Dynamics of ultrastructural changes in the yolk syncytial layer and its microenvironment during gastrulation and early postembryonic development of Hemichromis Bimaculatus

BACKGROUND: In fish embryogenesis, the yolk sac is the most important provisional organ. The period of its active functioning coincides with the embryonic and early postembryonic development of the embryo. The main function of this organ is trophic. Its implementation in Teleostei is associated with a specialized structure - the yolk syncytial layer (YSL). Ultrastructural transformations of this layer during gastrulation and early postembryonic development, as well as its interaction with migrating muscle fibers and melanophores, are probably necessary for the efficient use of nutrients by the developing embryo. The role of the yolk sac in the embryogenesis of Hemichromis Bimaculatus may be somewhat broader in comparison with existing ideas. AIM: to study the features of the Jewel Cichlid (Hemichromis Bimaculatus) yolk sac structural organization during the embryonic and early postembryonic periods of development. METHODS: The research was carried out on 22 embryos and larvae of Hemichromis Bimaculatus from 1 to 7 days after egg laying. The morphological features of the yolk sac were studied using light and transmission electron microscopy methods. RESULTS: On the 2nd day, the yolk sac was separated from the embryo by the trunk fold. Its main structure was the yolk syncytial layer, containing numerous nuclei, microvilli, mitochondria and phagolysosomes. The morphological features of the YSS were similar to those of the placental symplastotrophoblast and indicated its high functional activity. The yolk sac mesenchyme contained blood vessels, migrating melanophores and muscle fibers. The periderm, covered with a special shell, served as the primary skin of the embryo. The transition of Jewel Cichlid larvae to exogenous nutrition was accompanied by a significant decrease in yolk sac size with its subsequent involution. CONCLUSION: The trophic function of the yolk sac in Hemichromis Bimaculatus is realized thanks to the yolk syncytial layer, which during the period of gastrulation and postembryonic development interacts with a number of structures present in the wall of the organ. Migrating muscle fibers promote activation of yolk granules; accumulation of toxic metabolic products is associated with melanophores. The special structure of the periderm protects the yolk sac and the embryo itself from external influences.