Structure of the yolk syncytial layer during postembryonic development of Andinoacara rivulatus (Günther), 1860 (Cichlidae)

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.

The yolk syncytial layer of loach, Misgurnus fossilis (Teleostei) during early development

SummaryThe yolk syncytial layer (YSL) of Teleostei is a dynamic multifunctional temporary system. This paper describes the YSL structure of Misgurnus fossilis (Cobitidae) during its early developmental stages, studied using histological methods. YSL formation is prolonged. From the late blastula stage, the basal surface of the YSL is uneven and has protuberances, but becomes smoother during development. There are syncytial ‘islands’ with 1–2 yolk syncytial nuclei in the yolk mass. During epiboly, gastrulation and early segmentation, loach YSL is of different thickness in different regions along the dorso-ventral and antero-posterior axes of an embryo. The YSL is thickened in the dorsal region of gastrulae compared with the ventral region. Although the development of M. fossilis is similar to the development of zebrafish, there are important differences in YSL formation and organization that await further study and analysis. The study of YSL organization contributes to our knowledge of teleost developmental diversity and to the biology of temporary structures.

Injuries in pacu embryos (Piaractus mesopotamicus) after freezing and thawing

SummaryAlthough the sperm cryopreservation of freshwater and marine teleosts has been feasible for years, the cryopreservation of some fish embryos still remains elusive. Thus, the objective of this experiment was to analyze the embryo morphology after freezing and thawing 40 embryos of Piaractus mesopotamicus immersed into methanol and ethylene glycol, both at 7, 10 and 13% plus 0.1 M sucrose for 10 min. Soon after thawing, three embryos were treated with historesin, stained with hematoxylin–eosin and analyzed under an optical microscope. From every treatment, one palette containing embryos was thawed and incubated, but none of the eggs hatched. Samples containing two embryos were immersed into 10% methanol or 10% ethylene glycol both in association with sucrose, and embryos immersed into only water or sucrose solution were frozen, processed and analyzed using scanning electron microscopy (SEM). In both cases, the control group was immersed into only water. Although the embryos had the chorion, vitello, yolk syncytial layer and blastoderm, all of them were found altered under the optical microscope and by SEM. The chorion was irregular and injured; there was no individuality in the yolk granules; the yolk syncytial layer had an irregular shape, thickness and size; the blastoderm showed injuries in the nucleus shape and sometimes was absent; the blastoderm was located in atypical areas and absent in some embryos. In conclusion, no treatment was effective in preserving the embryos, and none of the embryos avoided injury from intracellular ice formation. These morphological injuries during the freezing process made the P. mesopotamicus embryos unfeasible for hatching.