ISSN:1059-910X    

DOI:10.1002/jemt.1070260402

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractAscidian embryos are useful for examining how events that occur during fertilization and cleavage affect gastrulation because they gastrulate early in development, during the seventh cleavage. In ascidians, both dorsal‐ventral and anterior‐posterior axes are determined before first cleavage. The dorsal‐ventral axis is fixed along the animal‐vegetal axis of the fertilized egg following the first phase of ooplasmic segregation, perhaps due to determinants moved to the vegetal pole in concert with the myoplasm and plasma membrane components. The first ooplasmic movements appear to be driven by the actin network in the cortical myoplasm. The anterior‐posterior axis becomes apparent after the second phase of ooplasmic segregation, when the cortical myoplasm becomes detached from the egg membrane, and moves to the posterior pole of the embryo. This movement is dependent on microtubules and has been attributed to the formatior and movement of the sperm aster. A major component of the cortical myoplasm, p58, is co‐localized along the microtubules emanating from the sperm aster. Gastrulation begins during the seventh cleavage with the invagination of the large endodermal cells at the vegetal pole of the embryo. The neural plate appears as a thickening of the epidermis on the dorsal side of the larva during the ninth cleavage; then the neural folds are formed, join, and close, elaborating the neural tube. Following neurulation, the tail is elongated as the neural tube and notochord cells intercalate at the midline of the embryo. Investigations using anural (tailless) ascidian larvae suggest that some of the processes underlying elongation can be restored by the zygotic genome. Although ascidian larvae contain fewer cells and cell types than vertebrate embryos, ascidian gastrulation and morphogenesis appear to employ similar mechanisms to those in vertebrate embryos. The extent of our current knowledge about the mechanisms involved in gastrulation and tail formation is summarized, and further experiments are suggested to explore the molecular mechanisms underlying these processes. © 1993 Wil

ISSN:1059-910X    

DOI:10.1002/jemt.1070260403

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractComparative embryological studies using anural and urodele ascidians provide an experimental system to study interactions between developmental and evolutionary mechanisms that produce alternate life history strategies. In this paper, cellular features of anural morphogenesis inMolgula pacificaare compared to morphogenesis in species that develop tailed (urodele) larvae and other anural molgulid species. The results of these studies are discussed with regard to possible mechanisms responsible for the evolution of anural morphogenesis and the ecological consequences of anural development. Early developmental processes including ooplasmic segregation, cleavage patterns, and the site and timing of gastrulation were similar inM. pacificacompared to urodele embryos and embryos produced by other anural species. The limited extent of invagination caused by large, yolky cells that restricted vegetal pole cell movements inM. pacificagastrulae contrasted with the extensive movements of vegetal pole cells that accompanied invagination inM. provisionalisembryos and the embryos of four urodele species. The modified mode of gastrulation exhibited byM. pacificaembryos is likely due to the relatively high yolk content of their eggs.The developmental fates of muscle and epidermal progenitor cells inM. pacificaembryos were altered compared to urodele embryos. Ultrastructural studies and acetylcholinesterase histochemistry experiments indicate that muscle progenitor cells have lost their potential to develop muscle cell features. This loss in myogenic potential suggests that muscle progenitor cells were re‐programmed to die. However, this possibility was not supported by the results of ultrastructural studies. A second possibility is discussed in that muscle progenitor cells may have been re‐specified to differentiate into adult cells after metamorphosis. Evidence is presented suggesting that the timing mechanism responsible for controlling the onset of metamorphosis, first evident by the outgrowth of epidermal ampullae, was modified inM. pacifica. This paper concludes with a discussion of how anural morphogenesis altered the ancestoral urodele life cycle and the possible ecological benefits of these evolutionary alterations. © 1993 Wiley‐Lis

ISSN:1059-910X    

DOI:10.1002/jemt.1070260404

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractUltrastructural studies and lineage analyses of gastrulating mouse embryos have revealed that differnt morphogenetic tissue movements are involved in the formation of the three definitive germ layers. Definitive ectoderm is formed by epibolic expansion of the pre‐existing progenitor population in the embryonic ectoderm. Formation of the mesoderm and the endoderm is initiated by cellular ingression at the primitive streak. The mesodermal layer is established by cell migration and cell sheet spreading, but the endoderm is formed by replacing the original primitive endodermal population. To this date, genes that are expressed during mouse gastrulation mostly encode cell surface adhesion or signalling molecules, growth factors and their receptors, and putative transcriptional factors. Their precise role during gastrulation remains to be investigated. © 1993 Wiley‐Liss,

ISSN:1059-910X    

DOI:10.1002/jemt.1070260405

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

摘要:

AbstractMorphological data generated from light and electron microscopy form the basis of our understanding of avian morphogenesis. Because chicken embryos are readily and cheaply obtained and are easily accessible for experimental manipulation, morphogenetic processes have been studied extensively in this species. Such studies have allowed us to identify the cells involved during morphogenesis, observe the shape changes or cellular translocations that accompany a morphogenetic process, and determine the timing of these events. Elucidation of the molecular basis of morphogenesis has awaited the integration of several additional approaches. Among these are experimental embryology, which has allowed us to understand cellular behavior associated with morphogenesis; immunocytochemistry, which has identified the macromolecular cues that regulate cell movements and the environmental factors that control them; and molecular techniques, which will permit us eventually to clarify the genetic regulation of morphogenesis. Although current research in development is heavily biased towards molecular biology, morphological studies continue to frame the questions that are now being addressed using molecular techniques.This review focuses on the cells of the neural crest as a model system where questions of avian morphogenesis have been profitably addressed. © 1993 Wiley‐Liss, I

ISSN:1059-910X    

DOI:10.1002/jemt.1070260406

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

ISSN:1059-910X    

DOI:10.1002/jemt.1070260407

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY

ISSN:1059-910X    

DOI:10.1002/jemt.1070260401

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1993

数据来源: WILEY