bluuebunny/crossref_metadata_2025 huggingface dataset:
The developmental specification of the vertebrate skull
Authors: Thorogood Peter
Published: April 2021

The initial form of the embryonic bony skull is determined in two ways; cranially, by the relative growth of the developing brain, and facially, by the chondrocranium. Both are essentially acting as structural templates around which the bony components of the skull are assembled. Assuming, therefore, that the specification of form and pattern in the facial skeleton occurs at the formation of the chondrocranium, this paper will focus on precisely how the chondrocranium forms. Any acceptable explanation of chondrocranial morphogenesis must satisfy at least two prerequisites. First, given the constancy of chondrocranial form in vertebrates, any model proposed should be equally applicable to all vertebrates. Second, it should enable us to answer questions of homology concerning the skull and, in particular, provide explanation for those instances where ‘homologous’ structures have a different (lineage) composition.

From studies limited to a small number of amphibian, avian and mammalian species, it is apparent that chondrogenesis in the vertebrate skull is largely, if not entirely, elicited by epitheliomesenchymal tissue interactions. Analysis of such interactions (and of those promoting osteogenesis) reveals that these are matrixmediated and, recently, the expression of certain ‘relevant’ matrix components has been shown to be developmentally regulated in a fashion that correlates with the location and timing of these interactions. From these, and related, observations a morphogenetic model, the so-called ‘Flypaper Model’, has been proposed to explain the specification of chondro-cranial form. A number of predictions arising from that model are currently being tested experimentally and the current status of the model is reviewed. Finally, the ability of this model to satisfy the prerequisites defined above is assessed.
Homeobox genes and the vertebrate head
Authors: Holland Peter W. H.
Published: April 2021

Several Drosophila genes important in the control of embryonic development contain a characteristic sequence of DNA, known as the homeobox. Homeobox sequences are also present in a family of vertebrate genes, which may therefore have regulatory roles during vertebrate embryogenesis. In this article, data concerning the spatial patterns of vertebrate homeobox gene expression are discussed in relation to recent descriptive and experimental analyses of head development. It is concluded that the patterns of gene expression are consistent with homeobox genes having roles in anteroposterior positional specification within the developing brain and possibly the neural crest. The data are not clearly consistent with these genes having direct roles in controlling the patterns of cranial segmentation, although further studies may reveal whether vertebrate segments are units of developmental specification.
Localization of c-myc expression during oogenesis and embryonic development in Xenopus laevis
Authors: Hourdry Jacques , Brulfert Annie , Gusse Michel , Schoevaert Damien , Taylor Michael V. , Mechali Marcel
Published: April 2021

The expression of the proto-oncogene c-myc during oogenesis and embryonic development was followed by in situ hybridization using a cytological protocol adapted to amphibian embryos. The c-myc RNA was highly expressed in the cytoplasm of young oocytes and was further diluted during oocyte growth without specific localization. From the neurula stage on, new myc transcripts were detected and the whole embryo appeared positive with antisense myc RNA probes relative to control sense RNA probes. In addition, a spatial localization of high levels of the transcript was also observed in specific areas of the developing embryo, including the epidermis, gill buds, optic vesicles and lens placodes. These observations might indicate a specific role of the c-myc gene during the differentiation of these tissues. Alternatively, this high level of myc expression might prevent such tissues from entering into terminal differentiation during the growth of the embryo.
Evidence for dedifferentiation and metaplasia in amphibian limb regeneration from inheritance of DNA methylation
Authors: Casimir Colin M. , Gates Phillip B. , Patient Roger K. , Brockes Jeremy P.
Published: April 2021

Amphibian limb regeneration is a process in which it has been suggested that cells of one differentiated type may dedifferentiate and give rise to cells of another type in the regenerate. We have used two tissue-specific hypomethylations in the newt cardioskeletal myosin heavy chain gene as lineage markers to follow the fate of cells during limb regeneration. Analysis of genomic DNA from different muscle cell populations allowed the assignment of one marker to the muscle (Hypo A) lineage and the other, more tentatively, to the ‘connective tissue’ (Hypo B) component of muscle. The contribution to regenerated limb cartilage and limb blastemal tissue by çells carrying these markers was estimated by quantitative analysis of Southern blot hybridizations using DNA from regenerate tissues. The results are consistent with a contribution of cells from both muscle and connective tissue lineages to cartilage in regenerated limbs. In addition, removal of the humerus at the time of amputation (eliminating any contribution from pre-existing cartilage), has provided evidence for an increased representation of cells carrying the connective tissue marker in regenerate cartilage but did not affect the representation of cells carrying the muscle cell marker.
Retinoic acid proximalizes level-specific properties responsible for intercalary regeneration in axolotl limbs
Authors: Crawford Karen , Stocum David L.
Published: April 2021

The objective of this study was to determine whether retinoic acid (RA) coordinately proximalizes positional memory and the cellular recognition system that detects pattern discontinuity in regenerating amphibian limbs. The strategy was to test the capacity of RA-treated blastemas to evoke intercalary regeneration when grafted to an amputation level proximal to their level of origin. Control wrist and ankle, or elbow and knee blastemas treated with the retinoid solvent, dimethylsulphoxide, evoked intercalary regeneration as effectively as untreated blastemas, when grafted to the midstylopodial amputation surface of host limbs. RA-treated wrist and ankle or elbow and knee blastemas were proximalized and formed complete limbs that were at an angle to, or continuous with, the midstylopodium of the host limb. No intercalary regeneration, from either graft or host, was observed in these cases. The results indicate that the cellular mechanism that recognizes disparities between nonneighbouring cells and initiates intercalary regeneration is coordinately proximalized with positional memory. Thus the recognition mechanism and positional memory are directly related. Intercalary regeneration and corrective displacement (affinophoresis), both of which restore a pattern of normal cell neighbours by different means in regenerating axolotl limbs, appear to use the same mechanism to recognize pattern discontinuity.
Clonal analysis of the crustacean segment: the discordance between genealogical and segmental borders
Authors: Dohle Wolfgang , Scholtz Gerhard
Published: April 2021

The post-naupliar germ bands of many higher crustaceans show a regular grid-like pattern of cells. This pattern is generated, in part but not in toto, by the proliferation of teloblasts. The exact lineage of all the cells of the post-naupliar germ band has been investigated in most of the orders belonging to the monophyletic unit Peracarida (Cumacea, Tanaidacea, Isopoda, Mysidacea, Amphipoda). The cell divisions and differentiation could be followed up to the formation of appendage buds, of ganglia and of intersegmental furrows.

The most-striking result is that the genealogical borders between cells of different clones do not correspond to the transverse intersegmental furrows. Instead, the genealogical borders divide a segment, and even the appendage buds, into anterior and posterior compartments.

There are different pathways for the formation of the cells of the post-naupliar germ band, though the subsequent differentiation may be nearly identical. It has been deduced from these findings that the fate of the cells is not determined by their origin. This supposition could be substantiated by a comparative analysis of the different orders. In Amphipoda, for instance, ectoteloblasts are not differentiated; the post-naupliar germ band is formed by an assemblage of blastoderm cells. Nevertheless, the cleavage pattern of these cells is for the most part identical to that of the other orders that possess ectoteloblasts.
The developmental expression of the heat-shock response in Xenopus laevis
Authors: Davis Robin E. , King Mary Lou
Published: April 2021

We have investigated the Xenopus heat-shock response of somatic cells, oocytes and embryos. Xenopus defolli-culated oocytes displayed a highly variable response to heat shock depending on the culture medium. Intact follicles, however, respond to heat stress by synthesizing an invariant pattern of hsps. Although a subset of the hsp70/68 complex is expressed constitutively in the absence of heat shock in oocytes and embryos (hsc70), actual induction of hsps in response to stress does not occur until the blastula stage when transcription of the zygotic genome is first activated. By gastrulation, the hsps of somatic cells, including members of the hsp30/26 complex, were expressed coordinately in response to heat shock. We further show that Xenopus hsps have different solubilities perhaps reflecting their different subcellular locations. The 26 000-30 000Mr complex (hsp30/26) was present almost exclusively in a detergent-insoluble fraction, as was 25−50 % of the hsp70/68 complex and greater than 50 % of hsp56, suggesting that these hsps may be associated with the cytoskeleton during a heat shock. In contrast, the other Xenopus hsps (hsp86, hsp75 and hsp61) were totally solubilized in a low-salt buffer.
Expression and role of E- and P-cadherin adhesion molecules in embryonic histogenesis II. Skin morphogenesis
Authors: Hirai Yohei , Nose Akinao , Kobayashi Shigeyasu , Takeichi Masatoshi
Published: April 2021

Expression and the role of E- and P-cadherin in the histogenesis of the surface epidermis and hair follicles were examined using the upper lip skin of the mouse. P-cadherin is expressed exclusively in the proliferating region of these tissues, that is in the germinative layer of the surface epidermis, the outer root sheath and the hair matrix. E-cadherin is coexpressed in these layers but this molecule was also detected in non-proliferating regions such as the intermediate layer of the surface epidermis and the immature regions of the inner root sheath. Neither P- nor E-cadherin was detected in fully keratinized layers such as the horny layer of the surface epidermis, the outermost layer of the outer root sheath and the mature hair fibres. These two cadherins were not detected in dermal cells. We cultured pieces of the upper lip skin in vitro in the absence or presence of a monoclonal antibody to E-cadherin (ECCD-1) or to P-cadherin (PCD-1). In control cultures, skin morphogenesis normally occurred in a pattern whereby the hair follicles grew and dermal cells were condensed to form the dermal sheath. A mixture of ECCD-1 and PCD-1, however, induced abnormal morphogenesis in the skin in several respects. (1) The cuboidal or columnar arrangement of basal epithelial cells was distorted. (2) Hair follicles were deformed. (3) Condensation of dermal cells was suppressed, causing a homogeneous distribution of these cells. These results suggest that cadherins present in epidermal cells are involved not only in maintaining the arrangement of these cells but also in inducing dermal condensation.
Pattern regulation during the development of the ventral abdomen in the flesh fly Sarcophaga agryostoma
Authors: Smith Hazel
Published: April 2021

The adult abdominal epidermis of the flesh fly, Sarco-phaga agryostoma, arises from small nests of diploid histoblasts which grow out and replace the polytene larval epidermal cells during metamorphosis. Extirpation experiments are used to investigate the roles of interactions with neighbouring nests or larval epidermis in determining the fates of the ventral histoblast nests. The extirpation of both left and right ventral histoblast nests deletes the sternite in the corresponding adult segment, but when only one nest is extirpated the hemisternite on that side of the adult is regenerated by the remaining nest. Regenerated hemisternites are smaller and bear fewer bristles than normal hemisternites. Extirpation of the larval epidermis between the two ventral nests leads to the production of small duplicate hemisternites, which resemble the regenerated hemisternites produced by single ventral nests. These midventral extirpations were also found to delay ventral nest fusion.

The results suggest that each ventral nest has an autonomous capacity to form both hemisternites, but that interactions between the nests after fusion normally limit their development and prevent pattern duplication. The possible nature of such interactions is discussed and related to models of pattern regulation and growth control in imaginal disc development.
Genetic analysis of developmental mechanisms in hydra XIX. Stimulation of regeneration by injury in the regeneration-deficient mutant strain, reg-16
Authors: Kobatake Eucaly , Sugiyama Tsutomu
Published: April 2021

A mutant strain of Hydra magnipapillata, reg-16, has a very low regenerative capacity. After head removal, it usually restores 10-20% of the original number of tentacles in 7 days.

A procedure was found to markedly improve tentacle regeneration in this strain. The closed wound located at the apical regenerating tip of the decapitated polyp was gently reopened using a pair of forceps. Reg-16 polyps treated in this way at 24 and 48 h after head removal restored nearly all of the original number of tentacles in 7 days.

A lateral tissue transplantation procedure was employed to examine the effect of wound reopening on the morphogenetic potential of decapitated reg-16 polyps. Wound reopening produced a significant increase in head activation level without producing a preceding decrease in head inhibition level. This and other observations suggest that the coupled activationinhibition changes that normally occur after head removal from the wild-type hydra do not occur in this strain.

Mechanisms responsible for the wound reopening effect and the absence of activation-inhibition coupling in the mutant strain reg-16 are discussed.
Molecular differences between the rostral and caudal halves of the sclerotome in the chick embryo
Authors: Norris Wendie E. , Stern Claudio D. , Keynes Roger J.
Published: April 2021

It is known that both neural crest cell migration and motor axon outgrowth in most vertebrate embryos are segmented because of restrictions imposed upon their distribution by the neighbouring sclerotomes, each of which is divided into a rostral and a caudal half. The caudal half does not allow crest migration or axon outgrowth, while the rostral half does. In this paper, we investigate the expression of proteins and glycoproteins in the two halves of the sclerotome of the chick embryo at stages between 20 and 32 pairs of somites by two-dimensional SDS-polyacrylamide gel electrophoresis. We find that the patterns of expression are complex, and that polypeptides and glycoproteins vary both spatially and temporally: of those that are expressed differentially by the sclerotome, some differ quantitatively and others qualitatively. Some macromolecules change their spatial distribution with developmental age, and some appear or disappear as the embryos become older.
An assessment of the developmental potential of embryonic stem cells in the midgestation mouse embryo
Authors: Beddington R. S. P. , Robertson E. J.
Published: April 2021

Embryonic stem cells (ES) cells were injected into host blastocysts either in groups of 10–15 cells or as single cells in order to test their developmental potential in the developing embryo. The analysis of midgestation chimaeras, by electrophoretic separation of glucose phosphate isomerase (GPI) isozymes, showed that ES cells were capable of colonizing trophectoderm and primitive endoderm derivatives at a low frequency, as well as producing a high rate of chimaerism in tissues of the fetus and extraembryonlc mesoderm.
The specification of heart mesoderm occurs during gastrulation in Xenopus laevis
Authors: Sater Amy K. , Jacobson Antone G.
Published: April 2021

The establishment of heart mesoderm during Xenopus development has been examined using an assay for heart differentiation in explants and explant combinations in culture. Previous studies using urodele embryos have shown that the heart mesoderm is induced by the prospective pharyngeal endoderm during neurula and postneurula stages. In this study, we find that the specification of heart mesoderm must begin well before the end of gastrulation in Xenopus embryos. Explants of prospective heart mesoderm isolated from mid- or late neurula stages were capable of heart formation in nearly 100% of cases, indicating that the specification of heart mesoderm is complete by midneurula stages. Moreover, inclusion of pharyngeal endoderm had no statistically significant effect upon either the frequency of heart formation or the timing of the initiation of heartbeat in explants of prospective heart mesoderm isolated after the end of gastrulation. When the superficial pharyngeal endoderm was removed at the beginning of gastrulation, experimental embryos formed hearts, as did explants of prospective heart mesoderm from such embryos. These results indicate that the inductive interactions responsible for the establishment of heart mesoderm occur prior to the end of gastrulation and do not require the participation of the superficial pharyngeal endoderm.
Oligodendrocyte progenitors isolated directly from developing telencephalon at a specific phenotypic stage: myelinogenic potential in a defined environment
Authors: Gard A. L. , Pfeiffer S. E.
Published: April 2021

Oligodendroglia differentiate asynchronously in the developing central nervous system, passing through a series of stages identified by the sequential expression of specific differentiation antigens, culminating in the formation of the myelin sheath. In the work presented here, oligodendrocyte progenitors at a temporally narrow and well-defined phenotypic stage of development have been isolated in high purity and yield directly from postnatal rat telencephalon. This stage is identified by the expression of the O4 antigen, the earliest recognized surface marker specific for the oligodendroglial lineage, but the absence of the differentiation marker galactosylcerebroside (GalC). These O4+GalC– progenitors first appear at birth (105/telencephalon), 2–3 days before O4+GalC+ oligodendrocytes. The work presented here demonstrates that a major subpopulation of O4+GalC– progenitors (80 %), which we have termed ‘proligoden-drocytes’, is fully committed to terminal oligodendrocyte differentiation. A relatively small, maximal set of nutritional supplements are sufficient for proligodendro-cytes to carry out the myelinogenic cascade of differentiated gene expression in a temporally normal manner, in quantitatively significant amounts, in normal ratios of myelin protein isoforms, and in a regulated relationship to the inclusion of myelin-specific products into myelinlike membrane sheets. An important corollary is that this step of myelinogenesis does not require contact with other cell types, in particular neurones and astrocytes, nor does it require unknown growth factors unique to these cell types. Additionally under these conditions, there exists a developmentally quiescent subpopulation (20 %) of O4+GalC– cells that may have significance for understanding the progenitors previously described in adult brain and suggested to be instrumental in remyelination under pathological conditions.
Muscle-specific (CArG) and serum-responsive (SRE) promoter elements are functionally interchangeable inXenopusembryos and mouse fibroblasts
Authors: Taylor Michael , Treisman Richard , Garrett Nigel , Mohun Tim
Published: April 2021

The Xenopus cardiac actin gene contains four copies of a promoter element, the CArG box, which is conserved amongst striated muscle actin genes and is essential for tissue-specific expression in the developing Xenopus embryo. Our aim is to identify embryo and muscle proteins that interact with the CArG box as a step towards understanding the molecular basis of this developmentally regulated gene expression. The CArG box shares some sequence similarity with the Serum Response Element (SRE), which mediates the transcriptional activation by serum of genes such as c-fos and cytoskeletal actin. We show here that the most proximal cardiac actin CArG box is recognized by the same binding activity as the cytoskeletal actin SRE in nuclear extracts from both Xenopus embryos and mammalian muscle cells. This activity is indistinguishable from the previously characterized HeLa cell SRE-binding activity, Serum Response Factor (SRE). Importantly, we extend these in vitro studies to demonstrate that the CArG box and SRE are functionally interchangeable, both in Xenopus embryos and mouse fibroblasts. This implies that the CArG box and SRE can bind the same protein in vivo, as well as in vitro. Our results identify an SRF-like protein as a CArG box-binding factor and we discuss the implication that a common mechanism may be utilized in both muscle-specific gene expression and serum-responsive transcription.
Influence of extracellular matrix proteins on the expression of neuronal polarity
Authors: Chamak B. , Prochiantz A.
Published: April 2021

The influence of laminin (LN) and fibronectin (FN) on the differentiation of individual neurones from the embryonic rat central nervous system was studied in vitro. In control cultures or in the presence of soluble FN, most neurones had several dendrite-like and one axon-like processes. On substratum-bound LN, multipolar and unipolar cells were present. Soluble LN and bound FN induced a very simple neuronal morphology, most neurones having only one axon-like neurite as defined by morphological and immunocytochemical characteristics. The significant reduction of neuronal adhesion and spreading in conditions leading to the growth inhibition of dendrite-like processes suggests that, contrary to that of axons, dendrite growth strongly depends on neuronal adhesion. We propose a model in which the different dependency of axonal and dendritic outgrowth towards adhesion and spreading is explained by the respective physical properties of the two types of neurites.
Temporal changes in the expression of the insulin-like growth factor II gene associated with tissue maturation in the human fetus
Authors: Brice A.L. , Cheetham J. E. , Bolton V. N. , Hill N. C. W. , Schofield P. N.
Published: April 2021

The insulin-like growth factors are broadly distributed in the human conceptus and are thought to play a role in the growth and differentiation of tissues during development. Using in situ hybridization we have shown that a wide variety of specific cell types within tissues express the gene for insulin-like growth factor O at times of development from 18 days to 14 weeks of gestation. Examination of blastocysts produced by in vitro fertilization showed no expression, thus bracketing the time of first accumulation of IGF-II mRNA to between 5 and 18 days postfertilization. The pattern of IGF-II expression shows specific age-related differences in different tissues. In the kidney, for example, expression is found in the cells of the metanephric blastema which is dramatically reduced as the blastema differentiates. The reverse is also seen, and we have noted an increase in expression of IGF-II in the cytotrophoblast layer of the placenta with gestational age. The sites of expression do not correlate with areas of either high mitotic activity or specific types of differentiation, but the observed pattern of expression in the kidney, adrenal glands and liver suggests an explanation for the abnormally high IGF-II mRNA expression in developmental tumours such as Wilms’ tumour.
Homing behaviour of regenerating axons in the amphibian limb
Authors: Wilson Steve , Tonge D. A. , Holder Nigel
Published: April 2021

Following peripheral nerve deviation in the limbs of urodele amphibians axons regrow distally toward their previous target muscles (Holder et al. 1984; Proc. Roy. Soc. Lond. B 222, 477-489). This study describes analysis of this axon regeneration over time following deviation of the forearm flexor nerve in Trituras cristatus and the extensor cranialis nerve in the axolotl. Using horseradish peroxidase (HRP) axonal tracing, electro-physiology and electron microscopy, we describe the sequence of events leading to reestablishment of functional innervation. HRP fills reveal axons leaving the deviated nerve via a number of possible routes and they invariably grow distally. Many axons take a path close to that of the original nerve but others fasciculate forming parallel paths. Electrophysiology and electron microscopy show that axons in the deviated region of the nerve degenerate extensively compared with cut, but undeviated, controls. The results are discussed in terms of the possible axon-growth-promoting mechanisms that result in directed growth.
J1 /tenascin-related molecules are not responsible for the segmented pattern of neural crest cells or motor axons in the chick embryo
Authors: Stern Claudio D. , Norris Wendie E. , Bronner-Fraser Marianne , Carlson Geoffrey J. , Faissner Andreas , Keynes Roger J. , Schachner Melitta
Published: April 2021

It has been suggested that substrate adhesion molecules of the tenascin family may be responsible for the segmented outgrowth of motor axons and neural crest cells during formation of the peripheral nervous system. We have used two monoclonal antibodies (M1B4 and 578) and an antiserum [KAF9(1)] to study the expression of Jl/tenascin-related molecules within the somites of the chick embryo. Neural crest cells were identified with monoclonal antibodies HNK-1 and 20B4. Young somites are surrounded by Jl/tenascin immunoreactive material, while old sclerotomes are immunoreactive predominantly in their rostral halves, as described by other authors (Tan et al. 1987 - Proc. natn. Acad. Sci. U.S.A. 84, 7977; Mackie et al. 1988 - Development 102, 237). At intermediate stages of development, however, immunoreactivity is found mainly in the caudal half of each sclerotome. After ablation of the neural crest, the pattern of immunoreactivity is no longer localised to the rostral halves of the older, neural-crest-free sclerotomes. SDS–polyacrylamide gel electrophoresis of affinity-purified somite tissue, extracted using M1B4 antibody, shows a characteristic set of bands, including one of about 230×103, as described for cytotactin, Jl-200/220 and the monomeric form of tenascin. Affinity-purified somite material obtained from neural-crest-ablated somites reveals some of the bands seen in older control embryos, but the high molecular weight components (120-230X103) are missing. Young epithelial somites also lack the higher molecular mass components. The neural crest may therefore participate in the expression of Jl/tenascin-related molecules in the chick embryo. These results suggest that these molecules are not directly responsible for the segmented outgrowth of precursors of the peripheral nervous system.
Cell death of motoneurons in the chick embryo spinal cord: XI. Acetylcholine receptors and synaptogenesis in skeletal muscle following the reduction of motoneuron death by neuromuscular blockade
Authors: Oppenheim Ronald W. , Bursztajn Sherry , Prevette David
Published: April 2021

Treatment of chick embryos with neuromuscular blocking agents such as curare during periods of naturally occurring motoneuron death results in a striking reduction of this normal cell loss. Inactivity-induced changes in motoneuron survival were found to be associated with increased levels of AChRs and AChR-clusters in skeletal muscle and with increased focal sites of AChE that are innervated (‘synaptic sites’). Treatment of embryos with curare after the normal cell death period (E12–E15) resulted in no change in motoneuron survival. Although AChR-clusters and focal sites of AChE were increased in these embryos on E16, many of these sites were uninnervated. Treatment of embryos with nicotine or decamethonium (E6–E10) also reduced neuromuscular activity but did not alter motoneuron survival nor did such treatment alter AChRs. The different effects of curare vs nicotine and decamethoniam on motoneuron survival and AChRs may be related to the fact that the former is a competitive blocker whereas the latter two drugs are depolarizing blockers. Finally, treatment of embryos (E6–9) with doses of curare (1 mg daily) that allow for the almost complete recovery of neuromuscular activity a few days following treatment (by E16) resulted in the gradual loss of the excess motoneurons that were present on E10, and by E16 the number of remaining AChR clusters and focal sites of AChE were also decreased to levels comparable to control values. Inactivity-induced changes in AChRs or AChR-clusters may be an important factor in the reduced motoneuron death that accompanies neuromuscular blockade during critical stages of development. These receptor changes very likely reflect increased synaptogenesis in the muscles of paralyzed embryos which in turn may act to reduce motoneuron death by providing increased access to muscle-derived neurotrophic molecules.
Development of adult sensilla on the wing and notum of Drosophila melanogaster
Authors: Hartenstein Volker , Posakony James W.
Published: April 2021

We have investigated the temporal pattern of appearance, cell lineage, and cytodifferentiation of selected sensory organs (sensilla) of adult Drosophila. This analysis was facilitated by the discovery that the monoclonal antibody 22C10 labels not only the neuron of the developing sensillum organ, but the accessory cells as well. The precursors of the macrochaetes and the recurved (chemosensory) bristles of the wing margin divide around and shortly after puparium formation, while those of the microchaetes and the stout and slender (mechanosensory) bristles of the wing margin divide between 9h and 18 h after puparium formation (apt). The onset of sensillum differentiation follows the terminal precursor division within a few hours. Four of the cells in an individual microchaete organ are clonally related: A single first-order precursor cell divides to produce two second-order precursors; one of these divides into the neuron and thecogen cell, the other into the trichogen cell and tormogen cell. Along the anterior wing margin, two rounds of division generate the cells of the mechanosensory sensilla; here, no strict clonal relationship seems to exist between the cells of an individual sensillum. At the time of sensillum precursor division, many other, non-sensillum-producing cells within the notum and wing proliferate as well. This mitotic activity follows a spatially non-random pattern.
Genetic differences in glucose phosphate isomerase activity among mouse embryos
Authors: West John D. , Flockhart Jean H.
Published: April 2021

We have compared mouse embryos of three heterozygous, congenic genotypes (with high, medium and low levels of oocyte-coded glucose phosphate isomerase (GPI-1) activity respectively) to test whether 1) the survival time of oocyte-coded GPI-1 activity in the early embryo is affected by its activity level in the oocyte and 2) whether embryo-coded GPI-1 is detected earlier in embryos that inherit low levels of oocyte-coded GPI-1. The oocyte-coded GPI-1 was entirely GPI-1 A allozyme in the high and medium groups but was the less stable GPI-1C allozyme in the low group. We determined total GPI-1 activity and the ratio of different GPI-1 allozymes in early embryos and calculated the activity of oocyte-coded and embryo-coded GPI-1. In all three groups, the oocyte-coded enzyme activity remained at a more or less constant level for the first days. Some oocyte-coded GPI-1 remained in day embryos from the high and medium groups but was gone by days. Very little remained in day embryos that inherited low levels of a less stable form of the enzyme (GPI-1C allozyme). Despite a 4- to 5-fold difference in initial oocyte-coded GPI-1 activity, no differences were seen among the three genotypically distinct groups of embryos in the time of activation of the embryonic Gpi-1s genes. The embryo-coded GPI-1 was first detectable in day compacted morulae in all three groups. The level of oocyte-coded GPI-1, in the high group, when embryo-coded GPI-1 was first detected was higher than the level in the low group at any stage prior to detection of embryo-coded GPI-1. These results imply that embryonic Gpi-1s genes are not activated by a mechanism that depends on oocyte-coded GPI-1 enzyme activity falling below a critical threshold. Once both embryonic Gpi-ls alleles were activated. they were usually equally expressed in Gpi-lsa/Gpi-lsb heterozygous embryos. However, the maternal: paternal expression was closer to 2:1 in 5 % (3/60) of the and day embryos. These could have been digynic triploid embryos.
Spatial control of hairy protein expression during embryogenesis
Authors: Hooper Katharine L. , Parkhurst Susan M. , Ish-Horowicz David
Published: April 2021

We have used a polyclonal antiserum specific for the Drosophila segmentation gene, hairy (h), to analyse its expression during embryogenesis. The pattern of wildtype expression resembles that of h transcription, being expressed in stripes at the blastoderm stage, h is also expressed later in the stomodæum, proctodæum, tracheal pits and mesoderm. We demonstrate that h protein stripes show consistent phase relationships to those of the even-skipped (eve) pair-rule gene. We examine h protein patterns in embryos mutant for other segmentation genes, including h itself. We show that lack of h activity appears not to affect h striping, arguing that h expression is not under autoregulatory control. We also show that h activity is not needed for tracheal invagination. Mutations that are rearranged upstream of the h gene cause the loss of specific stripes, indicating that the h promoter includes activating elements that respond to specific spatial cues. Our observations suggest that pairrule striping may be under redundant control, and we discuss possible implications for hierarchical models of pair-rule gene action.
The maternal store of the xlgv7 mRNA in full-grown oocytes is not required for normal development in Xenopus
Authors: Kloc Malgorzata , Miller Mill , Carrasco Andres E. , Eastman Eric , Etkin Laurence
Published: April 2021

We have attempted to analyze the function of a maternal mRNA xlgv7 which is distributed as an animal - vegetal gradient in stage 6 oocytes using a combination of antisense oligodeoxynucleotide injection into oocytes followed by in vitro maturation and fertilization. Injection of 20 ng of the antisense oligodeoxynucleotide resulted in the destruction of the xlgv7 mRNA to undetectable levels. Upon maturation and fertilization the resulting embryos develop with no specific defects suggesting that the maternal store of xlgv7 in stage 6 oocytes is not required and that the embryo can develop solely with the maternal store of the xlgv7 protein. Also, these results demonstrate the feasibility of this approach in destroying a specific maternal RNA and assaying its effect on development.
Tissue-specific regulation of junctional communication in the skin of mouse fetuses homozygous for the repeated epilation (Er) mutation
Authors: Kam Ephraim , Pitts John D.
Published: April 2021

Mouse embryos homozygous for the repeated epilation (Er) gene have abnormally developed skin characterised by hyper-proliferation and incomplete differentiation of the epidermis. In this report, we have studied the patterns of junctional communication in the skin of these mutants to see if the loss of control of proliferation/ differentiation is associated with any altered patterns of communication. Using the dye-injection technique we have shown that, compared to normal skin, junctional communication among dermal cells of Er/Er mutants is greatly reduced and the frequency of dermal-epidermal communication is, on the other hand, increased. These results support our previously proposed model, which suggests that selective regulation of junctional communication can be a component of proliferative control in a complex tissue.
Transactivation of the adenovirus Ella promoter in the absence of adenovirus E1A protein is restricted to mouse oocytes and preimplantation embryos
Authors: Dooley Thomas P. , Miranda Miriam , Jones Nicholas C. , DePamphilis Melvin L.
Published: April 2021

Undifferentiated mouse embryonal carcinoma (EC) cells are capable of transactivating the adenovirus Ella promoter in the absence of its normal transactivator, E1A protein, suggesting that EC cells contain an E1A-like activity. In an effort to identify where this activity appears during normal mouse development, mouse oocytes and preimplantation embryos were injected with plasmids containing the Ella promoter coupled to various reporter genes. These expression vectors were fully active in human 293 cells where E1A is present, but were inactive in differentiated fibroblast cell lines unless cotransfected with the El A gene. In mouse oocytes and preimplantation embryos, Ella promoter activity in the absence of adenovirus E1A protein was equivalent to or greater than activity of the HSV thymidine kinase promoter coupled to a strong enhancer. Coinjection of the E1A gene failed to stimulate Ella activity further, perhaps because c-myc protein, which has been reported to transactivate this promoter, was already present at high levels in mouse oocytes. Activation of the Ella promoter in the absence of E1A was unique to mouse oocytes and preimplantation embryos because gene expression from an Ella promoter introduced into transgenic mice was observed only in the adult ovary, and particularly in the oocytes. In addition, postimplantation transgenic embryos failed to express the ElA-activatable reporter gene, thereby indicating that expression from the Ella promoter is restricted to the relatively undifferentiated stages of oogenesis and preimplantation development. These data suggest that cellular promoters of the class that can be transactivated by E1A may serve uniquely to initiate transcription of genes that are needed for preimplantation development.
Mutations affecting skeletal muscle myofibril structure in the zebrafish*
Authors: Felsenfeld Adam L. , Walker Charline , Westerfield Monte , Kimmel Charles , Streisinger George
Published: April 2021

We describe embryonic lethal mutations in the zebrafish, Brachydanio rerio, which affect organization of skeletal muscle myofibrils. The mutations, fub-l(b45) and fub-l( b126), were independently isolated from progeny of gamma-irradiated females. Each segregates as a single recessive gene; b45 is located about 23 map units from its centromere. The b126 mutation has a similar but slightly larger apparent gene–centromere distance and a less severe phenotype. The two mutations fail to complement, suggesting that they are allelic. Homozygous b45 mutant embryos are paralyzed, and their axial skeletal muscle cells are unstriated, containing severely disorganized myofibrillar components. Gel-electrophoretic comparisons of b45 mutant and wildtype muscle proteins failed to reveal absent or altered major myofibrillar proteins. Embryos genetically mosaic for b45 were also phenotypically mosaic, suggesting that the defect is cell-autonomous. We suggest that these mutations identify a gene required for proper organization of skeletal muscle myofibrils, and that the more severe mutation may represent a null allele.
Insulin and insulin-like growth factor I (IGF I) in early mouse embryogenesis
Authors: Spaventi Radan , Antica Mariastefania , Pavelić Krešimir
Published: April 2021

Growth factors have an important role in the regulation of cell growth, division and differentiation. They are also involved in the regulation of embryonic growth and differentiation. Insulin and insulin-like growth factor I (IGF I) play an important part in these events in the later stages of embryogenesis, when organogenesis is completed. In this study, we are presenting evidence that insulin and IGF I are also secreted by embryonic tissues during the prepancreatic stage of mouse development. We found measurable amounts of insulin and IGF I in 8- to 12-day-old mouse embryos. We also showed that embryonic cells derived from 8-, 9- and 10-day-old mouse embryos secrete insulin, IGF I and/or related molecules. Furthermore, the same growth factors, when added to the culture of 9-day-old mouse embryonic cells, stimulate their proliferation. These results lead to the conclusion that insulin can stimulate the growth of embryonic cells during the period when pancreas is not yet formed, which is indirect evidence for a paracrine (or autocrine) type of action.
The early development of retinal ganglion cells with uncrossed axons in the mouse: retinal position and axonal course
Authors: Colello R. J. , Guillery R. W.
Published: April 2021

The carbocyanine dye, Dil, has been used to study the retinal origin of the uncrossed retinofugal component of the mouse and to show the course taken by these fibres through the optic nerve and chiasm during development. Optic axons first arrive at the chiasm at embryonic day 13 (E13) but do not cross the midline until E14. After this stage, fibres taking an uncrossed course can be selectively labelled by unilateral tract implants of Dil. The earliest ipsilaterally projecting ganglion cells are located in the dorsal central retina. The first sign of the adult pattern of distribution of ganglion cells with uncrossed axons located mainly in the ventrotemporal retina is seen on embryonic day 16.5, thus showing that the adult line of decussation forms early in development. A small number of labelled cells continue to be found in nasal and dorsal retina at all later stages.

At early stages (E14-15), retrogradely labelled uncrossed fibres are found in virtually all fascicles of the developing nerve, intermingling with crossed axons throughout the length of the nerve. At later stages of development (E16–17), although uncrossed fibres pass predominantly within the temporal part of the stalk, they remain intermingled with crossed axons. A significant number of uncrossed axons also lie within the nasal part of the optic stalk. The position of uncrossed fibres throughout the nerve in the later developmental stages is comparable to that seen in the adult rodent (Baker and Jeffery, 1989). The distribution of uncrossed axons thus indicates that positional cues are not sufficient to account for the choice made by axons when they reach the optic chiasm.
Calcium and cell cycle control
Authors: Whitaker Michael , Patel Rajnikant
Published: April 2021

The cell division cycle of the early sea urchin embryo is basic. Nonetheless, it has control points in common with the yeast and mammalian cell cycles, at START, mitosis ENTRY and mitosis EXIT.

Progression through each control point in sea urchins is triggered by transient increases in intracellular free calcium. The Cai transients control cell cycle progression by translational and post-translational regulation of the cell cycle control proteins pp34 and cyclin. The START Cai transient leads to phosphorylation of pp34 and cyclin synthesis. The mitosis ENTRY Cai transient triggers cyclin phosphorylation. The motosis EXIT transient causes destruction of phosphorylated cyclin.

We compare cell cycle regulation by calcium in sea urchin embryos to cell cycle regulation in other eggs and oocytes and in mammalian cells.
Cell movements during epiboly and gastrulation in zebrafish
Authors: Warga Rachel M. , Kimmel Charles B.
Published: April 2021

Beginning during the late blastula stage in zebrafish, cells located beneath a surface epithelial layer of the blastoderm undergo rearrangements that accompany major changes in shape of the embryo. We describe three distinctive kinds of cell rearrangements. (1) Radial cell intercalations during epiboly mix cells located deeply in the blastoderm among more superficial ones. These rearrangements thoroughly stir the positions of deep cells, as the blastoderm thins and spreads across the yolk cell. (2) Involution at or near the blastoderm margin occurs during gastrulation. This movement folds the blastoderm into two cellular layers, the epiblast and hypoblast, within a ring (the germ ring) around its entire circumference. Involuting cells move anteriorwards in the hypoblast relative to cells that remain in the epiblast; the movement shears the positions of cells that were neighbors before gastrulation. Involuting cells eventually form endoderm and mesoderm, in an anteriorposterior sequence according to the time of involution. The epiblast is equivalent to embryonic ectoderm. (3) Mediolateral cell intercalations in both the epiblast and hypoblast mediate convergence and extension movements towards the dorsal side of the gastrula. By this rearrangement, cells that were initially neighboring one another become dispersed along the anterior-posterior axis of the embryo. Epiboly, involution and convergent extension in zebrafish involve the same kinds of cellular rearrangements as in amphibians, and they occur during comparable stages of embryogenesis.
Origin and organization of the zebrafish fate map
Authors: Kimmel Charles B. , Warga Rachel M. , Schilling Thomas F.
Published: April 2021

We have analyzed lineages of cells labeled by intracellular injection of tracer dye during early zebrafish development to learn when cells become allocated to particular fates during development, and how the fate map is organized. The earliest lineage restriction was described previously, and segregates the yolk cell from the blastoderm in the midblastula. After one or two more cell divisions, the lineages of epithelial enveloping layer (EVL) cells become restricted to generate exclusively periderm. Following an additional division in the late blastula, deep layer (DEL) cells generate clones that are restricted to single deep embryonic tissues. The appearance of both the EVL and DEL restrictions could be causally linked to blastoderm morphogenesis during epiboly. A fate map emerges as the DEL cell lineages become restricted in the late blastula. It is similar in organization to that of an amphibian embryo. DEL cells located near the animal pole of the early gastrula give rise to ectodermal fates (including the definitive epidermis). Cells located near the blastoderm margin give rise to mesodermal and endodermal fates. Dorsal cells in the gastrula form dorsal and anterior structures in the embryo, and ventral cells in the gastrula form dorsal, ventral and posterior structures. The exact locations of progenitors of single cell types and of local regions of the embryo cannot be mapped at the stages we examined, because of variable cell rearrangements during gastrulation.
Changes in the allelic methylation patterns of c-H-ras-1, insulin and retinoblastoma genes in human development
Authors: Ghazi H. , N.Magewu A. , Gonzales F. , Jones P.A.
Published: April 2021

The methylation status of the c-H-ras-1, insulin and retinoblastoma genes was determined in human sperm, hydatidiform mole, fetal tissues, adult lymphocytes and adult kidney. Individual alleles of c-H-ras-1 and insulin were distinguishable due to presence of endogenous variable number of tandem repeat (VNTR) polymor-phisms. Both alleles of the latter two genes were extensively methylated in sperm compared to the other tissues. Several sites within these genes were less methylated in fetal tissues and the two alleles were differentially methylated in some cases. The retinoblas-to ma gene was highly methylated in all tissues examined, with the exception of a single site that was under-methylated in sperm only. The sperm-specific methyl-ation patterns in all three genes could represent imprinting of the parental chromosomes. Since 5-methylcytosine is inherently mutagenic, it is possible that methylation imprinting could alter the susceptibili-ties of human genes to point mutations.