies on meta-cis interactions, that is it may necessitate gene conti guity. Such a mechanism would be based on DNA-specific, rather than gene-specific, features such as chromatin con figurations or DNA replication. The existence of such a meta-cis mechanism would explain the extraordinary con servation of this genetic system during evolution as its basic properties would be linked to that of the genetic material itself. Consequently, it is hypothesized that, in vertebrates, the resistance of this mechanism to evolutionary variations may be the reason for the existence of a short develop mental window of morphological invariance (the phylo typic progression).
Collagen Formation and Wound Contraction during Repair of Small Excised Wounds in the Skin of Rats
Authors: Abercrombie M. , Flint M. H. , James D. W.
Published: April 2021

Standard wounds of approximately 17 mm.2 area have been made in the dorsolumbar region of rats by cutting out a small square of skin within a standard tattoo-mark. By this technique, at 5, 10, 15, and 25 days after operation, the amount of wound contraction (disregarding epithelialization) has been measured, and the area of repair has been removed for collagen estimation by the hydroxyproline method of Neuman & Logan (1950). Contraction had occurred significantly 5 days after operation, was very active between 5 and 10 days, and did not proceed farther after 10 days. There was a decrease in wet weight of the wound content parallel to its contraction in area. During contraction the wound changed in shape from a square to a 4-pointed star. The rim of old tissue surrounding the wound diminished in length and in area. When contraction had ceased the surrounding skin was under abnormal tension, judged by the expansion when the wounded area,was excised. These observations can be accounted for by supposing that the content of the wounded area exerts a contractile force, and that the rim of original skin has some rigidity. Chemically measurable collagen formation had started 5 days after operation and it continued beyond 10 days, i.e. after contraction had ceased. The result was a progressive rise in both concentration and absolute amount of collagen in the wound. By 25 days the area under repair was, however, not nearly back to normal in collagen per unit area or per unit wet weight.
The inductors of differentiation of prefollicular tissue and the follicular epithelium in ovarioles of Pyrrhocoris apterus (Heteroptera)
Authors: Masner Petr
Published: April 2021

Differentiation of the tissues of the telotrophic ovarioles from the embryonic primordium occurs gradually throughout larval development of bugs. All tissues are established already at the beginning of the last larval stage as ob-served by Wick & Bonhag (1955) in Oncopeltus fasciatus. During the last instar, when the corpora allata are morphogenetically inactive, the synthetic activity of the cells of first ovarian nutritive system, the trophocytes, takes place. The nutrients released by trophocytes at the final emergence activate the oocytes, starting previtellogenesis (Masner, 1966). The function of this nutritive system is known to be independent of any humoral control (Wigglesworth, 1936, and others). Food intake seems to be the only inductor of trophocyte growth. The developing oocyte grows into the prefollicular syncytium. The surrounding tissue starts to organize into a unilayered epithelium.
Fine structure of degenerating taste buds after denervation
Authors: Farbman Albert I.
Published: April 2021

It is well known that taste buds are dependent on an intact nerve supply, and when experimentally denervated they degenerate and disappear (von Vintschgau & Honigschmied, 1877; von Vintschgau, 1880; Griffini, 1887; Meyer, 1897; Olmsted, 1920a, b, 1921, 1922; May, 1925; Whiteside, 1927; Torrey, 1934, 1936; Wagner, 1953; Guth, 1957, 1958, 1963; Beidler, 1962, 1963). Olmsted (1920 b) has suggested that the degenerating taste bud cells are cleared away by macrophages invading the epithelium; Guth (1957, 1958, 1963) has demon-strated sloughing of degenerating taste buds from the epithelial surface, and others believe that taste bud cells dedifferentiate to become lining epithelium (Meyer, 1897; Wagner, 1953). Because of this disagreement and because recent evidence for cell turnover in taste buds has indicated that cell death and replace-ment is a normal occurrence (Beidler, 1962, 1963; DeLorenzo, 1963; Beidler & Smallman, 1965), it is pertinent to study the fine structure of degenerating taste buds in the hope of elucidating the process by which taste bud cells and nerve terminals degenerate.
Is the presumptive notochord responsible for somite genesis in the chick?
Authors: Nicolet G.
Published: April 2021

Fragments of Hensen’s node or head process were implanted into the primitive streak 0·8 mm behind the node. The posterior region was isolated from all influences emanating from the anterior part of the blastoderm, either by cutting off a large rectangular piece just in front of the implant or by explanting the region combined with the notochord implant into the area opaca. As controls, corresponding regions of the primitive streak were isolated or explanted.

In controls, somite formation never took place, while the implantation of presumptive notochord into this posterior region was followed by the formation of somites in most cases.

In these experiments, the action of the presumptive notochord in its new situation was similar to that of the chorda-factor in amphibians.
An ultrastructural study of primordial germ cells, oogonia and early oocytes in Xenopus laevis
Authors: Al-Mukhtar Kawakib A. K. , Webb Andrew C.
Published: April 2021

Electron-microscope observations on the differentiation of germ cells in Xenopus laevis have revealed that the Balbiani body, cytoplasmic nucleolus-like bodies and groups of mitochondria associated with granular material previously reported only in older amphibian oocytes, are also present in the primordial germ cells, oogonia and early meiotic (pre-diplotene) oocytes of this species.

Although there is considerable morphological reorganization of the gonad as a whole at the time of sex determination, little visible change in the ultrastructure of the primordial germ cells appears to take place during their transition to oogonia. Both primordial germ cells and oogonia have highly lobed nuclei and their cytoplasm contains a conspicuous, juxtanuclear organelle aggregate (consisting for the most part of mitochondria), which is considered to represent the precursor of the Balbiani body.

In marked contrast, the transition from oogonium to oocyte in Xenopus is characterized by a distinctive change in nuclear shape (from lobed to round) associated with the onset of meiosis.

During leptotene the oocyte chromatin becomes visibly organized into electron-dense axial elements (representing the single unpaired chromosomes) which are surrounded by a fibrillar network. Towards the end of leptotene, these axial elements become attached to the inner surface of the nuclear membrane in a localized region adjacent to the juxtanuclear mitochondrial aggregate. Zygotene is marked by the initiation of axial element pairing over short regions, resulting in the typical synaptonemal complex configuration of paired homologous chromosomes. The polarization of these tripartite ribbons within the nucleus becomes more pronounced in late zygotene, producing the familiar Bouquet arrangement. The synaptonemal complexes are more extensive as synapsis reaches a climax during pachytene, whereas the polarization is to some extent lost. The fine structure of synaptonemal complexes in the Xenopus oocyte is essentially the same as that described in numerous other plant and animal meiocytes. It is not until the beginning of the extended diplotene phase that any appreciable increase in cell diameter takes place. During early diplotene (oocyte diameter approximately 50/μm), the compact Balbiani body characteristic of the pre-vitellogenic anuran oocyte is formed by condensation of the juxtanuclear mitochondrial aggregate.

Electron-dense, granular material appears to pass between nucleus and cytoplasm via nuclear pores in all stages of Xenopus germ cell differentiation studied. There is a distinct similarity in electron density and granular content between this ‘nuage material’ associated with the nuclear pores and the cytoplasmic aggregates of granular material in association with mitochondria or in the form of nucleolus-like bodies.
The growth of the retina in Xenopus laevis: an autoradiographic study
Authors: Feldman Joan D. , Gaze R. M.
Published: April 2021

The retina of Xenopus laevis has previously been shown, using autoradiographic methods, to develop in the normal animal by the annular addition of cells at the ciliary margin.

The development of the retina in animals with surgically produced ‘compound eyes’ was subsequently studied. In these animals the eye cup was split along the dorsoventral axis and the resulting half-eyes were recombined so as to form animals with a double-nasal eye.

The retina in experimental animals was found to develop as in the normal animal. No labelling of cells with radioactive thymidine was seen along the cut edge of each half-eye; thus in terms of cell division each half of the compound eye remains a half.
Ultrastructural studies of the mouse blastocyst substages
Authors: Nadijcka Mary , Hillman Nina
Published: April 2021

The mouse blastocyst stage covers approximately 2 days. During this period, embryonic development advances through four ultrastructurally distinct substages. Specific ultra-structural characteristics, such as changes in cell shape and/or the distribution of intracellular organelles, may be used to characterize the various cell types from each other at each sub-stage (e.g. trophoblast from inner cell mass cells), as well as to distinguish the same cells from each other at different substages (e.g. substage 1 abembryonic trophoblast cells from substage 2 abembryonic trophoblast cells). A distinguishing characteristic of these blastocyst substages are the membrane contacts (tight junctions, desmosomes, focal tight junctions) between cells. The apical surfaces of adjacent trophoblast cells, from the time of cavity formation until the initial stage of implantation, are connected by a tight junction and a desmosome. The basal region of the apposed trophoblast membranes, however, vary both in their spatial inter-relationship with each other and in their defined cellular contacts. Likewise, the cell contacts between the inner cell mass cells, and between these cells and the embryonic trophoblast cells, are distinctive for each blastocyst substage.
Zur Histogenese des Somiten beim Hühnchen
Authors: Mestres P. , Hinrichsen K.
Published: April 2021

The histogenesis of somites in the chick

The somites of the thoracic region of chick embryos incubated for 2 (stages 13–15) and 3 days (stages 17–19) have been examined cytologically and ultrastructurally. Cell contacts and cell arrangement were studied first in spherical somites. The further development of the spherical somite is characterized by changes in the cell arrangement. According to these changes two somitic cell populations can be distinguished: a ventral one, which during the third day loses its epithelial arrangement and becomes disaggregated, and a dorsal one, the dermatome, which remains epithelial for a longer time. Some cells of the ventral group reaggregate on the apical surface of the dermatome cells, and constitute the myotome. The contacts between dermatome and myotome cells as well as between the myotome cells themselves are decribed. Although the myotome cells appear to be arranged in an epithelial-like order and develop a basement membrane at the sclerotome side, the characteristic contacts and cell polarity of epithelia were not observed. These observations are discussed in relation to the process of cell adhesion and cell communication during the morphogenesis of the secondary mesenchyme.
The role of Schwann cells in paradoxical regeneration in the axolotl
Authors: Maden M.
Published: April 2021

The experiments described here examine further the conditions under which paradoxical regeneration occurs and provide support for the hypothesis that a proximal migration of Schwann cells is responsible for the phenomenon. When only the hand is shielded from irradition and the limb is denervated, amputation through the forearm or upper arm sometimes results in regeneration. The effects of variation in the time interval between denervation and amputation, the level of amputation and the method and number of denervations on the incidence of regeneration were investigated. The presence or absence of viable Schwann cells at the amputation plane was deduced from the remyelination of nerves under conditions which do or do not permit paradoxical regeneration. The nerves of totally irradiated and denervated limbs remain unmyelinated following regrowth of axons and such limbs do not regenerate after amputation. When only the hand was shielded from irradiation before the limbs were denervated, the new axons became completely remyelinated and some of these limbs regenerated when amputated. It is suggested that under these conditions Schwann cells can migrate proximally and can then proliferate further to form a blastema, since they would be the only unirradiated tissue present at the amputation plane.
Factors affecting the time of formation of the mouse blastocoele
Authors: Smith Rosita , McLaren Anne
Published: April 2021

In normal mouse embryos developing in vivo, the first appearance of the blastocyst cavity was found to be associated more closely with developmental age, judged by cell number, than with chronological age, i.e. elapsed time since ovulation. When development was slowed by in vitro culture, formation of the blastocoele was delayed. However, cell number itself was not a critical factor, since the number of cells per embryo could be doubled or tripled or halved by experimental manipulation without substantially affecting the timing of blastocoele formation. Experiments in which one cell division was suppressed with cytochalasin-B, leading to tetraploidy, showed that the number of cell divisions since fertilization was also not critical. A possible role is suggested either for nucleocytoplasmic ratio, or for the number of nuclear or chromosomal divisions or DNA replications since fertilization, all of which increase during cleavage.
Participation of neural crest-derived cells in the genesis of the skull in birds
Authors: Lièvre Christiane S. Le
Published: April 2021

The differentiation of cephalic neural crest cells into skeletal tissue in birds has been observed using the quail-chick nuclear marking system, which is based on specific differences in the distribution of the nuclear DNA. Chimaeras were formed by replacing a fragment of cephalic neural primordium of a 2- to 12-somite chicken embryo by the corresponding fragment isolated from an equivalent quail embryo. The participation of the graft-derived cells in the formation of the skull of these embryos was studied on histological sections after Feulgen and Rossenbeck staining.

Cells from the prosencephalic neural crest migrate into the frontal nasal process and mix with the mesencephalic neural crest cells in the lateral nasal processes, around the optic cupule and beneath the diencephalon. In addition, the mesencephalic neural crest cells form the bulk of the mesenchyme of the maxillary processes and mandibular arch, whereas the rhombencephalic neural crest cells become located in the branchial arches.

The origin of cartilages of the chondrocranium and bones of the neurocranium and viscerocranium has been shown in the chimaeric embryos: the basal plate cartilages, occipital bones, sphenoid bones and the cranial vault are mainly of mesodermal origin. However some parts have a dual origin: rhombo-mesencephalic neural crest cells are found in the otic capsule, and the frontal bone, the rostrum of parasphenoid and the orbital cartilages contain diverse amounts of prosencephalo-mesencephalic neural crest cells. The squamosals and the columella auris are formed from mesectodermic cells as are the nasal skeleton, the palatines and the maxillar bones. The mesectodermal origin of mandibular and hyoid bones and cartilages was already known.

From these results it appears that the cephalic neural crest is particularly important in the formation of the facial part of the skull, while the vault and dorsal part are mesodermal and cartilages and bones found in the intermediary region are of mixed origin. The presence of mixed structures implies that the mesoderm and the mesectoderm are equally competent towards the specific inducers of these bones and cartilages. This correlates with the equivalence in differentiation capacities already shown for cephalic mesodermal and mesectodermal mesenchymes.
Has collagen a role in muscle pattern formation in the developing chick wing? 1. An immunofluorescence study
Authors: Shellswell G. B. , Bailey A. J. , Duance V. C. , Restall D. J.
Published: April 2021

We have used antibodies to three of the isomorphic forms of collagen, types I, III and V, in an immunofluorescence microscopy study of myogenesis in the embryonic chick wing, concentrating on the period between stages 27 and 30 (5 to 7·5 days incubation) which is when the dorsal and ventral muscle masses separate into discrete muscles.

We have demonstrated the presence of all three collagen types at the ectoderm-mesenchyme junction from stage 27 onwards. Type I collagen and then type III collagen are found in progressively deeper layers of the dermis at the later stages. Both types I and V collagen are initially present in the cartilage elements, but type 1 collagen becomes restricted to the periphery of these structures at later stages. The developing muscle areas show a lack of staining at all stages and it is only at the latest stages that types I and III collagen first appear in the surrounding epimysium.

We discuss possible mechanisms for the division of the muscle masses in the light of this information on the distribution of collagen types.
The eye margin and compound-eye development in the cockroach: evidence against recruitment
Authors: Nowel Mark S. , Shelton Peter M. J.
Published: April 2021

The compound eye of the cockroach nymph grows from stadium to stadium by the addition of new ommatidia to the growing edge of the eye. By a series of transplant operations on Periplaneta americana and from SEM studies on Gromphadorhina portentosa it is shown that the proliferating region of the eye margin is a budding zone. There is no recruitment of larval head-capsule epidermis into the eye.
The haemoglobins of developing duck embryos
Authors: Cirotto C. , Arangi I. , Panara F.
Published: April 2021

Three haemoglobins were isolated by ion-exchange chromatography from the haemolysates of embryonic duck erythrocytes up to 8 days of development. The component globins were characterized both by electrophoresis in dissociating conditions and by finger-printing analysis. The major haemoglobin fraction El appears to be an embryonic tetramer since its constituent globins are different from all the others synthesized during embryonic and adult life. The two minor fractions E2 and E3 show a-type subunits that are very similar to those of the two adult haemoglobins Al and A2 respectively. They are present all through embryonic life, as demonstrated by chromatographic analysis. For these reasons they have been considered foetal.

The two haemoglobins typical of the adult animal are found in the red cells of the embryo from 8 days of incubation. Their relative amounts change continuously during embryonic development and reach the adult value after hatching.
In vitro development of inner cell masses isolated from t0/t0 and tw5/tw5 mouse embryos
Authors: Hogan Brigid , Spiegelman Martha , Bennett Dorothea
Published: April 2021

Inner cell masses (ICMs) isolated immunosurgically from mouse blastocysts segregating the homozygous lethal mutants t0/t0 and tw5 /tw5 were cultured in vitro. Presumed ICMs fail to grow after three days in culture (equivalent gestational day 7 · 5) when they consist of an outer layer of endoderm cells surrounding about 30 epiblast cells. Presumed homozygous tw5 / tw5 ICMs develop to a more advanced stage in culture and on the seventh day (equivalent gestational day 11 · 5) consist of an inner core of disorganized ectoderm cells with a small proamniotic cavity, surrounded by multiple layers of endoderm cells.