KIT-KIT ligand in the growth of porcine oocytes in primordial follicles.

Mammalian ovaries are endowed with a huge number of small oocytes in primordial follicles (primordial oocytes). The mechanism regulating initiation of oocyte growth and follicular development is not well understood. Several growth factors and cytokines are known to be involved in oocyte growth and follicular development. Herein, the involvement of KIT, a receptor tyrosine kinase, and its ligand, KIT ligand (KL), in the initiation of porcine oocyte growth was examined. At first, KIT expression was examined immunohistochemically in primordial oocytes from neonatal (10-20 days) and prepubertal (about 6 months) pigs. Similar expression of KIT was detected in all oocytes from both the neonatal and prepubertal pigs. Next, to examine the growth of primordial oocytes, ovarian tissues containing primordial oocytes were xenotransplanted into immunodeficient SCID mice. Primordial oocytes from the neonatal pigs grew with follicular development as described previously, whereas those from the prepubertal pigs did not initiate growth in the xenografts after 2 months. To stimulate the growth of primordial oocytes from the prepubertal pigs, they were cultured in a medium supplemented with KL (50 and 100 ng/ml) for 1 or 3 days before xenografting. After 2 months, however, the oocytes did not grow, and the primordial follicles did not develop, although a higher number of primordial oocytes survived in the KL-treated tissues. These results suggest that KIT-KL might not be associated with the growth initiation of porcine primordial oocytes, although they do enhance the survival of the oocytes.