Fig. 4. The small luteal cells that are derived from the thecal layer are particularly responsive to luteinizing hormone and human chorionic gonadotropin; they are called theca lutein cells. The large luteal cells that are derived from the granulosa cells secrete a steady-state (basal) amount of progesterone and are not responsive to luteinizing hormone or human chorionic gonadotropin; they are called granulosa lutein cells. (Adapted from O’Hara A, Mori T, Taii S, et al: Functional differentation in steroidogenesis of two types of luteal cells isolated from mature human corpora lutea of menstrual cycle. J Clin Endocrinol Metabol 65:1192, 1987.)