Kit ligand actions on ovarian stromal cells: effects on theca cell recruitment and steroid production. 
Factors that control recruitment of theca cells from ovarian stromal-interstitial cells are important for early follicle development in the ovary.  During recruitment, theca cells organize into distinct layers around early developing follicles and establish essential cell-cell interactions with granulosa cells.  Recruitment of theca cells from ovarian stromal stem cells is proposed to involve cellular proliferation, as well as induction of theca cell-specific functional markers.  Previously, the speculation was made that a granulosa cell-derived "theca cell organizer" is involved in theca cell recruitment.  Granulosa cells have been shown to produce kit-ligand/stem cell factor (KL).  KL is known to promote stem cell proliferation and differentiation in a number of tissues.  Therefore, the hypothesis was tested in the current study that granulosa cell-derived KL may help recruit theca cells from undifferentiated stromal stem cells during early follicle development.  The actions of KL were examined using adult bovine ovarian fragment organ culture and isolated ovarian stromal-interstitial cells.  In organ culture KL significantly increased the number of theca cell layers around primary follicles.  Experiments using purified stromal-interstitial cell cultures showed that KL stimulated ovarian stromal cell proliferation in a dose-dependent manner.  Stromal cell differentiation into theca cells was analyzed by the induction of theca cell functional markers (i.e., androstenedione and progesterone production).  Bovine ovarian stromal cells produced low levels of androstenedione (5-40 ng/microg DNA) and progesterone (5-30 ng/microg DNA) in vitro that were approximately 20-fold lower than theca cells under similar conditions.  Treatment with KL did not affect ovarian stromal cell androstenedione or progesterone production.  Interestingly, hormones such as estrogen and hCG did stimulate stromal cell steroid production.  The results in this study suggest that granulosa cell-derived KL appears to promote the formation of theca cell layers around small (i.e., primary) ovarian follicles.  KL directly stimulated ovarian stromal cell proliferation but alone did not induce functional differentiation (i.e., high steroid production).  Therefore, KL is proposed to promote early follicle development by inducing proliferation and organization of stromal stem cells around small follicles.  Observations suggest that KL may act as a granulosa-derived "theca cell organizer" to promote stem cell recruitment of ovarian stromal cells in a manner similar to the way that KL promotes hematopoietic and lymphoid stem cells in bone marrow and the thymus.