Structure of the Yolk Syncytial Layer in the Larvae of Whitefishes: A Histological Study

The yolk syncytial layer (YSL) is a symplastic provisory system that forms at the early stages of embryogenesis of teleosts. The YSL serves morphogenetic, trophic, and immune functions. Despite its important role in development, data on the structure of YSL is scarce. In the present study, comparative histological research on the features of YSL structure in the development of larvae of four economically important species of Coregonidae (Salmoniformes)-Coregonus peled, Coregonus muksun, Coregonus nasus, and Stenodus leucichthys nelma-is presented. The YSL of the larvae of Coregonidae has a complex, differentiated structure. Functional regionalization of YSL cytoplasm, possibly determined by the specific features of nutrient assimilation, is typical for all aforementioned species. Cytoplasm that encircles a large oil globule appears striated. A division into an inner area, filled with yolk inclusions, and an outer, smoother homogeneous area, can be noted in the cytoplasm surrounding the yolk mass. The oil globule is retained after complete utilization of the yolk mass. The inequality of thickness of YSL along anteroposterior and dorsoventral axes also indicates the functional regionalization. The dorsomedial area of YSL, located under the intestine, is the least thick. Dorsolateral areas are strongly incrassated and envelop the intestine from two sides. Gigantic nuclei of exceptionally complex form are typical for YSL. Specific features apply to the form of YSL and its nuclei. Based on the obtained results, a fundamental similarity in organization of the YSL of larvae of the studied whitefishes can be concluded; however, its specific variations distinguishable on a histological level can be discussed.