bluuebunny/crossref_metadata_2025 huggingface dataset:
Developmental expression of the syndecans: possible function and regulation
Authors: Bernfield Merton , Hinkes Michael T. , Gallo Richard L.
Published: April 2021

Recent work has made clear that heparan sulfate at the cell surface is essential for a wide variety of interactions of cells with their microenvironment, including the action of growth factors, extracellular matrix, proteases and protease inhibitors. A major source of this cell surface heparan sulfate is a multigene family of proteoglycans, the syndecans, that are expressed developmentally in association with changes in tissue organization and morphology and induced during wound repair. In this review, we describe mechanisms underlying the differential expression of the syndecans, focusing on syndecan-1. The induction of syndecan-1 can result from soluble extracellular factor(s) acting at multiple levels of cellular regulation. At the transcriptional level, the promoter of the murine syndecan-1 gene contains potential recognition sites for several well-known regulatory genes, including Hox and MyoD family members. Because changes in syndecan expression enable cells to become more or less responsive to their microenvironment, understanding these regulatory mechanisms can lead to an improved understanding of how cellular behavior is controlled during development and wound repair.
crumbs and stardust, two genes of Drosophila required for the development of epithelial cell polarity
Authors: Knust Elisabeth , Tepaß Ulrich , Wodarz Andreas
Published: April 2021

Loss-of-function mutations in the Drosophila genes crumbs and stardust are embryonic lethal and cause a breakdown of ectodermally derived epithelia during organogenesis, leading to formation of irregular cell clusters and extensive cell death in some epithelia. The mutant phenotype develops gradually and affects the various epithelia to different extents, crumbs encodes a large transmembrane protein with 30 EGF-like repeats and four laminin A G-domain-like repeats in its extra- cellular domain, suggesting its participation in protein- protein interactions. The CRUMBS protein is exclusively expressed on the apical membrane of all ectodermally derived epithelia, the tissues affected in crumbs and stardust mutant embryos. The gene function is com- pletely abolished by a crumbs mutation that causes pro- duction of a protein with a truncated cytoplasmic domain. Instead of being apically localized as in wild- type, the mutant CRUMBS protein is diffusely distributed in the cytoplasm; this occurs before any morphologically detectable cellular phenotype is visible, suggesting that targeting of proteins is affected in crumbs mutant embryos. Later, the protein can be detected on the apical and basolateral membranes. Mutations in stardust produce a phenotype nearly identical to that associated with crumbs mutations, suggesting that both genes are functionally related. Double imitant combinations and gene dosage studies suggest that both genes are part of a common genetic pathway, in which stardust acts down- stream of crumbs.
Dissecting Wnt signalling pathways and Wnf-sensitive developmental processes through transient misexpression analyses in embryos of Xenopus laevis
Authors: Moon Randall T. , Christian Jan L. , Campbell Robert M. , McGrew L. Lynn , DeMarais Alyce A. , Torres Monica , Lai Cheng-Jung , Olson Daniel J. , Kelly Gregory M.
Published: April 2021

We review evidence that Xenopus Wnts (Xwnts) have activities consistent with their hypothesized roles as secreted signalling factors involved in multiple developmental processes. Transient misexpression of different Xwnts has distinct effects upon early development, and upon the formation of tissues in UV-irradiated embryos. Misexpression of Xwnts also has distinct effects on the in vitro differentiation of blastula cap expiants. Cellular responses to Xwnt signals include changes in gap junctional permeability, altered responsiveness to growth factors, and possibly changes in cell adhesion. Current data suggest that a maternal Xwnt- or noggin-like activity is involved in the Nieuwkoop center activity during mesoderm induction, that Xwnt-8 participates in a pathway of differentiation as ventral mesoderm, and that Xwnt-5A is a potential modulator of morphogenetic movements.
Analysis of the formation of the animal pole plasm in the eggs of Limnaea stagnalis
Authors: Raven Chr. P. , Wal U. P. Van Der
Published: April 2021

The animal pole plasm in the eggs of Limnaea stagnalis becomes visible about 1 hr. before the first cleavage. At this time the male pronucleus is on its way toward the animal pole, while the female pronucleus is being formed by the fusion of karyomeres resulting from the swelling of the egg chromosomes after the completion of the second maturation division.
Periodic sensitivity of mechanisms of cytodifferentiation in cleaving eggs of Limnaea stagnalis
Authors: Geilenkirchen W. L. M.
Published: April 2021

Eggs of Limnaea stagnalis, of ages differing by 15-min. intervals between oviposition and the third cleavage division, have been centrifuged at 640 g. for 10 min. Centrifugation before first cleavage did not have any effect on development. Centrifugation at first, second and third cleavage caused a strong morphogenetic effect. Centrifugation during prophase of second and third division caused delay of cleavage. The mitotic figure is disrupted. It is suggested that the morphogenetic effect of centrifugation can be ascribed to a disturbance of the normal distribution of morphogenetic ‘substance’ at or in the cell surface, by random insertion or resorption of membrane after stretching the cell.
The effect of plucking hairs during different phases of the follicular cycle
Authors: Johnson Elizabeth , Ebling F. J.
Published: April 2021

Hair growth is intermittent; periods of activity when the follicle is producing the hair alternating with periods of rest when the dead hair is retained. In the rat, hair growth occurs in a series of waves which start ventrally and pass over the flanks to the back (Dry, 1926; Butcher, 1934; Johnson, 1958).
Lactate dehydrogenase isoenzymes in foetal and neonatal tissues
Authors: Latner A. L. , Skillen A. W.
Published: April 2021

The Existence of isoenzymes, one class of the multiple molecular forms of enzymes, is now well recognized. Isoenzyme distribution patterns, particularly those of lactate dehydrogenase (LDH), have been extensively studied and are often tissue and species specific (Markert & Møller, 1959; Wieland, Pfleiderer, Haupt & Wörner, 1959; Plagemann, Gregory & Wroblewski, 1960; Tsao, 1960). Various techniques, such as zone electrophoresis and column chromatography, have been used to separate isoenzymes of lactate dehydrogenase. Using starch-gel electrophoresis combined with visual demonstration of the isoenzyme patterns, we have detected the usual five zones of activity in human tissues (Latner & Skillen, 1961). They are termed LD1 LD2, LD3, LD4 and LD5 where, according to the terminology we use, the latter is the isoenzyme which migrates furthest towards the anode and is most prominent in heart muscle.
Cell movements, cell division and growth in the hydroid Clytia johnstoni
Authors: Hale L. J.
Published: April 2021

In Clytia johnstoni, mitosis is almost exclusively a function of the ectoderm cells; only very rarely does an endoderm cell divide. Thus new endoderm cells must be produced by the ectoderm cells. Cell divisions may occur anywhere in the stolonic ectoderm except for approximately the first 0·3 mm. from a growing tip. During the development of a hydranth mitoses are rare except for a local wave of ectodermal cell division contributing to the formation of the tentacles and buccal region. Growth only takes place in the first 6–12 mm. of a stolon. The ectoderm cells in the first 2–3 mm. move towards the tip; those on the ‘top’ of a stolon (i.e. away from the substratum) are concerned in the initiation of hydranths and those at the sides and bottom with stolon growth. In the remainder of this region ectoderm cells on the top of the stolon migrate into and form the ectoderm of the hydranths. The hydroplasm contains cells, mostly endoderm, which are derived from non-growing regions of the stolon and from regressing parts of the animal. During the ebb and flow of the hydroplasm, such cells are transported to, and adhere to the growing regions of stolons and to growing hydranths. The endoderm of hydranths is almost entirely supplied by this means. Cells differentiate into the various regions of the hydranth and stolon according to their positions relative to the growing tip.
Patterns of pigmentation in experimentally produced mouse chimaerae
Authors: Tarkowski Andrzej K.
Published: April 2021

Distribution of pigment-forming cells has been analysed in sixteen individuals (six embryos, nine newborn young, one adult animal) developed from fused eggs differing in genotype for pigmentation (pink eye dilution versus agouti). The following structures were examined: the outer layer of the retina (all individuals), choroid and Harderian gland and skin from various regions of the body (postnatal individuals). In the six embryos, pigment was revealed in their retinae but since the development of pigmentation was still in progress at that age the genotype of non-pigmented cells could not be identified. In seven out of the nine newborn young the outer layer of the retina was a mosaic with pigmented and non-pigmented cells intermingled. In the two other animals the outer layer of the retina was homogenous, being of’pink’ type in one animal and of ‘agouti’ type in the other. In the adult animal the outer layer of the retina of one eye seems to be composed of two types of cells. The difference of pigmentation between them is quantitative only, the colour and shape of the granules being the same and typical for the agouti (hybrid) component. To postulate the chimaeric constitution it would be necessary to assume, therefore, that the cells derived from the LAB Grey component (pink-eyed) have been in some way induced by the neighbouring hybrid cells to synthesize pigment typical of the latter. In all newborn animals, except the one with non-pigmented eyes, melanocytes from the agouti component were detected in the choiroid, Harderian gland and the samples of skin examined. However, the presence of melanocytes of the other genotype could be neither proved nor rejected. On the basis of the composition of the retinae and the distribution of pigmented melanocytes in the skin it is inferred that the majority of animals developed from fused eggs are real chimaeras, at least as far as the neural derivatives are concerned. Since pigmentation does not reveal the genetic constitution of structures of different developmental history and derivation, the chimaeric character of animals with homogenous retinae cannot be excluded.
The lin-3/let-23 pathway mediates inductive signalling during male spicule development in Caenorhabditis elegans
Authors: Chamberlin Helen M. , Sternberg Paul W.
Published: April 2021

During Caenorhabditis elegans male spicule development, four pairs of precursor cells respond to multiple positional cues and establish a pattern of fates that correlates with relative anterior-posterior cell position. One of the extracellular cues is provided by the F and U cells, which promote anterior fates. We show that the genes in the lin-3/let-23 signalling pathway required for hermaphrodite vulval induction also mediate this F/U signal. Reduction-of-function mutations in lin-3, let-23, sem-5, let-60 or lin-45 disrupt the fate of anterior cells. Likewise, activation of the pathway with ubiquitously produced signal results in posterior cells inappropriately adopting the anterior fates even in the absence of F and U. We have further used this genetic pathway to begin to understand how multiple posi-tional cues are integrated to specify cell fate. We demon-strate that lin-15 acts in spicule development as it does in vulval induction, as a negative regulator of let-23 receptor activity. A second extracellular cue, from Y.p, also acts antagonistically to the lin-3/let-23 pathway. However, this signal is apparently integrated into the lin-3/let-23 pathway at some step after lin-45 raf and is thus functionally distinct from lin-15. We have also investigated the role of lin-12 in forming the anterior/posterior pattern of fates. A lin-12 gain-of-function defect is masked by redundant positional information from F and U.
Role of the teashirt gene in Drosophila midgut morphogenesis: secreted proteins mediate the action of homeotic genes
Authors: Mathies Laura D. , Kerridge Stephen , Scott Matthew P.
Published: April 2021

Homeotic genes control the development of embryonic structure by coordinating the activities of downstream ‘target’ genes. The identities and functions of target genes must be understood in order to learn how homeotic genes control morphogenesis. Drosophila midgut development is regulated by homeotic genes expressed in the visceral mesoderm, where two of their target genes have been identified. Both encode secreted proteins. The Ultrabithorax (Ubx) homeotic gene activates transcription of the decapentaplegic (dpp) gene, which encodes a TGFβ class protein, while in adjacent mesoderm cells the abdominal-A (abd-A) homeotic gene activates transcription of the wingless (wg) gene, which encodes a Wnt class protein. The homeotic genes Antennapedia (Antp) and Sex combs reduced (Scr) act in more anterior midgut regions. Here we report the identification of another homeotic gene target in the midgut mesoderm, the teashirt (tsh) gene, which encodes a protein with zinc finger motifs. tsh is necessary for proper formation of anterior and central midgut structures. Antp activates tsh in anterior midgut mesoderm. In the central midgut mesoderm Ubx, abd-A, dpp, and wg are required for proper tsh expression. The control of tsh by Ubx and abd-A, and probably also by Antp, is mediated by secreted signaling molecules. By responding to signals as well as localized transcription regulators, the tsh transcription factor is produced in a spatial pattern distinct from any of the homeotic genes.
Soma-germline asymmetry in the distributions of embryonic RNAs in Caenorhabditis elegans
Authors: Seydoux Geraldine , Fire Andrew
Published: April 2021

Early embryogenesis in Caenorhabditis elegans is characterized by a series of unequal cleavages that mark the stepwise separation of somatic and germ lineages. We have developed an in situ hybridization protocol to examine the localization of specific maternal and embryonically transcribed messenger RNAs during these early cleavages. We detected three classes of maternal RNAs: RNAs that are maintained in all cells, RNAs that are maintained in germline cells but are lost from somatic cells, and a population of RNAs that are associated with the germlinespecific P granules. We observed embryonically transcribed RNAs in somatic cells as early as the 4-cell stage. These transcripts were not detected in germline cells. These observations suggest that mechanisms which distinguish between soma and germline cause asymmetries in mRNA stability and transcription within the first few cleavages of C. elegans embryogenesis.
GLP-1 is localized to the mitotic region of the C. elegans germ line
Authors: Crittenden Sarah L. , Troemel Emily R. , Evans Thomas C. , Kimble Judith
Published: April 2021

In C. elegans, germline mitosis depends on induction by the somatic distal tip cell (DTC) and on activity of the glp-1 gene. Using antibodies to GLP-1 protein, we have examined GLP-1 on western blots and by immunocytochemistry. GLP-1 is tightly associated with membranes of mitotic germline cells, supporting its identification as an integral membrane protein. Furthermore, GLP-1 is localized within the germ line to the mitotic region, consistent with the model that GLP-1 acts as a membrane receptor for the distal tip cell signal. Unexpectedly, GLP-1 and the zone of mitosis extend further than the DTC processes. We present three models by which the DTC may influence GLP-1 activity and thereby determine the zone of mitosis. The spatial restriction of GLP-1 appears to be controlled at the translational level in hermaphrodites. We suggest that down-regulation of GLP-1 may be required to effect the transition from mitosis into meiosis.
RK2, a glial-specific homeodomain protein required for embryonic nerve cord condensation and viability in Drosophila
Authors: Campbell Gerard , Göring Harald , Lin Tina , Spana Eric , Andersson Siv , Doe Chris Q. , Tomlinson Andrew
Published: April 2021

We report the identification of RK2, a glial-specific home-odomain protein. RK2 is localized to the nucleus of virtually all embryonic and imaginal glial cells, with the exception of midline glia. Embryos mutant for the gene encoding RK2 are embryonic lethal but normal for early gliogenesis (birth, initial divisions and migration of glia) and axonogenesis (neuronal pathfinding and fasciculation). However, later in development, there are significantly fewer longitudinal glia that are spatially disorganized; in addition, there is a slight disorganization of axon fascicles and a defective nerve cord condensation. This suggests that RK2 is not required for early glial determination, but rather for aspects of glial differentiation or function that are required for embryonic viability.
Genesis of an organ: molecular analysis of the pha-1 gene
Authors: Granato Michael , Schnabel Heinke , Schnabel Ralf
Published: April 2021

The organisation of organ formation is still an unsolved problem. Mutations in the zygotic lethal gene pha-1 affect a late step during organ development in the nematode C. elegans. In mutant embryos all tissues in the pharynx fail to undergo terminal differentiation and morphogenesis. The expression of an early differentiation marker in pharyngeal muscle precursors is not impaired in mutant embryos, which suggests that pharynx cells still acquire their identity. Therefore the gene defines an organ-specific terminal differentiation function. We cloned and sequenced the pha-1 gene and found that the deduced protein sequence contains features characteristic of the bZIP family of transcription factors. During embryogenesis a transgenic pha-1 reporter construct is expressed transiently in all pharynx precursor cells at the time when these cells become restricted to form the pharynx organ. A mosaic analysis of the requirement of pha-1 activity during pharynx formation is consistent with the notion that pha-1 acts cell-autonomously in all cells of the pharynx primordium. The data suggest that pha-1 initiates and coordinates programs required for cytodifferentiation and morphogenesis in all cell types of the entire organ on the transcriptional level. We propose that organs are independent developmental units whose identity is reflected on the gene regulatory level.
Pituitary ontogeny of the Snell dwarf mouse reveals Pit-1-independent and Pit-1-dependent origins of the thyrotrope
Authors: Lin Sheng-Cai , Li Sen , Drolet Daniel W. , Rosenfeld Michael G.
Published: April 2021

The anterior pituitary provides a model to study the molecular mechanisms responsible for emergence of distinct cell types within an organ. Dwarf mice (Snell) that express a mutant form of the tissue-specific POU-domain transcription factor Pit-1 fail to generate three cell types, including the thyrotrope (S. Li, E. B. Crenshaw, E. J. Rawson, D. S. Simmons, L. Swanson and M. G. Rosenfeld (1990), Nature 347, 528-533). Analyses of wild-type and Pit-1-defective mice, presented here, have revealed that thyrotropes unexpectedly arise from two independent cell populations. The first population is Pit-1-independent and appears on e12 in the rostral tip of the developing gland, but phenotypically disappears by the day of birth. The second is Pit-1-dependent and arises subsequently in the caudomedial portion of the developing gland (e15.5), following the initial expression of Pit-1 in this region. The failure of caudomedial thyrotrope cells to appear in the Snell dwarf, and the observation that Pit-1 can bind to and transactivate the TSHβ promoter, apparently enhanced by its phosphorylation, suggests that Pit-1 is directly required for the appearance of this distinct population that serves as the precursors of the mature thyrotrope cell type. These data suggest that different molecular mechanisms, based on the actions of distinct transcription factors, can serve to independently generate a specific cell phenotype during mammalian organogenesis.
Novel tenascin variants with a distinctive pattern of expression in the avian embryo
Authors: Tucker R. P. , Spring J. , Baumgartner S. , Martin D. , Hagios C. , Poss P. M. , Chiquet-Ehrismann R.
Published: April 2021

Previous studies have shown that several forms of the glycoprotein tenascin are present in the embryonic extracellular matrix. These forms are the result of alternative splicing, which generates tenascin variants with different numbers of fibronectin type III repeats. We have used degenerate primers and PCR to isolate a novel tenascin exon from an avian genomic library. Genomic clones contained a sequence encoding a fibronectin type III repeat that corresponds to repeat ‘C’ from the variable domain of human tenascin. To demonstrate that tenascin containing repeat ‘C’ is actually synthesized by avian cells, a mono-specific antiserum was raised against a repeat ‘C’ fusion protein. This antiserum recognized a novel high-molecular-weight variant on immunoblots of tenascin isolated from chicken embryo fibroblast-conditioned medium, and stained tendons on frozen sections of chicken embryos. A cDNA probe specific for mRNA encoding repeat ‘C’ was used for in situ hybridization. This probe hybridized in a subset of the embryonic tissues labelled with a universal tenascin probe, including tendons, ligaments and mesenchyme at sites of epithelial-mesenchymal interactions. Finally, we provide evidence that additional fibronectin type III repeats, one corresponding to a recently discovered human repeat as well as one entirely novel sequence, also exists in chicken tenascin mRNA. These data indicate that tenascin is present in the embryonic matrix in a multitude of forms and that these forms have distinctive distributions that may reflect more than one function for tenascin in development.
Regulation of oligodendrocyte differentiation: a role for retinoic acid in the spinal cord
Authors: Noll Elizabeth , Miller Robert H.
Published: April 2021

During development, oligodendrocyte precursors undergo sequential stages of differentiation characterized by expression of distinct cell surface properties and proliferative responses. Although both PDGF and bFGF are mitogenic for these cells, the factors that regulate the progression of oligodendrocyte precursors through their differentiative program remain unclear.

One factor present in the embryonic spinal cord that may regulate differentiation of oligodendrocyte precursors is retinoic acid. Here we show that retinoic acid inhibits the maturation of embryonic spinal cord oligodendrocyte precursors in vitro at an early, highly motile stage of differentiation, characterized by the expression of A2B5 immunoreactivity. Basic FGF acts both as a mitogen and an inhibitor of spinal cord oligodendrocyte precursor maturation, but at a significantly later stage of differentiation, characterized by the expression of O4 immunoreactivity. In the presence of RA both the mitogenic and differentiation inhibiting effects of bFGF are abolished, consistent with RA acting as an early regulator of oligodendrocyte differentiation. During embryonic development, oligodendrocyte precursors arise initially from a distinct group of cells at the ventral ventricular zone of the spinal cord. Myelination of the entire spinal cord is dependent on the migration of immature precursor cells to peripheral developing white matter. Since the embryonic spinal cord has the capacity to release relatively high levels of retinoids, we propose that RA inhibits oligodendrocyte differentiation during early embryonic development permitting their dispersal throughout the entire spinal cord.
bundle sheath defective, a mutation that disrupts cellular differentiation in maize leaves
Authors: Langdale Jane A. , Kidner Catherine A.
Published: April 2021

Post-primordial differentiation events in developing maize leaves produce two photosynthetic cell types (bundle sheath and mesophyll) that are morphologically and biochemically distinct. We have isolated a mutation that disrupts the differentiation of one of these cell types in light-grown leaves. bundle sheath defective 1-mutable 1 (bsd1-m1) is an unstable allele that was induced by transposon mutagenesis. In the bundle sheath cells of bsd1-m1 leaves, chloroplasts differentiate aberrantly and C4 photosynthetic enzymes are absent. The development of mesophyll cells is unaffected. In dark-grown bsd1-m1 seedlings, morphological differentiation of etioplasts is only disrupted in bundle sheath cells but photosynthetic enzyme accumulation patterns are altered in both cell types. These data suggest that, during normal development, the Bsd1 gene directs the morphological differentiation of chloroplasts in a lightindependent and bundle sheath cell-specific fashion. In contrast, Bsd1 gene action on photosynthetic gene expression patterns is cell-type independent in the dark (C3 state) but bundle sheath cell-specific in the light (C4 state). Current models hypothesize that C4 photosynthetic differentiation is achieved through a light-induced interaction between bundle sheath and mesophyll cells (J. A. Langdale and T. Nelson (1991) Trends in Genetics 7, 191–196). Based on the data shown in this paper, we propose that induction of the C4 state restricts Bsd1 gene action to bundle sheath cells.
Sex determination of the Drosophila germ line: tra and dsx control somatic inductive signals
Authors: Steinmann-Zwicky Monica
Published: April 2021

In Drosophila, the sex of germ cells is determined by cell-autonomous and inductive signals. XY germ cells autonomously enter spermatogenesis when developing in a female host. In contrast, XX germ cells non-autonomously become spermatogenic when developing in a male host. In first instar larvae with two X chromosomes, XX germ cells enter the female or the male pathway depending on the presence or absence of transformer (tra) activity in the surrounding soma. In somatic cells, the product of tra regulates the expression of the gene double sex (dsx) which can form a male-specific or a female-specific product. In dsx mutant larvae, XX and XY germ cells develop abnormally, with a seemingly intersexual phenotype. This indicates that female-specific somatic dsx products feminize XX germ cells, and male-specific somatic dsx products masculinize XX and XY germ cells. The results show that tra and dsx control early inductive signals that determine the sex of XX germ cells and that somatic signals also affect the development of XY germ cells.

XX germ cells that develop in pseudomales lacking the sex-determining function of Sxl are spermatogenic. If, however, female-specific tra functions are expressed in these animals, XX germ cells become oogenic. Furthermore, transplanted XX germ cells can become oogenic and form eggs in XY animals that express the female-specific function of tra. Therefore, TRA product present in somatic cells of XY animals or in animals lacking the sex-determining function of Sxl, is sufficient to support developing XX germ cells through oogenesis.
Interacting functions of snail, twist and huckebein during the early development of germ layers in Drosophila
Authors: Reuter Rolf , Leptin Maria
Published: April 2021

Two zygotic genes, snail (sna) and twist (twi), are required for mesoderm development, which begins with the formation of the ventral furrow. Both twi and sna are expressed ventrally in the blastoderm, encode transcription factors and promote the invagination of the ventral furrow by activating or repressing appropriate target genes. However, sna and twi alone do not define the position of the ventral furrow, since they are also expressed in ventral cells that do not invaginate. We show that huckebein (hkb) sets the anterior and the posterior borders of the ventral furrow, but acts by different modes of regulation. In the posterior part of the blastoderm, hkb represses the expression of sna in the endodermal primordium (which we suggest to be adjacent to the mesodermal primordium). In the anterior part, hkb antagonizes the activation of target genes by twi and sna. Here, bicoid permits the co-expression of hkb, sna and twi, which are all required for the development of the anterior digestive tract. We suggest that mesodermal fate is determined where sna and twi but not hkb are expressed. Anteriorly hkb together with sna determines endodermal fate, and hkb together with sna and twi are required for foregut development.
Mef2 gene expression marks the cardiac and skeletal muscle lineages during mouse embryogenesis
Authors: Edmondson Diane G. , Lyons Gary E. , Martin James F. , Olson Eric N.
Published: April 2021

Members of the MEF2 family of transcription factors bind a conserved A/T-rich sequence in the control regions of many skeletal and cardiac muscle genes. To begin to assess the roles of the different Mef2 genes in the control of muscle gene expression in vivo, we analyzed by in situ hybridization the expression patterns of the Mef2a, Mef2c and Mef2d genes during mouse embryogenesis. We first detected MEF2C expression at day 7.5 postcoitum (p.c.) in cells of the cardiac mesoderm that give rise to the primitive heart tube, making MEF2C one of the earliest markers for the cardiac muscle lineage yet described. By day 8.5, MEF2A, MEF2C and MEF2D mRNAs are all detected in the myocardium. By day 9.0, MEF2C is expressed in rostral myotomes, where its expression lags by about a day behind that of myf5 and several hours behind that of myogenin. MEF2A and MEF2D are expressed at a lower level than MEF2C in the myotome at day 9.5 and are detected in more embryonic tissues than MEF2C. Expression of each of the MEF2 transcripts is observed in muscle-forming regions within the limbs at day 11.5 p.c. and within muscle fibers throughout the embryo at later developmental stages. The expression of MEF2C in the somites and fetal muscle is distinct from that of MEF2A, MEF2D and the myogenic bHLH regulatory genes, suggesting that it may represent a distinct myogenic cell type. Neural crest cells also express high levels of MEF2 mRNAs between days 8.5 and 10.5 of gestation. After day 12.5 p.c., MEF2 transcripts are detected at high levels in specific regions of the brain and ultimately in a wide range of tissues. The distinct patterns of expression of the different Mef2 genes during mouse embryogenesis suggest that these genes respond to unique developmental cues and support the notion that their products play roles in the regulation of muscle-specific transcription during establishment of the cardiac and skeletal muscle lineages.
Extracellular cAMP can restore development in Dictyostelium cells lacking one, but not two subtypes of early cAMP receptors (cARs). Evidence for involvement of cAR1 in aggregative gene expression
Authors: Soede Ron D. M. , Insall Robert H. , Devreotes Peter N. , Schaap Pauline
Published: April 2021

Extracellular cAMP induces expression of several classes of developmentally regulated genes in Dictyostelium. Four highly homologous surface cAMP receptors (cARs) were identified earlier, but involvement of specific cARs in gene regulation has not been clarified. Cells lacking the chemotactic receptor, cAR1, neither aggregate nor express developmentally regulated genes. Expression of aggregative genes is in wild-type cells induced by nanomolar cAMP pulses and repressed by persistent micromolar cAMP stimuli, which induce expression of prespore and prestalkenriched genes during the postaggregative stages of development. We show here that in cell lines carrying a cAR1 gene disruption, nanomolar pulses cannot induce aggregative gene expression. Remarkably, micromolar cAMP can induce expression of aggregative genes in car1− cells as well as expression of prespore and prestalk-enriched genes, and furthermore restores their ability to form normal slugs and fruiting bodies. These data indicate that cAR1 mediates aggregative but not postaggregative gene expression and morphogenesis, and suggest that after gene disruption, its function is partially taken over by a lower affinity receptor that is not subjected to desensitization. The absence of another early cAMP receptor, cAR3, does not affect development. However, in a car1−/car3− double mutant, cAMP stimulation cannot restore any developmental gene expression, indicating that cAR3 may have substituted for cAR1 in car1− cell lines.
βHeavy-spectrin has a restricted tissue and subcellular distribution during Drosophila embryogenesis
Authors: Thomas Claire M. , Kiehart Daniel P.
Published: April 2021

The components of the membrane skeleton play an important role in maintaining membrane structure during the dynamic changes in cell shape that characterize development. βHeavy-spectrin is a unique β-spectrin from Drosophila melanogaster that is closer in size (Mr=430×103) to dystrophin than to other β-spectrin members of the spectrin/α-actinin/dystrophin gene super-family. Here we establish that both the subcellular localization of the βHeavyspectrin protein and the tissue distribution of βHeavyspectrin transcript accumulation change dramatically during embryonic development.

Maternally loaded protein is uniformly distributed around the plasma membrane of the egg. During cellularization it is associated with the invaginating furrow canals and in a region of the lateral membranes at the apices of the forming cells (apicolateral). During gastrulation the apicolateral staining remains and is joined by a new apical cap, or plate, of βHeavy-spectrin in areas where morphogenetic movements occur. These locations include the ventral and cephalic furrows and the posterior midgut invagination. Thus, dynamic rearrangement of the subcellular distribution of the protein is precisely coordinated with changes in cell shape.

Zygotic message and protein accumulate after the germ band is fully extended, in the musculature, epidermis, hindgut, and trachea of the developing embryo. βHeavyspectrin in the epidermis, hindgut, and trachea is apically localized, while the protein in the somatic and visceral musculature is not obviously polarized.

The distribution of βHeavy-spectrin suggests roles in establishing an apicolateral membrane domain that is known to be rich in intercellular junctions and in establishing a unique membrane domain associated with contractile processes.
FlyBase -The Drosophila genetic database
Authors: Ashburner Michael , Drysdale Rachel
Published: April 2021

The first recorded scientific publication on Drosophila was 310 years ago (Mentzel, 1684). By 1980 about 35,000 papers on Drosophila had been published and at the time of writing this total had risen to over 60,000. By the year 2000, there will be over 80,000 Drosophila publications — and the on-going publication rate will be more than 4,000 a year. There is nothing unique in this rate of growth — it is typical for any “active” subject to double its output every fifteen years (see de Solla Price, 1986). Sooner or later, of course, the curve must plateau but, until it does, the individual scientist faces an obvious problem. Not all of the papers published will be of the standard of those in Development. Nevertheless, just sifting those that are worthy of reading from those that are not will be (indeed is) a daunting task. What is to be done? The answer is obvious, we must exploit the power of computers to point us to papers that we need to read. We must also exploit the power of computers to provide us with basic data about our organism. Luckily, there is every prospect that the power of engines to process and access these data will increase, and their relative cost decrease, with the growth in scientific information.
Lazarillo, a new GPI-linked surface lipocalin, is restricted to a subset of neurons in the grasshopper embryo
Authors: D. Ganfornina María , Sánchez Diego , Bastiani Michael J.
Published: April 2021

Lazarillo, a protein recognized by the monoclonal antibody 10E6, is expressed by a subset of neurons in the developing nervous system of the grasshopper. It is a glycoprotein of 45×103Mr with internal disulfide bonds and linked to the extracellular side of the plasma membrane by a glycosylphosphatidylinositol moiety. Peptide sequences obtained from affinity purified adult protein were used to identify an embryonic cDNA clone, and in situ hybridizations confirmed that the distribution of the Lazarillo mRNA paralleled that of the monoclonal antibody labeling on embryos. Sequence analysis defines Lazarillo as a member of the lipocalin family, extracellular carriers of small hydrophobic ligands, and most related to the porphyrinand retinol-binding lipocalins. Lazarillo is the first example of a lipocalin anchored to the plasma membrane, highly glycosylated, and restricted to a subset of developing neurons.
Transforming growth factor α disrupts the normal program of cellular differentiation in the gastric mucosa of transgenic mice
Authors: Sharp Richard , Babyatsky Mark W. , Takagi Hitoshi , Tågerud Sven , Wang Timothy C. , Bockman Dale E. , Brand Stephen J. , Merlino Glenn
Published: April 2021

Transforming growth factor alpha (TGFα) evokes diverse responses in transgenic mouse tissues in which it is over-expressed, including the gastric mucosa, which experiences aberrant growth and a coincident repression of hydrochloric acid production. Here we show that ectopically expressed TGFα induces an age-dependent cellular reorganization of the transgenic stomach, in which the surface mucous cell population in the gastric pit is greatly expanded at the expense of cells in the glandular base. Immunohistochemical analysis of BrdU incorporation into DNA demonstrated that although mature surface mucous cells were not proliferating, DNA synthesis was enhanced by approximately 67% in the glandular base and isthmus, where progenitor cells reside. RNA blot and in situ hybridization were employed to determine temporal and spatial expression patterns of specific markers representing a variety of exocrine and endocrine gastric cell types. Mature parietal and chief cells were specifically depleted from the glandular mucosa, as judged by a 6- to 7-fold decrease in the expression of genes encoding H+,K+-ATPase, which is required for acid secretion, and pepsinogen C, respectively. The reduction of these markers coincided in time with the activation of TGFα transgene expression in the neonatal stomach. The rate of cell death in the glandular region was not overtly different. Significantly, the loss of parietal and chief cells occurred without a concomitant loss of their respective cellular precursors. In contrast to exocrine cells, D and G endocrine cells were much less severely affected, based on analysis of somatostatin and gastrin expression. Analysis of these dynamic changes indicates that TGFα can induce selective alterations in terminal differentiation and proliferation in the gastric mucosa, and suggests that TGFα plays an important physiological role in the normal regulation of epithelial cell renewal.
Lamina–specific cues guide outgrowth and arborization of retinal axons in the optic tectum
Authors: Yamagata Masahito , Sanes Joshua R.
Published: April 2021

In the chick, retinal axons enter the optic tectum through a superficial lamina, then branch into distinct deeper retinorecipient laminae, where they arborize and form synapses. To study factors that guide this laminar selectivity, we devised an organotypic culture system in which a transverse tectal section is overlaid with a retinal explant large enough to allow unimpeded access to all tectal laminae. Outgrowth, branching, and arborization patterns of retinal axons on tectal slices were lamina-selective, indicating the existence of localized cues that guide retinal axons. Further studies suggested that some of these cues are: (1) associated with cell membranes or extracellular matrix (because axons grew selectively on chemically fixed tectal sections); (2) intrinsic to the tectum (because axons grew selectively on tectal sections prepared from enucleated embryos); (3) distinct from topographic cues (because axons from nasal and temporal retina behaved similarly on anterior tectal slices); and (4) selective for retinal axons (because axons growing from other explants exhibited different laminar preferences).
Expression of floricaula in single cell layers of periclinal chimeras activates downstream homeotic genes in all layers of floral meristems
Authors: Hantke Sabine S. , Carpenter Rosemary , Coen Enrico S.
Published: April 2021

We show that the flowering sectors on plants mutant for floricaula (flo), a meristem identity gene in Antirrhinum majus, are periclinal chimeras expressing flo in either the L1, L2 or L3 cell layer. Flower morphology is almost normal in L1 chimeras, but altered in L2 and L3 chimeras. Expression of flo in any one cell layer results in the expression of organ identity genes, deficiens (def) and plena (ple) in all three cell layers of the chimeras, showing that flo acts inductively to promote gene transcription. The activation of both def and ple is delayed, and the expression domain of def is reduced, accounting for some of the phenotypic properties of the chimeras. Furthermore, we show that flo exhibits some cell-autonomy with respect to autoregulation.
Axogenesis in the embryonic brain of the grasshopper Schistocerca gregaria: an identified cell analysis of early brain development
Authors: Boyan George , Therianos Stavros , Williams J. Leslie D. , Reichert Heinrich
Published: April 2021

Axogenesis in the embryonic brain was studied at the single cell level in the grasshopper Schistocerca gregaria. A small set of individually identifiable pioneer neurons establishes a primary axon scaffold during early embryogenesis. At the beginning of scaffold formation, pioneering axons navigate along and between glial borders that surround clusters of proliferating neuroblasts. In each brain hemisphere, an axonal outgrowth cascade involving a series of pioneer neurons establishes a pathway from the optic ganglia to the brain midline. At the midline the primary preoral commissural interconnection in the embryonic brain is pioneered by a pair of midline-derived pioneer neurons. A second preoral commissural connection is pioneered by two pairs of pars intercerebralis pioneer neurons. Descending tracts are pioneered by the progeny of identified neuroblasts in the pars intercerebralis, deutocerebrum and tritocerebrum; the postoral tritocerebral commissure is pioneered by a pair of tritocerebral neurons. All of the pioneering brain neurons express the cell adhesion molecule fasciclin I during initial axon outgrowth and fasciculation. Once established, the primary axon scaffold of the brain is used for fasciculation by subsequently differentiating neurons and, by the 40% stage of embryogenesis, axonal projections that characterize the mature brain become evident. The single cell analysis of grasshopper brain development presented here sets the stage for manipulative cell biological experiments and provides the basis for comparative molecular genetic studies of embryonic brain development in Drosophila.
The dorsal involuting marginal zone stiffens anisotropically during its convergent extension in the gastrula of Xenopus laevis
Authors: Moore Steven W. , Keller Raymond E. , Koehl M. A. R.
Published: April 2021

Physically, the course of morphogenesis is determined by the distribution and timing of force production in the embryo and by the mechanical properties of the tissues on which these forces act. We have miniaturized a standard materials-testing procedure (the stress-relaxation test) to measure the viscoelastic properties of the dorsal involuting marginal zone, prechordal mesoderm, and vegetal endoderm of Xenopus laevis embryos during gastrulation. We focused on the involuting marginal zone, because it undergoes convergent extension (an important and wide-spread morphogenetic process) and drives involution, blastopore closure and elongation of the embryonic axis. We show that the involuting marginal zone stiffens during gastrulation, stiffening is a special property of this region rather than a general property of the whole embryo, stiffening is greater along the anteroposterior axis than the mediolateral axis and changes in the cytoskeleton or extra-cellular matrix are necessary for stiffening, although changes in cell-cell adhesions or cell-matrix adhesions are not ruled out. These findings provide a baseline of data on which future experiments can be designed and make specific, testable predictions about the roles of the cytoskeleton, extracellular matrix and intercellular adhesion in convergent extension, as well as predictions about the morphogenetic role of convergent extension in early development.
Formin isoforms are differentially expressed in the mouse embryo and are required for normal expression of gf-4 and shh in the limb bud
Authors: Chan David C. , Wynshaw-Boris Anthony , Leder Philip
Published: April 2021

Mice homozygous for the recessive limb deformity (ld) mutation display both limb and renal defects. The limb defects, oligodactyly and syndactyly, have been traced to improper differentiation of the apical ectodermal ridge (AER) and shortening of the anteroposterior limb axis. The renal defects, usually aplasia, are thought to result from failure of ureteric bud outgrowth. Since the ld locus gives rise to multiple RNA isoforms encoding several different proteins (termed formins), we wished to understand their role in the formation of these organs. Therefore, we first examined the embryonic expression patterns of the four major ld mRNA isoforms. Isoforms I, II and III (all con-taining a basic amino terminus) are expressed in dorsal root ganglia, cranial ganglia and the developing kidney including the ureteric bud. Isoform IV (containing an acidic amino terminus) is expressed in the notochord, the somites, the apical ectodermal ridge (AER) of the limb bud and the developing kidney including the ureteric bud. Using a lacZ reporter assay in transgenic mice, we show that this differential expression of isoform IV results from distinct regulatory sequences upstream of its first exon. These expression patterns suggest that all four isoforms may be involved in ureteric bud outgrowth, while isoform IV may be involved in AER differentiation. To define further the developmental consequences of the ld limb defect, we analyzed the expression of a number of genes thought to play a role in limb development. Most signifi-cantly, we find that although the AERs of ld limb buds express several AER markers, they do not express detectable levels of fibroblast growth factor 4 (fgf-4), which has been proposed to be the AER signal to the mesoderm. Thus we conclude that one or more formins are necessary to initiate and/or maintain fgf-4 production in the distal limb. Since ld limbs form distal structures such as digits, we further conclude that while fgf-4 is capable of support-ing distal limb outgrowth in manipulated limbs, it is not essential for distal outgrowth in normal limb development. In addition, ld limbs show a severe decrease in the expression of several mesodermal markers, including sonic hedgehog (shh), a marker for the polarizing region and Hoxd-12, a marker for posterior mesoderm. We propose that incomplete differentiation of the AER in ld limb buds leads to reduction of polarizing activity and defects along the anteroposterior axis.