Fish development

Abstract: Complete dispersion and subsequent reaggregation of pre-embryonic blastomeres, features characteristic of annual fish development, are analyzed in Austrofundulus myersi Dahl. Cleavage produces a typical teleost blastoderm. During the mid-blastula stages, blastomeres segregate into two populations; deep blastomeres which disperse, and outer blastomeres which form the enveloping cell layer. When epiboly of the enveloping cell layer and periblast commences, the deep blastomeres come together as a consolidated mass and then migrate outward as amoeboid cells. When epiboly has concluded, the deep blastomeres are completely dispersed. After a few days, these cells come together to form a definitive aggregate within which embryogenesis occurs. The reaggregation process was quantified in vivo by determining “coefficients of dispersion.” Amoeboid blastomeres were found to be: uniformly distributed when epiboly is completed (day 2); randomly distributed on day 3; and aggregated on day 4. The origin of the annual fish developmental pattern has involved the spatial and temporal dissociation of the processes of epiboly and embryogenesis. This has been accomplished by the interposition of the dispersion-reaggregation phases. They may represent an exaggeration of the phases of incipient dispersion and localized aggregation which occur in most teleosts. The dispersion-reaggregation process is considered to be a control mechanism operating on the principle of a requirement for “critical cell mass” as a necessary prelude to the primary events of embryogenesis. The survival value of the dispersed phase is discussed. Comparative studies of 43 closely related species have shown that the dispersion-reaggregation pattern is present only in those cyprinodont fishes with annual life cycles. This group includes members of the following genera: Aphyosemion (annual species only); Austrofundulus; Cynolebias (including Cynopoecilus); Nothobranchius; Pterolebias; Rachovia; and Roloffia. Non-annual cyprinodonts develop as do other fishes. Some Aphyosemion species have transitional developmental patterns.

Abstract: Annual fish development differs from that of other teleosts because a phase of blastomere dispersion-reaggregation spatially and temporally separates epiboly from embryogenesis. The fate of dispersed blastomeres was assessed in diblastodermic eggs of the annual fishes Cynolebias whitei and C. nigripinnis. In typical teleosts, blastomere determination and the events of primary embryonic induction occur prior to or during epiboly, so diblastodermic eggs produce partially or completely duplicated embryos. In the diblastodermic eggs of Cynolebias, the two blastoderms are completely separate from the one cell stage to the high blastula. Blastoderm fusion begins during midepiboly. By the end of epiboly, blastoderm fusion has been completed, and the deep, embryo-forming blastomeres of both blastoderms have completely dispersed and intermingled to form a single cell population. A typical annual fish dispersed blastomere phase ensues. Blastomeres reaggregate into a single mass, in which one embryo develops. When hatched, the young fish have no obvious structural or functional abnormalities. We suggest that the dispersed blastomeres of annual fish eggs are equivalent and that induction or determination takes place within the reaggregate. Alternatively, dispersed cells are partially determined but highly regulative, so that, when two populations fuse, the cells sort out according to tissue type and form a single embryo. In either instance, the formation of a single, normal embryo seems to corroborate the hypothesis that the dispersed cell phase of annual fishes is an adaptation that prevents environmentally induced developmental defects. © 1993 Wiley-Liss, Inc.

Abstract: The present report firstly describes a pilot study in which, during early development of embryos of the common carp, Cyprinus carpio, the cellular adhesion to fibronectin (FN) was blocked by administration of GRGDS peptide (which binds to the FN-receptor). As this treatment resulted in developmental aberrations, suggesting a functional role for FN, the major part of the work was focussed on the distribution of reactivity of anti-FN antibodies during epiboly and gastrulation. GRGDS treatment had a concentration dependent effect on development. Incubation of embryos in 1.5 mg/ml from the 32-cell stage onwards caused a retardation of epiboly, which did not proceed beyond 60%. The embryos did not show involution, as was confirmed by histological study. These preliminary results suggest that FN is involved in both epiboly and gastrulation of carp embryos. During cleavage, no specific extracellular binding of anti-FN antiserum could be observed. However, binding to a number of cell membranes took place from early epiboly onwards. After the onset of gastrulation, we observed a gradually increasing number of the deepest epiblast cells, showing immunostaining on part of their surface, facing the yolk syncytial layer (YSL) or the involuted cells. During early epiboly, anti-FN binding was restricted to areas in front of the migratory hypoblast cells. Later on, binding was found at the border of hypoblast and epiblast cells. At 100% epiboly, some contact areas of epiblast and hypoblast showed a discontinuous lining of reactivity, whilst other areas appeared devoid of anti-FN binding sites. The results indicate that FN is involved in the migration and guidance of hypoblast cells during gastrulation in carp.