bluuebunny/crossref_metadata_2025 huggingface dataset:
Expression of the fibroblast growth factor-5 gene in the mouse embryo
Authors: Haub Olivia , Goldfarb Mitchell
Published: April 2021

Fibroblast growth factors (FGFs) are structurally related mitogens that can regulate the differentiation of a wide variety of cells. As a step towards elucidating the developmental roles played by one of these factors, we have used in situ hybridization methods to examine expression of the murine Fgf-5 gene during embryogenesis. Fgf-5 RNA was detected at seven distinct sites in the developing mouse embryo: (1) postimplantation epiblast (embryonic day , (2) lateral splanchnic mesoderm , (3) lateral somatic mesoderm , (4) myotomes , (5) mastication muscle , (6) limb mesenchyme , and (7) acoustic ganglion . At several of these sites, expression is spatially restricted within the tissues. We offer several hypotheses regarding the roles of FGF-5 in murine development.
Proteinases of the mammary gland: developmental regulation in vivo and vectorial secretion in culture
Authors: Talhouk Rabih S. , Chin Jennie R. , Unemori Elaine N. , Werb Zena , Bissell Mina J.
Published: April 2021

The extracellular matrix (ECM) is an important regulator of mammary epithelial cell function both in vivo and in culture. Substantial remodeling of ECM accompanies the structural changes in the mammary gland during gestation, lactation and involution. However, little is known about the nature of the enzymes and the processes involved. We have characterized and studied the regulation of cell-associated and secreted mammary gland proteinases active at neutral pH that may be involved in degradation of the ECM during the different stages of mammary development. Mammary tissue extracts from virgin and pregnant CD-I mice resolved by zymography contained three major proteinases of 60K (K=103Mr), 68K and 70K that degraded denatured collagen. These three gelatinases were completely inhibited by the tissue inhibitor of metalloproteinases. Proteolytic activity was lowest during lactation especially for the 60K gelatinase which was shown to be the activated form of the 68K gelatinase. The activated 60K form decreased prior to parturition but increased markedly after the first two days of involution. An additional gelatin-degrading proteinase of 130K was expressed during the first three days of involution and differed from the other gelatinases by its lack of inhibition by the tissue inhibitor of metalloproteinases.

The activity of the casein-degrading proteinases was lowest during lactation. Three caseinolytic activities were detected in mammary tissue extracts. A novel 26K cell-associated caseinase - a serine arginine-esterase - was modulated at different stages of mammary development. The other caseinases, at 92K and a larger than 100K, were not developmentally regulated. To find out which cell type produced the proteinases in the mammary gland, we isolated and cultured mouse mammary epithelial cells. Cells cultured on different substrata produced the full spectrum of gelatinases and caseinases seen in the whole gland thus implicating the epithelial cells as a major source of these enzymes. Analysis of proteinases secreted by cells grown on a reconstituted basement membrane showed that gelatinases were secreted preferentially in the direction of the basement membrane. The temporal pattern of expression of these proteinases and the basal secretion of gelatinases by epithelial cells suggest their involvement in the remodelling of the extracellular matrix during the different stages of mammary development and thus modulation of mammary cell function.
The embryonic origin of imaginal discs in Drosophila
Authors: Bate Michael , Arias Alfonso Martinez
Published: April 2021

The thoracic imaginal discs of Drosophila melanogaster can be observed during embryogenesis as clusters of cells with particular shapes, sizes and behaviours. These structures can be detected soon after germ band shortening and their development appears to be tightly linked to that of the larval epidermis.
Substitution for mesenchyme by basement-membrane-like substratum and epidermal growth factor in inducing branching morphogenesis of mouse salivary epithelium
Authors: Nogawa Hiroyuki , Takahashi Yu
Published: April 2021

Mouse salivary epithelium cannot undergo branching morphogenesis in the absence of the surrounding mesenchyme. To clarify the nature of the mesenchymal influence on the epithelium, we have investigated the culture conditions in which the epithelium could normally branch in the absence of mesenchymal cells. Combination of basement-membrane-like substratum (Matrigel) and epidermal growth factor (EGF) could substitute for the mesenchyme, the epithelium showing typical branching morphogenesis. Transforming growth factor a had the same effect as EGF. Matrigel plus basic fibroblast growth factor or transforming growth factor β1 and collagen gel plus EGF were not sufficient to support the branching of the epithelium. These results clearly reveal that the role of mesenchyme in salivary morphogenesis is both to provide the epithelium with an appropriate substratum and to accelerate growth of the epithelium.
Identification of multiple stages in the conversion of maize meristems from vegetative to floral development
Authors: Irish Erin E. , Nelson Timothy M.
Published: April 2021

Vegetative growth in most lines of maize is terminated at a predictable stage in development by the conversion of the shoot apical meristem into an inflorescence, the tassel. The conversion from vegetative to floral development is under developmental control, the basis of which is obscure. We have assayed the developmental potential of the shoot apical meristem in order to identify the stage at which it is determined to form a tassel. We show, using shoot apex culture, that meristems are not determined to form a tassel until after all vegetative nodes have been initiated. We also show that floral determination is a separate, later event in the development of a maize inflorescence. Intermediate stages can be distinguished in which the meristem is determinate and has the phyllotaxis of a tassel when cultured but develops vegetative shoots from buds that normally give rise to sets of florets.
Differential stability of expression of similarly specified endogenous and exogenous genes in the sea urchin embryo
Authors: Livant Donna L. , Hough-Evans Barbara R. , Moore James G. , Britten Roy J. , Davidson Eric H.
Published: April 2021

The object of these experiments was to determine whether competitive titration in vivo of factors required for expression of the Cyllla · CAT fusion gene would affect expression of the endogenous Cyllla gene in the same embryos. Earlier work showed that expression of this fusion gene after injection into sea urchin eggs is stoichiometrically reduced when low molar excesses of DNA fragments containing only its regulatory domain are coinjected. In order to compare endogenous (i.e. Cyllla) and exogenous (i.e. Cyllla · CAT) expression simultaneously in embryos bearing excess competitor regulatory DNA, we developed, and here describe, a new procedure for generating transgenic sea urchin embryos in which all of the cells in many embryos, and most in others, bear the exogenous DNA. Such large reduction of mosaicism can be achieved by multiple injection of the exogenous DNA fragments into fertilized eggs. Using this method, we demonstrate that at a level of competitor DNA incorporation which reduces Cyllla·CAT expression by 85%, endogenous Cyllla mRNA levels are wholly unaffected. Nor is spatial expression of the endogenous Cyllla gene disturbed. Since the Cyllla · CAT genes are properly expressed under control of the Cyllla regulatory sequences, they must participate in the same set of necessary DNA-protein interactions. However, we infer from the results that we report here that the regulatory complexes in the endogenous Cyllla gene are greatly stabilized relative to those of the exogenous Cyllla · CAT genes.
Restricted spatial and temporal expression of G-protein α subunits during Drosophila embryogenesis
Authors: Wolfgang William J. , Quan Franklin , Thambi Nina , Forte Michael
Published: April 2021

Of the known signal transduction mechanisms, the most evolutionarily ancient is mediated by a family of heterotrimeric guanine nucleotide binding proteins or G proteins. In simple organisms, this form of sensory transduction is used exclusively to convey signals of developmental consequence. In metazoan organisms, however, the developmental role of G-protein-coupled sensory transduction has been more difficult to elucidate because of the wide variety of signals (peptides, small molecules, odorants, hormones, etc.) that use this form of sensory transduction. We have begun to examine the role of G-protein-coupled signaling during development by investigating the expression during Drosophila embryogenesis of a limited set of G proteins. Since these proteins are a common component of all G-protein-coupled signaling systems, their developmental pattern of expression should indicate when and where programmed changes in gene activity are initiated by, or involve the participation of, G-protein-coupled signaling events. We have focused on the spatial and temporal expression pattern of three different Drosophila G-protein α subunits by northern blot analysis, in situ hybridization and immunocytochemistry using antibodies directed to peptides specifically found in each α subunit. From the spatial and temporal restriction of the expression of each protein, our results suggest that different forms of G-protein-coupled sensory transduction may mediate developmental interactions during both early and late stages of embryogenesis and may participate in a variety of specific developmental processes such as the establishment of embryonic position, the ontogeny of the nervous system and organogenesis.
The origin of reactive oxygen species in mouse embryos cultured in vitro
Authors: Nasr-Esfahani M. Mohammad , Johnson Martin H.
Published: April 2021

The increase in production of reactive oxygen species such as H2O2 at the G2/M phase of the second cell cycle may be related to the in vitro block to development of mouse 2-cell embryos. The occurrence of the H2O2 rise is independent of the activation of the embryonic genome and of passage through the S, G2 and M phases of the first cell cycle and G1 and M phases of the second cell cycle, but does require the activation of the unfertilized oocyte. The H2O2 is produced via dismutation of superoxide by the enzyme superoxide dismutase. Production of superoxide via mitochondrial, NADPH-oxidase and xanthine/xanthine oxidase systems has been investigated. The evidence suggests that superoxide, and thereby H2O2, is produced by the xanthine/xanthine oxidase system, but an involvement of the other superoxide generating systems has not been excluded. The relation between H2O2 and development in vitro is discussed.
Hox-7 expression during murine craniofacial development
Authors: Mackenzie Alasdair , Ferguson Mark W. J. , Sharpe Paul T.
Published: April 2021

We have used in situ hybridisation to establish the temporal and spatial expression patterns of the mouse homeobox-containing gene; Hox-7, in the developing embryonic cranium and nervous system of the mouse between embryonic days 9.5 (E9.5) and E15.5. Hox-7 has previously been associated with areas of mesenchymal – epithelial interaction and cell migration especially in neural crest ectomesenchymal cells. Aside from the expression patterns seen in the facial anlage at E9.5, Hox-7 transcripts were also detected in the neuroepithelium including cells of the dorsal midline of the neural tube. This expression pattern persisted throughout the embryonic time span studied. At E11.5, expression of Hox-7 became obvious in the neuroepithelium of the forming tela choroida and the telencephelii in areas destined to form the choroid plexus before any atrophy of the neuroepithelium took place. High expression of Hox-7 was also present in the mesenchyme cells invading the pouch formed by the involuting choroid plexus neuroepithelium. A second major site where Hox-7 was expressed was the anlage of the anterior pituitary; the Rathke’s pouch. Expression became obvious at E10.5 throughout the pouch but by E12.5 became more regionalised in areas of the pouch destined to form the pars distalis. Hox-7 was also expressed in the forming meninges and skull bone precursors from E10.5 onwards. Expression of the Hox-7 gene is also seen in the external ear, the forming eye, the nasal pits and forming Jacobson’s organs.

When these expression patterns are considered together with characterised human and mouse retinoic acid embryopathies and the congenital malformations seen in human children associated with deletion of chromosome 4pl6.1 (Wolf-Hirschhorn syndrome), Hox-7 may be a good candidate as one of the genes involved in the initiation of the choroid plexus phenotype and its subsequent formation, the formation of the outer ear, formation of the dentition and the differentiation of the cell types of the anterior pituitary. The expression pattern of Hox-7 in the dorsal midline of the neural tube further suggests that it may also be involved in the specification of the dorsal-ventral axis of the developing nervous system.
Perturbation of neuronal differentiation and axon guidance in the spinal cord of mouse embryos lacking a floor plate: analysis of Danforth’s shorttail mutation
Authors: Bovolenta Paola , Dodd Jane
Published: April 2021

The floor plate of the vertebrate nervous system has been implicated in the guidance of commissural axons at the ventral midline. Experiments in chick have also suggested that at earlier stages of development the floor plate induces the differentiation of motor neurons and other neurons of the ventral spinal cord. Here we have examined the development of the spinal cord in a mouse mutant, Danforth’s short-tail, in which the floor plate is absent from caudal regions of the neuraxis. In affected regions of the spinal cord, commissural axons exhibited aberrant projection patterns as they reached and crossed the ventral midline. In addition, motor neurons were absent or markedly reduced in number in regions of the spinal cord lacking a floor plate. Our results suggest that the floor plate is indeed an intermediate target in the projection of commissural axons and support the idea that several different mechanisms operate in concert in the guidance of axons to their cellular targets in the developing nervous system. In addition, these experiments suggest that the mechanisms that govern the differentiation of the floor plate and other ventral cell types in the neural tube are common to mammals and lower vertebrates.
Early ablation of target muscles modulates the arborisation pattern of an identified embryonic Drosophila motor axon
Authors: Sink Helen , Whitington Paul M.
Published: April 2021

The Drosophila RP3 motor axon establishes a stereotypic arborisation along the adjoining edges of muscles 6 and 7 by the end of embryogenesis. The present study has examined the role of the target muscles in determining this axonal arborisation pattern. Target muscles were surgically ablated prior to the arrival of the RP3 axon. Following further development of the embryo in culture medium, the morphology of target-deprived RP3 motor axons was assayed by intracellular injection with the dye Lucifer Yellow. Axonal arborisations were formed on a variety of non-target muscles when muscles 6 and 7 were removed and these contacts were maintained into stage 16. The pattern of axonal arborisations over non-target muscles varied between preparations in terms of the number of muscles contacted, and the distribution of arborisations on individual muscles. Following removal of muscle 6, the RP3 motor axon frequently contacted muscle 7, and axonal arborisations were present along the distal edge of the muscle. In the absence of muscle 7, the RP3 axon often did not contact muscle 6 and when muscle 6 was contacted, the arborisation of RP3 was poorly developed. Axonal processes were retained on non-target muscles when only one target muscle was present. Therefore, the establishment of a stereotypic arborisation by the RP3 motor axon is apparently dependent on growth cone contact with both target muscles.
Control of expression of the homeotic labial (lab) locus of Drosophila melanogaster. evidence for both positive and negative autogenous regulation
Authors: Chouinard Scott , Kaufman Thomas C.
Published: April 2021

The homeotic gene labial (lab) is required for proper development of the embryonic and adult head in Drosophila melanogaster. The lab gene product accumulates in a complex pattern in both embryonic and imaginal tissue. During embryogenesis, lab is expressed in the endodermally derived cells of the midgut, in ectodermally derived cells of the procephalon and dorsal ridge, and in a small subset of progenitor sensory cells. imaginal expression is restricted to a narrow region of the peripodial membrane of the eye-antennal disc. As part of our continuing effort to understand the role of lab in development, we have begun a dissection of the regulatory elements of the lab transcription unit and used germ line transformation experiments to determine which aspects of the observed expression pattern are essential for proper head development and viability. Transgenic embryos harboring an abridged lab gene are able to overcome the embryonic lethality associated with the loss of lab function and survive to adulthood. Interestingly, in these transgenic lines the lab protein accumulates only in a subset of those embryonic cells that normally express the gene, namely the procephalon and the anterior midgut. We also find that, once initiated, lab expression is maintained by positive autoregulation. Although lab minigene activity is sufficient to rescue the embryonic lethality of lab mutations, the transgenes fail to rescue defects in the adult head capsule. However, the defects observed in this study encompass a broader domain than those seen using somatic recombination to generate lab−1 clonal tissue. The failed rescue and observed cuticular defects are, at least in part, explained by the observation that the transgenes, rather than failing to be expressed, are associated with ectopic accumulation of lab protein in the peripodial membrane of the antennal disc. Moreover, this aberrant expression pattern is correlated with the abnormal expression of two other homeotic genes, Deformed (Dfd) and Sex combs reduced (Scr) in the eye-antennal disc. These results are only observed when the transgene is resident in a lab− genotype and ectopic expression of lab and misregulation of Dfd and Scr are not seen in a lab+ background. This result suggests that the wild-type lab gene product is necessary for the normal regulation of the locus in the imaginal discs, but unlike the case in the embryo, the event is negative. We discuss the biological implications of these results in relation to the role of lab in development.
Hensen’s node induces neural tissue in Xenopus ectoderm. Implications for the action of the organizer in neural induction
Authors: Kintner C. R. , Dodd J.
Published: April 2021

The development of the vertebrate nervous system is initiated in amphibia by inductive interactions between ectoderm and a region of the embryo called the organizer. The organizer tissue in the dorsal lip of the blastopore of Xenopus and Hensen’s node in chick embryos have similar neural inducing properties when transplanted into ectopic sites in their respective embryos. To begin to determine the nature of the inducing signals of the organizer and whether they are conserved across species we have examined the ability of Hensen’s node to induce neural tissue in Xenopus ectoderm. We show that Hensen’s node induces large amounts of neural tissue in Xenopus ectoderm. Neural induction proceeds in the absence of mesodermal differentiation and is accompanied by tissue movements which may reflect notoplate induction. The competence of the ectoderm to respond to Hensen’s node extends much later in development than that to activin-A or to induction by vegetal cells, and parallels the extended competence to neural induction by axial mesoderm. The actions of activin-A and Hensen’s node are further distinguished by their effects on lithium-treated ectoderm. These results suggest that neural induction can occur efficiently in response to inducing signals from organizer tissue arrested at a stage prior to gastrulation, and that such early interactions in the blastula may be an important component of neural induction in vertebrate embryos.
Determination of the embryonic axes of Drosophila*
Authors: Nüsslein-Volhard Christiane
Published: April 2021

The principles of embryonic pattern formation have been studied extensively in many systems using classical experimental approaches. In Drosophila, a powerful combination of genetics and transplantation experiments, as well as molecular biology, have helped to elucidate the mechanisms that operate during oogenesis and early embryogenesis to establish a set of positional cues required for axis determination in the early embryo.

In systematic searches for maternal effect mutations a small number of about 30 genes have been identified that specifically affect the process of determination of the embryonic axes. These ‘coordinate’ genes define four systems that determine the anteroposterior (AP) axis (three systems) and the dorsoventral (DV) axis (one system) independently. In the anteroposterior axis, the anterior system determines the segmented region of head and thorax, the posterior system determines the segmented abdominal region, and the terminal system is responsible for the formation of the nonsegmented termini at the anterior and posterior egg tips, the acron and telson. In contrast, pattern along the dorsoventral axis is determined by one system only. Although all four systems use different biochemical mechanisms, they share several properties. (1) The product of one gene in each system is localized in a specific region of the freshly laid egg and functions as a spatial signal. (2) In each system, this spatial information finally results in the asymmetrical distribution of one gene product that functions as a transcription factor. (3) This transcription factor is distributed in a concentration gradient that defines the spatial limits of expression of one or more zygotic target genes.

The combined action of these three anteroposterior systems as well as the dorsoventral system defines the expression of zygotic target genes in at least seven distinct regions along the anteroposterior and at least three in the dorsoventral axis. These longitudinal and transverse domains provide a coarse spatial prepattern which is then further refined by the action and interaction of zygotic pattern genes.
A genetic and molecular model for flower development in Arabidopsis thaliana
Authors: Meyerowitz Elliot M. , Bowman John L. , Brockman Laura L. , Drews Gary N. , Jack Thomas , Sieburth Leslie E. , Weigel Detlef
Published: April 2021

Cells in developing organisms do not only differentiate, they differentiate in defined patterns. A striking example is the differentiation of flowers, which in most plant families consist of four types of organs: sepals, petals, stamens and carpels, each composed of characteristic cell types. In the families of flowering plants in which these organs occur, they are patterned with the sepals in the outermost whorl or whorls of the flower, with the petals next closest to the center, the stamens even closer to the center, and the carpels central. in each species of flowering plant the disposition and number (or range of numbers) of these organs is also specified, and the floral ‘formula’ is repeated in each of the flowers on each individual plant of the species. We do not know how cells in developing plants determine their position, and in response to this determination differentiate to the cell types appropriate for that position. While there have been a number of speculative proposals for the mechanism of organ specification in flowers (Goethe, 1790; Goebel, 1900; Heslop-Harrison, 1964; Green,1988), recent genetic evidence is inconsistent with all of them, at least in the forms in which they were originally presented (Bowman et al. 1989; Meyerowitz et al. 1989). We describe here a preliminary model, based on experiments with Arabidopsis thaliana. The model is by and large consistent with existing evidence, and has predicted the results of a number of genetic and molecular experiments that have been recently performed.
How does the cytoskeleton read the laws of geometry in aligning the division plane of plant cells?
Authors: Lloyd Clive W.
Published: April 2021

Since Robert Hooke observed the froth-like texture of sectioned plant tissue, there have been numerous attempts to describe the geometrical properties of cells and to account for the patterns they form. Some aspects of biological patterning can be mimicked by compressed spheres and by liquid foams, implying that compression or surface tension are physical bases of patterning.

The 14-sided semi-regular tetrakaidecahedron encloses a given volume most efficiently and packs to fill space. However, observations of real plant tissue (and of soap bubbles) in the first half of this century established that plant cells only rarely form this mathematically ideal figure composed predominantly of 6-sided polygons. Instead, they tend to form a topologically transformed variant having mainly pentagonal faces although there is variability in the number of sides and the angles formed. But the one irreducible component of normal cell and tissue geometry is that only three edges meet at a point in a plane. In solid space, this gives rise to tetrahedral junctions and it is from this that certain limitations on sidedness flow. For three edges to meet at a point means that there must be an avoidance mechanism which prevents a new cell plate from aligning with an existing 3-way junction. Sinnott and Bloch (1940) saw that the cytoplasmic strands which precede the cell plate, predicted its alignment and also avoided 3-way junctions in unwounded tissues. Recently, F-actin and microtubules have been detected in these pre-mitotic, transvacuolar strands. The question considered here is why those cytoskeletal elements avoid aligning with the vertex where a neighbouring cross wall has already joined the mother wall. An hypothesis is discussed in which tensile strands - against a background of cortical re-organization during pre-mitosis - tend to seek the minimal path between nucleus and cortex. In this way, it is suggested that unstable strands are gradually drawn into a transvacuolar baffle (the phragmosome) within which cell division occurs. Vertices are avoided by the strands because they constitute unfavoured longer paths. The demonstrable tendency of tensile strands to contact mother walls perpendicularly would seem to account for Hofmeister’s and Sachs’ rules involving right-angled junctions. As others have discussed, such right-angled junctions give way to co-equal 120°angles between the three walls during subsequent cell growth. It is this asynchrony of cell division - where attachment of a cell plate causes the neighbouring wall to buckle - that forms a vertex to be avoided by subsequent pre-mitotic strands in that neighbouring cell. In this way, successive division planes would not co-align. It is therefore suggested that the exceptional formation of 4-way junctions in wounded tissue results from the fact that adjacent cells divide simultaneously; the lack of prebuckling of a common wall under these circumstances means that there is no vertex to be avoided by the minimal path mechanism.
Cell polarity and tissue patterning in plants
Authors: Sachs Tsvi
Published: April 2021

Cell polarization is the specialization of developmental events along one orientation or one direction. Such polarization must be an early, essential stage of tissue patterning. The specification of orientation could not occur only at the level of the genetic system and it must express a coordination of events in many cells. There is a positive feedback relation between cell polarization and the transport of the known hormone auxin: polarity determines oriented auxin transport while transport itself induces both new and continued polarization. Since cell polarization increases gradually, this feedback leads to the canalization of transport - and of the associated cell differentiation - along defined strands of specialized cells. Recent work has shown that the same canalized flow can also be an important determinant of cell shape. In primordial, embryonic regions cell growth is oriented along the flow of auxin from the shoot towards the root. In later developmental stages the cells respond to the same flow by growing in girth, presumably adjusting the capacity of the tissues to the flow of signals. Finally, disrupted flow near wounds results in the development of relatively unorganized callus. Continued callus development appears to require the participation of the cells, as sources and sinks of auxin and other signals. The overall picture to emerge suggests that cell patterning can result from competition between cells acting as preferred channels, sources and sinks for developmental signals.
Development of the spatial pattern of retinoic acid receptor-β transcripts in embryonic chick facial primordia
Authors: Rowe Annie , Richman Joy M. , Brickell Paul M.
Published: April 2021

Retinoic acid causes a range of embryonic defects, including craniofacial abnormalities, in both birds and mammals and is believed to have a number of roles in normal development. We have previously shown that the distribution of retinoic acid receptor-β (RAR-β transcripts is spatially restricted within the neural-crestderived upper beak primordia of the chick embryo. We have now used in situ hybridisation to trace the distribution of RAR-β transcripts during the migration of cranial neural crest cells and during formation of these primordia.

RAR-β transcripts were present in a subset of migrating neural-crest-derived cells in the head of the stage 10 embryo. These cells were situated in pathways followed by cells that migrate from the neural crest overlying the posterior prosencephalic/anterior mesencephalic region of the developing brain. Cells containing RAR-β transcripts accumulated around the developing eyes and in the regions of the ventral head from which the upper beak primordia later develop. We mapped the distribution of RAR-β transcripts as the facial primordia were forming, with particular reference to the development of the maxillary primordia. We found that these form in a region of the ventral head that includes the boundary between regions of high and low levels of RAR-β transcripts. The boundary between these two groups of cells persisted as the maxillary primordia developed.

The restriction of RAR-β transcripts to a subset of migrating neural crest cells which arise from a specific region of the neural crest, and which give rise to precisely distributed populations of cells, provides further evidence that there is some form of prepatterning in the neural crest from which the facial primordia originate.
c-ros: the vertebrate homolog of the sevenless tyrosine kinase receptor is tightly regulated during organogenesis in mouse embryonic development*
Authors: Tessarollo Lino , Nagarajan Lalitha , Parada Luis F.
Published: April 2021

The c-ros proto-oncogene is the vertebrate homologue of the Drosophila sevenless tyrosine kinase receptor. Examination of c-ros mRNA transcripts in the mouse embryo reveals a stringent pattern of expression. Only kidney, intestine and lung exhibit ros-specific RNA using sensitive techniques such as RNAse protection and in situ hybridization. The temporal and spatial arrangement of c-ros transcripts is coincident with the phenotypic induction and proliferation of epithelium during organogenesis of the kidney and intestine. The data provide evidence for a role of c-ros in the obligate cell-cell interactions that characterize the morphogenic induction and proliferation of epithelial cells in the kidney, intestine and lung. The c-ros tyrosine kinase receptor may provide a signal transduction pathway for epithelial-mesenchymal interactions.
The prion protein gene: a role in mouse embryogenesis?
Authors: Manson J. , West J. D. , Thomson V. , Mcbride P. , Kaufman M. H. , Hope J.
Published: April 2021

The neural membrane glycoprotein PrP (prion protein) has a key role in the development of scrapie and related neurodegenerative diseases. During pathogenesis, PrP accumulates in and around cells of the brain from which it can be isolated in a disease-specific, protease-resistant form. Although the involvement of PrP in the pathology of these diseases has long been known, the normal function of PrP remains unknown. Previous studies have shown that the PrP gene is expressed tissue specifically in adult animals, the highest levels in the brain, with intermediate levels in heart and lung and low levels in spleen. Prenatally, PrP mRNA has been detected in the brain of rat and hamster just prior to birth. In this study we have examined the expression of the PrP gene during mouse embryonic development by in situ hybridization and observed dramatic regional and temporal gene expression in the embryo. Transcripts were detected in developing brain and spinal cord by 13.5 days. In addition, PrP gene expression was detected in the peripheral nervous system, in ganglia and nerve trunks of the sympathetic nervous system and neural cell populations of sensory organs. Expression of the PrP gene was not limited to neuronal cells, but was also detected in specific non-neuronal cell populations of the 13.5 and 16.5 day embryos and in extra-embryonic tissues from 6.5 days. This cell-specific expression suggests a pleiotropic role for PrP during development.
Regulation and role of PDGF receptor a-subunit expression during embryogenesis
Authors: Schatteman Gina C. , Morrison-Graham Kathleen , Koppen Antje Van , Weston James A. , Bowen-Pope Daniel F.
Published: April 2021

The platelet-derived growth factor receptor a-subunit (PDGFR α) is the form of the PDGF receptor that is required for binding of PDGF A-chain. Expression of PDGFR α within the early embryo is first detected as the mesoderm forms, and remains characteristic of many mesodermal derivatives during later development. By 9.5 days of development, embryos homozygous for the Patch mutation (a deletion of the PDGFRα) display obvious growth retardation and deficiencies in mesodermal structures, resulting in the death of more than half of these embryos. Mutant embryos that survive this first critical period are viable until a new set of defects become apparent in most connective tissues. For example, the skin is missing the dermis and connective tissue components are reduced in many organs. By this stage, expression of PDGFRα mRNA is also found in neural crest-derived mesenchyme, and late embryonic defects are associated with both mesodermal and neural crest derivatives. Except for the neural crest, the lens and choroid plexus, PDGFRα mRNA is not detected in ectodermal derivatives until late in development in the central nervous system. Expression is not detected in any embryonic endodermal derivative at any stage of development. These results demonstrate that PDGFRα is differentially expressed during development and that this expression is necessary for the development of specific tissues.
Histone H1 subtype synthesis in neurons and neuroblasts
Authors: Dominguez Valentin , Pina Benjamin , Suau Pedro
Published: April 2021

Rat cerebral cortex neurons contain the five histone HI subtypes Hla-e and the subtype HIO present in other mammalian somatic tissues. The four subtypes Hla-d decay exponentially during postnatal development and are partially or totally replaced by Hle that becomes the major Bl subtype in adults. Hl0 accumulates in a period restricted to neuronal terminal differentiation. Here we study the synthesis of the Bl subtypes in cortical neurons and their neuroblasts by in vivo labeling with [14C]lysine. The subtype synthesis pattern of neuroblasts has been determined by labeling gravid rats during the period of proliferation of cortical neurons and synthesis in neurons has been studied by postnatal labeling. The subtype Hla is synthesized in neuroblasts but not in neurons and is therefore rapidly removed from neuronal chromatin. The synthesis of Hlb and Hld is much lower in neurons than in neuroblasts so that these subtypes are replaced to a large extent during postnatal development. Ole is synthesized at levels much higher than the other subtypes both in neurons and neuroblasts, but its very high turnover, about one order of magnitude faster than that of Hle in neurons, favors its partial replacement during postnatal development. Comparison of the synthesis rates of Hl0 in newborn and 30-day-old rats shows that the accumulation of HIO in differentiating neurons is due to an increased level of synthesis.
Control of proliferation in the retina: temporal changes in responsiveness to FGF and TGFα
Authors: Lillien Laura , Cepko Connie
Published: April 2021

Proliferation in the rat retina, as in other parts of the nervous system, occurs during a restricted period of development. In addition to regulating cell number, the mechanisms that control proliferation influence the patterning of tissues, and may affect the determination of cell type. To begin to determine how proliferation is controlled, several growth factors found in the retina were tested for effects on progenitor cell division in culture. Proliferation was enhanced by TGFα, bFGF and aFGF, and many of the dividing cells later differentiated into cells with the antigenic phenotypes of retinal neurons and glial cells. The mitotic response of retinal cells to these factors changed during development: progenitor cells from younger retinas (embryonic day 15 to 18; E15-E18) were more responsive to FGF’s, while progenitor cells from older retinas (>E20) were more responsive to TGFα. Progenitor cells stopped dividing in vitro, even when treated with excess mitogen. These observations suggest that proliferation in the retina may be stimulated by multiple mitogenic signals provided by TGFα, FGF, or related factors, and that proliferation is not controlled by limiting concentrations of mitogen alone. Rather, these data demonstrate that retinal cells change during development in their responsiveness to mitogenic signals. Such changes may contribute to the regulation of proliferation.
DVR-4 (Bone Morphogenetic Protein-4) as a posterior-ventralizing factor in Xenopus mesoderm induction
Authors: Jones C. Michael , Lyons Karen M. , An Peter M. Lap , Wright Christopher V.E. , Hogan Brigid L.M.
Published: April 2021

Establishment of mesodermal tissues in the amphibian body involves a series of inductive interactions probably elicited by a variety of peptide growth factors. Results reported here suggest that mesodermal patterning involves an array of signalling molecules including DVR-4, a TGF-β-like molecule. We show that ectopic expression of DVR-4 causes embryos to develop with an overall posterior and/or ventral character, and that DVR-4 induces ventral types of mesoderm in animal cap explants. Moreover, DVR-4 overrides the dorsalizing effects of activin. DVR-4 is therefore the first molecule reported both to induce posteroventral mesoderm and to counteract dorsalizing signals such as activin. Possible interactions between these molecules resulting in establishment of the embryonic body plan are discussed.
Developmental expression of the Xenopus int-2 (FGF-3) gene: activation by mesodermal and neural induction
Authors: Tannahill D. , Isaacs H. V. , Close M. J. , Peters G. , Slack J. M. W.
Published: April 2021

We have used a probe specific for the Xenopus homologue of the mammalian proto-oncogene int-2 (FGF-3) to examine the temporal and spatial expression pattern of the gene during Xenopus development. int-2 is expressed from just before the onset of gastrulation through to prelarval stages. In the early gastrula, it is expressed around the blastopore lip. This is maintained in the posterior third of the prospective mesoderm and neuroectoderm in the neurula. A second expression domain in the anterior third of the neuroectoderm alone appears in the late gastrula, which later resolves into the optic vesicles, hypothalamus and midbrain-hindbrain junction region. Further domains of expression arise in tailbud to prelarval embryos, including the stomodeal mesenchyme, the endoderm of the pharyngeal pouches and the cranial ganglia flanking the otocyst.

It is shown, by treatment of blastula ectoderm with bFGF and activin, that int-2 can be expressed in response to mesoderm induction. By heterotypic grafting of gastrula ectoderm into axolotl neural plate, we have also demonstrated that int-2 can be expressed in response to neural induction.

These results suggest that int-2 has multiple functions in development, including an early role in patterning of the anteroposterior body axis and a later role in the development of the tail, brain-derived structures and other epithelia.
Developmental regulation of villin gene expression in the epithelial cell lineages of mouse digestive and urogenital tracts
Authors: Maunoury Roger , Robine Sylvie , Pringault Eric , Léonard Nadine , Gaillard Jean Alfred , Louvard Daniel
Published: April 2021

The expression of villin, an actin-binding protein and major structural component of the brush border of specialized absorptive cells, was studied during mouse embryogenesis. We show that the ontogeny of villin expression is limited to the epithelial cell lineages of the digestive and uro-genital tracts and accounts for the tissue-specific expression observed in adult mice. This spatiotemporal pattern of villin expression is distinctive in sequence, intensity, regional distribution and polarization. During the development of the primitive gut, villin is faintly and discontinuously expressed in the invaginating foregut but it is expressed in every cell bordering the hindgut pocket. Later, villin expression increases along the developing intestine and concentrates in the brush border of the epithelium bordering the villi. In gut derivatives, villin is present in liver and pancreas primordia but only biliary and pancreatic cells maintain a faint villin expression as observed in adults. In the urogenital tract, mesonephric tubules are the first mesodermal derived structures to express villin. This expression is maintained in the ductuli efferentes, paradidymis and epoöphoron. Villin then appears in the proximal metanephric tubules and later increases and concentrates in the brush border of the renal proximal tubular epithelial cells. Thus villin expression can be considered as an early marker of the endodermal cell lineage during the development of the digestive system. Conversely, during the development of the excretory and genital system, villin is only expressed after the mesenchyme/epithelium conversion following the appearance of tubular structures.

These observations emphasize the multiple levels of regulation of villin gene activity that occur during mouse embryogenesis and account for the strict pattern of tissue-specific expression observed in adults. In the future, regulatory elements of the villin gene may be used to target the early expression of oncogenes to the digestive and urogenital tracts of transgenic mice.
The torpedo (DER) receptor tyrosine kinase is required at multiple times during Drosophila embryogenesis
Authors: Clifford Robert , Schüpbach Trudi
Published: April 2021

The torpedo (DER) gene of Drosophila, which encodes a receptor tyrosine kinase of the EGF receptor subfamily, is essential for oogenesis, embryogenesis and imaginal disc development. To gain insight into the nature of the signals transduced by the torpedo product, we have characterized the gene’s loss-of-function phenotype in the embryo. Through the induction of germline clones, we provide a genetic demonstration that maternal torpedo product does not contribute to zygotic development. Thus, the embryonic lethal phenotypes examined accurately reflect the consequences of eliminating all gene activity from the zygote.

Temperature-shift experiments with the conditional allele top1F26 show that torpedo is required at two distinct times during embryonic development: the gene is first needed for germband retraction and for the production of anterior, posterior and ventral cuticle, then later for the secretion of ventral denticles. Since denticle formation can be severely disrupted in top1F26 animals without affecting cuticle production, the early and late requirements for torpedo appear to be functionally unrelated.

torpedo, therefore, is required at multiple times in the development of the ventral epidermis, and may trans duce qualitatively different signals. Since the early requirement for torpedo correlates with the first visible defect in embryonic development, increased cell death in the amnioserosa, cephalic ectoderm and ventral epider mis, the abnormalities in cuticle production and germ band shortening seen in the mutant may be secondary consequences of a primary defect in cell viability. Given that the onset of cell death in torpedo embryos is not preceded by any obvious defects in mitogenesis, the establishment of cell identities or the maintenance of gene expression, it is possible that torpedo transduces a signal necessary for cell survival per se during early embryogenesis. During late embryogenesis, torpedo may mediate the reception of a second signal which regulates ventral epidermal cell differentiation.
MHox: a mesodermally restricted homeodomain protein that binds an essential site in the muscle creatine kinase enhancer
Authors: Cserjesi Peter , Lilly Brenda , Bryson Laura , Wang Yaoqi , Sassoon David A. , Olson Eric N.
Published: April 2021

Myogenic helix-loop-helix (HLH) proteins, such as myo-genin and MyoD, can activate muscle-specific transcription when introduced into a variety of nonmuscle cell types. Whereas cells of mesodermal origin are especially permissive to the actions of these myogenic regulators, many other cell types are refractory to myogenic conversion by them. Here we describe a novel home-odomain protein, MHox, that binds an A+T-rich element in the muscle creatine kinase (MCK) enhancer that is essential for muscle-specific transcription and trans-activation by myogenic HLH proteins. MHox is completely restricted to mesodermally derived cell types during embryogenesis and to established cell lines of mesodermal origin. In contrast to most other homeobox genes, MHox expression is excluded from the nervous system, with the highest levels observed in limb bud and visceral arches. In adult mice, MHox is expressed at high levels in skeletal muscle, heart and uterus. The DNA-binding properties and pattern of MHox expression are unique among homeobox genes and suggest a role for MHox as a transcriptional regulator that participates in the establishment of diverse mesodermal cell types.
TRP-2/DT, a new early melanoblast marker, shows that steel growth factor (c-kit ligand) is a survival factor
Authors: Steel Karen P. , Davidson Duncan R. , Jackson Ian J.
Published: April 2021

We have used a probe derived from TRP-2/DT to detect migratory melanoblasts shortly after they emerge from the neural crest, as early as 10 days post coitum (dpc). TRP-2/DT expression is otherwise restricted to the pre sumptive pigmented retinal epithelium, the developing telencephalon and the endolymphatic duct. The pattern of steel and c-kit hybridisation in the developing brain differed from that of TRP-2. TRP-1 and tyrosinase probes also detected melanoblasts but were both expressed later in development than TRP-2. We used the TRP-2/DT probe to investigate the way that the Steel-dickie (Sld) mutation interferes with melanocyte development, and found that the membrane-bound steel growth factor which is missing in Sld/Sld mutants is necessary for the survival of melanoblasts but not for their early migration and initial differentiation.