Summary In the dynamically changing structure and function of the mammalian ovary, growing follicles exhibit a common lineage of development with the OSE layer, and they mutually interact for physical support and exchange of nutrient/hormone/chemical messengers. As mentioned, the stem/progenitor cells of coelomic epithelium derive germ cells differentiating into oocytes and epithelial nests derive the surrounding pre-granulosa cells. In non-mammals oocytes develop in the ovarian cavity layered with surface epithelium. In mammals, OSE assists in the dissolution of the follicle apex and all the underneath connective tissue and helps in releasing the matured ovum. Further, the mitogens mostly present in follicular fluid influence the OSE proliferation, re-epithelization and tumorigenesis. Given this nature of the intimate relationship between the two integral tissues of the ovary, it was felt necessary to discuss in the following section the key features of mammalian follicle development. The oocytes and the surrounding cells of follicles constitute the fundamental reproductive unit of the ovary. In young women, the ovarian cortex is largely filled with progressively developing follicles, whereas with aging due to repeated ovulation and atresia most of the follicles are replaced by fibers. Growing follicles migrate deeper in the cortex and subsequently move back to the surface at the time of ovulation. Interestingly, the maximum number of ovarian follicles is found just prior to birth, which gradually declines during childhood and then drops sharply before menopause leaving just ~1000 follicles for the remaining life. Not only does the number declines but the quality of oocytes also deters at this stage, and several structural damages and aneuploidy emerge. According to Carlsson (2008), there are two distinct yet complementary processes that contribute to follicle development: Oogenesis The germ cells originating from embryonic ectoderm migrate during early gestation to the genital ridge. During the later gestation period, the primordial oocytes continue to multiply through mitosis, and just prior to birth they enter meiosis, arresting the first meiotic division at the diplotene stage of prophase. Such oocytes are termed primary oocytes. During 20 weeks of human gestation, about 7 million oocytes are produced; of which only 2 million remain at birth and the rest undergo apoptotic destruction. At puberty, approximately 400,000 primary oocytes still remain, and this is the quota for the reproductive phase of a woman. From this population of oocytes, only 400-500 (1%) follicles mature and ovulate, whereas the remainder becomes atretic. From the onset of puberty to menopause ca . 15-20 primary oocytes are released into the growing pool in each menstrual cycle, completing the meiosis-I and continuing through meiosis-II until metaphase. The second meiotic cycle completes upon fertilization. Folliculogenesis This is a sequential process in which the follicles mature from primordial to the pre-ovulatory stage. Two major processes accomplish this development,   * recruitment of the follicle into the growing follicle pool, and   * proliferation and differentiation of granulosa and theca cells. The first proc