Steroid Producing Cells during Ovarian Differentiation of the Tilapia, Sarotherodon niloticus

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Abstract
In order to examine the initial appearance and development of the steroid producing cells (SPCs) during the process of ovarian differentiation, histology and ultrastructure of tilapia (Sarotherodon niloticus) ovaries were investigated from 10 to 50 days after hatching. In gonads of fry at 23–26 days after hatching, initial ovarian differentiation was confirmed by the differentiation of stromal aggregations in the proximal and distal region of the gonad on the side facing the lateral wall. This represents the initial formation of the ovarian cavity. At the same time as ovarian differentiation, a few large cells appeared initially in the vicinity of blood vessels. They have some of the ultrastructural features characteristic of SPCs such as a moderate number of mitochondria with tubular cristae, a large amount of smooth endoplasmic reticulum and many free ribosomes. Based on these ultrastructural criteria, together with the present finding that these cells further differentiated into the typical SPCs at older stages, these cells were identified as SPCs. Thereafter, by 30–50 days, SPCs increased gradually in number in the area enclosing the blood vessels of ovaries. The increase in SPCs coincided with the development of germ cells, including the multiplication of oogonia and the transformation from oogonia to oocytes.