Theca cells are a group of endocrine cells in the ovary made up of connective tissue surrounding the follicle that has many diverse functions during folliculogenesis. These roles include synthesizing androgens, providing signal transduction between granulosa cells and oocytes during development by the establishment of a vascular system, providing nutrients, and providing structure and support to the follicle as it matures. The theca cells are responsible for the production of androstenedione, and indirectly the production of 17β estradiol, also called E2, by supplying the neighboring granulosa cells with androstenedione that with the help of the enzyme aromatase can be used as a substrate for this type of estradiol. FSH induces the granulosa cells to make aromatase that converts the androgens made by the theca interna into estradiol.
Theca cells with granulosa cells help form the corpus luteum. Theca cells are only correlated with developing ovarian follicles. They are the leading cause of endocrine-based infertility, as either hyperactivity or hypoactivity of the theca cells can lead to fertility problems; hyperactivity of theca cells causes hyperandrogenism, and hypoactivity leads to a lack of estrogen. Granulosa cell tumors, while rare (less than 5% of ovarian cancers), involve both the granulosa cells and the theca cells.