Ultrastructural observations on possible sites of steroid biosynthesis in the ovarian follicular epithelium of two species of cichlid fish,Cichlasoma nigrofasciatum and Haplochromis multicolor

Summary The follicular epithelial layers of the developing ovary of two cichlid species were examined by electron microscopy for evidence of steroid secretion. As each oocyte grew, its follicular cell layers increased in height, eventually becoming somewhat columnar; no development could be detected in follicle cells of non-activated oocytes. Isolated cells close to capillaries in the thecal layer developed large amounts of smooth membrane indicative of steroidogenesis, appearing similar at maturity to testicular Leydig cells. In Cichlasoma nigrofasciatum the mitochondria of differentiated thecal elements contained microtubule-like inclusions. It is suggested that these cells may produce estrogens during vitellogenesis.In developing granulosa cells, active synthesis of granular endoplasmic reticulum occurred. This membrane appeared to arise from the nuclear envelope, and in the pre-ovulatory stage was always intermediate between smooth and granular forms, being only partly associated with ribosomes. Evidence for steroid biosynthesis in the granulosa at this time was therefore equivocal. Evidence was found of transfer of micropinocytotic vesicles from the granulosa cells into the ooplasm.The fate of the post-ovulatory follicle was investigated in Cichlasoma. Thecal elements remained separate from granulosa and unchanged in ultrastructure for up to ten days. The granulosa cells proliferated and differentiated within a few hours after ovulation into a cell type containing much smooth reticulum, characteristic of steroidogenesis. However, after approximately three days numerous signs of degenerative processes became visible. The significance of the observed ultrastructural changes in relation to endocrine function is discussed.