Ovary

KIT/KIT Ligand in Mammalian Oogenesis and Folliculogenesis: Roles in Rabbit and Murine Ovarian Follicle Activation and Oocyte Growth

Abstract
In rodent ovaries KIT ligand (KITL) and its receptor KIT have diverse roles, including the promotion of primordial follicle activation, oocyte growth and follicle survival. Studies were undertaken to determine if KITL and KIT carry out similar activities in rabbits. Kitl and Kit mRNA and protein were localised to oocytes and granulosa cells, respectively, in the rabbit ovary. Ovarian cortical explants from juvenile rabbits and neonatal mouse ovaries were subsequently cultured with recombinant mouse KITL and/or KITL neutralizing antibody. Indices of follicle growth initiation were compared with controls and between treatment groups for each species. Recombinant mouse KITL had no stimulatory effect on primordial follicle recruitment in cultured rabbit ovarian explants. However, the mean diameter of oocytes from primordial, early primary, primary and growing primary follicles increased significantly in recombinant mouse KITL treated explants compared to untreated tissues. In contrast, recombinant mouse KITL promoted both primordial follicle activation and an increase in the diameter of oocytes from primordial and early primary follicles in the mouse and these effects were inhibited by co-culture with KITL neutralizing antibody. Recombinant mouse KITL had no effect on follicle survival for either species. These data demonstrate that KITL promotes the growth of rabbit and mouse oocytes and stimulates primordial follicle activation in the mouse but not in the rabbit. We propose that the physiological roles of KITL and KIT may differ between species and this has important implications for the design of in vitro culture systems for mammalian folliculogenesis, including the human.

Key words: Ovary • Follicle • Granulosa cells • Oocyte development • Signal transduction

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