Oogenesis:A Collaborative Effort

Oocyte-Accessory Cell Interactions

Roles of accessory cells: production of steroid hormones, transportation of essential components to the oocyte, and formation of accessory layers surrounding the oocyte (later, the egg).

There are two categories of accessory cells: follicle cells and nurse cells. What is the major distinction between these two categories of cells? Nurse cells are found in a number of invertebrate species. We shall discuss insect nurse cells in some detail.

In the mammalian follicle, each oocyte is surrounded by a multilayered cohort of follicle cells, often referred to as granulosa cells. Between the oocyte and granulosa cells is the acellular zona pellucida, which is secreted by the oocyte. The zona is penetrated by many short microvilli from the surface of the oocyte and cytoplasmic processes from the follicle cells. Desmosomes and gap junctions form at the points where the cytoplasmic process contact the oocyte surface. The gap junctions function in transfer of nutrients and regulatory molecules into the oocyte.

Granulosa cells secrete the fluid that accumulates in the extracellular spaces and bathes the oocyte. The spaces coalesce to form the antrum (Fig. 1). Follicles with large antra are called Graafian follicles. In the Graafian follicle a cluster of granulosa cells surround the accentrically-located oocyte. They are collectively called the cumulus oophorus. The cumulus is shed along with the egg at ovulation.

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