In avian species, the granulosa cells produce progesterone (P), but not testosterone (T) or estradiol (E). The theca folliculi in avian species produces T and E and is anatomically comprised of the theca interna and the theca externa. It is not known, however, whether both T and E are produced by the same cell type. In the present study, preovulatory follicles of adult female turkeys were separated into their anatomical subdivisions, the granulosa, theca interna, and theca externa, and the relative steroidogenic contribution by each layer was investigated. Granulosa cells from the largest (F1) and fifth largest (F5) preovulatory follicles produced increasing levels of P in response to ovine LH (oLH; 0-10 micrograms/ml); however, T and E were not detectable. Theca interna and theca externa cells from F5 follicles were cultured alone or in combination. Theca interna cells produced P and T, but not E. Theca externa cells produced P, T, and E. E production was greatly enhanced by coincubation of cells from the two theca layers. oLH, (0-100 ng/ml) stimulated theca interna P and T production and E production by the coincubation of theca interna and theca externa cells. When theca interna and theca externa cells from the F1 and F5 follicles were tested, theca externa cells from F5, but not F1, converted androstenedione and T, but not P into E, indicating a loss of aromatase activity in the theca externa with follicular maturation. Neither F1 nor F5 theca interna cells converted P, A, or T into estradiol, providing additional evidence that aromatase activity is limited exclusively to the theca externa. Addition of oLH (0-100 ng/ml) did not increase conversion of T into E by the F5 theca externa cells, suggesting that production of E by the theca externa is substrate driven and not influenced by direct LH action. Avian ovarian steroidogenesis is described using a three-cell model, with the principle sources of P, T, and E being the granulosa, theca interna, and theca externa cells, respectively.
