Temporary “extraembryonic” systems are important for the development of representatives of many Metazoan taxa and are characterized by significant diversity. The yolk syncytial layer (YSL), a component of embryonic and larval teleost yolk complex, is a provisory structure performing morphogenetic, nutritional and immune functions. Despite the crucial role of YSL in the early ontogeny of bony fishes, the data concerning YSL structure and morphological diversity is scant. Here, we describe YSL organization in the postembryonic development of the green terror cichlid, Andinoacara rivulatus, studied by the analysis of serial histological sections. YSL cytoplasm contains numerous yolk inclusions decreasing in size from basal to apical surface of YSL. The mode of yolk engulfment is one of the most variable YSL characteristics in the species studied. The yolk syncytial nuclei (YSN) have especially complex shape, and large nucleoli are well distinguished. The yolk mass consists of homogenous globules. It contains the oil globules, many of which are in contact with the basal surface of YSL. The YSL thickness is very uneven due to its interaction with oil globules. Significant part of the apical YSL surface is associated with the blood vessels and pigment cells. During the period of mixed feeding the yolk complex locates anterior to the liver. Thus, the yolk complex of green terror cichlid has a well-differentiated structure. Our results contribute to the knowledge of the morphology of fish temporary systems and allow clarifying their common and species-specific features.
A role for the extraembryonic yolk syncytial layer in patterning the zebrafish embryo suggested by properties of the hex gene
