theca cell

Theca cells form the outer wall of a growing ovarian follicle, just outside the granulosa cell layer. If granulosa cells are the egg's inner nursery, theca cells are the surrounding scaffolding that also serves as the follicle's chemical supplier, providing the raw material from which estrogen is eventually built.

Theca cells carry receptors for LH from the pituitary. Under LH stimulation they take cholesterol and convert it into androgens such as androstenedione and testosterone. They cannot perform the final aromatization step themselves; instead these androgens diffuse inward to the granulosa cells, which aromatize them into estradiol. This division of labor is the heart of the two-cell, two-gonadotropin model: LH drives theca androgen production, FSH drives granulosa estrogen production.

Theca cells also supply structural support and a blood supply to the otherwise avascular granulosa compartment. A clinically important caveat: when theca cells overproduce androgens — as in polycystic ovary syndrome, where elevated LH and insulin both stimulate them — the result is the excess androgen that causes acne, unwanted hair growth, and disrupted ovulation.