Granulosa cells

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Granulosa cells refer to the larger, cuboidal-shaped epithelial cells of the stratified follicular epithelium of ovarian follicles. It is present at all stages of follicle development except the primordial follicle which is surrounded by a single layer of follicular cells.

They are first observed in the multilayered primary follicle, where they surround the zona pellucida, a thick glycoprotein envelope between the granulosa cells and the enlarging primary oocyte surface. The granulosa cells continue to proliferate with further follicular development, forming the zona granulosa, a thick layer of cells separated from surrounding ovarian stromal cells by a basement membrane.

Granulosa cells communicate with each other via gap junctions. They also bear cytoplasmic processes called filopodia which penetrate the zona pellucida to contact microvilli of the oocyte, allowing communication between the cells through gap junctions as well.

Granulosa cells play an active role in fluid secretion and steroid hormone metabolism. As follicular development progresses, these cells are stimulated to produce prostaglandin and extracellular hyaluronan, increasing the volume, and viscosity of follicular fluid. Furthermore, granulosa cells convert estrogen precursors secreted by endocrine cells of the theca interna, which is a layer of differentiated stromal cells immediately outside the growing primary follicle, into estrogen by the action of the enzyme aromatase.

Following ovulation, granulosa cells along with the theca interna of the ruptured follicle develop into the corpus luteum. The granulosa cells and theca interna of the corpus luteum secrete progesterone and estrogen respectively, in preparation for implantation of a fertilized ovum.