Seasonal Reproductive Activity in the Ovaries of the <i>Cirrhinus mrigala</i> (Hamilton)

Abstract

The Indian major carp, Cirrhinus mrigala, is a seasonal breeder and spawns during monsoon season. On tlie basis of ovarian weights and histological studies, the ovarian cycle has been divided into four phases: (i) Postspawning period or the resting phase, (ii) Preparatory period, (iii) Prespawning period and (iv) the spawning period. Three types of primary oocytes have been identified in the stained ovarian sections, viz., stage 1 non-yolky oocytes, stage II vitellogenic oocytes and stage III yolky oocytes. Maximum ovarian weights were recorded during June-July, while maximum plasma vitellogenin titers were observed during May. Ova diameter was maximum during July and minimum during November-December. Morphological changes observed during follicular development include the appearance of many nucleoli and development of egg envelops. Yolk nucleus or Balbiani's vitelline body, a highly basophilic structure was observed adjacent to the nuclear envelope, which subsequently moved towards the periphery and finally disintegrated. Following vitellogenesis, the germinal vesicle (GV) migrates from a central position to the oocyte periphery and thereafter where subsequently it disappears. The phenomenon is called oocyte maturation. Theca, follicular epithelium and zona radiata surround the growing egg, A micropyle in zona radiata internus was also identified in the yolky and ovulated oocytes. A progressive decrease in the diameter of zona radiata, thinning and withdrawl of microvilli and stretching of the evelope perhaps appear to expell the egg resulting in ovulation.