Physiology of Finfish and Shellfish (2+1)

3.1.20. Causes

Various external and internal factors are believed to be responsible for the degeneration of oocytes. These are hormones, temperature, photoperiod, overcrowding, nutritional state of the breeders, physic-chemical nature of water, etc. Studies show that imbalance of the hormones and steroids causes degeneration of oocytes. Various environmental factors such as light, temperature, pH, salinity etc must be favourable for healthy growth of oocytes. Hence, any disturbance in these factors may cause atresia.

(In the female, gamete development also involves the manufacture and incorporation of yolk sac into the developing oocytes (vitellogenesis). During the early part of gonadal development the oogonia becomes associated with a number of pre-follicular cells and it is from these that the ovarian follicle is derived. In the salmonids for eg, the oocytes increase in diameter from about 50 µm to 500-1000 µm prior to the start of vitellogenesis. The incorporation of yolk that occurs during the vitellogenic phase leads to the diameter of the oocyte increasing to around 5000 µm (5 mm). thus, there is an enormous increase in the oocyte volume during vitellogenesis. The majority of this increase is due to the incorporation of exogenously synthesized yolk into the cytoplasm of the oocyte.

The yolk is derived from vitellogenin, a lipophospho-protein- ca complex with a molecular weight of approximately 440000. Vitellogenin is synthesized in the liver and is then released into the blood system. The vitellogenin is transported to the ovary in the blood from which is set aside by the oocyte. Following the uptake into the oocyte the vitellogenin is spilt into 2 major components- phosvitin and lipovitellin which forms the yolk stores).