Large number of primordial follicles present in mammalian ovaries at birth, grow during the course of reproductive life primarily in size but also in complexity of the follicular structure. Among all the primordial follicles present during birth, only 1% of follicles reach the stage of ovulation, while the rest undergo atresia. In order to rescue these ovarian primordial follicles that undergo atresia, in-vitro culture of these follicles to ovulatory stage is being attempted in different species. But the fertilization rates of oocytes obtained from the cultured preantral follicles were not satisfactory probably due to the synthesis of insufficient quantities of regulatory factors and / or paracrine signals. In order to verify this hypothesis, various gene expression pattern studies have been attempted in in-vivo and in-vitro cultured preantral and early antral follicles in sheep.