An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes hormones that influence stages of the menstrual cycle. At the time of puberty, women have approximately 200,000 to 300,000 follicles, each with the potential to release an egg cell (ovum) at ovulation for fertilization. These eggs are developed once every menstrual cycle with around 450–500 being ovulated during a woman's reproductive lifetime. Structure Ovarian follicles are the basic units of female reproductive biology. Each of them contains a single oocyte (immature ovum or egg cell). These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single competent oocyte in humans. They also consist of granulosa cells and theca of follicle. Oocyte Once a month, one of the ovaries releases a mature egg (ovum), known as an oocyte. The nucleus of such an oocyte is called a germinal vesicle (see picture). Cumulus oophorus Cumulus oophorus is a cluster of cells (called cumulus cells) that surround the oocyte both in the ovarian follicle and after ovulation. Membrana granulosa It contains numerous granulosa cells. Granulosa cell Granulosa cells or follicular cells are cells that surround the oocyte within the follicle; their numbers increase directly in response to heightened levels of circulating gonadotropins or decrease in response to testosterone. They also produce peptides involved in ovarian hormone synthesis regulation. Follicle-stimulating hormone (FSH) induces granulosa cells to express luteinizing hormone (LH) receptors on their surfaces; when circulating LH binds to these receptors, proliferation stops. Theca of follicle The granulosa cells, in turn, are enclosed in a thin layer of extracellular matrix – the follicular basement membrane or basal lamina (fibro-vascular coat in picture). Outside the basal lamina, the layers theca interna and theca externa are found. Development Primordial follicles are indiscernible to the naked eye. However, these eventually develop into primary, secondary and tertiary vesicular follicles. Tertiary vesicular follicles (also called "mature vesicular follicles" or "ripe vesicular follicles") are sometimes called Graafian follicles (after Regnier de Graaf). In humans, oocytes are established in the ovary before birth and may lie dormant awaiting initiation for up to 50 years. After rupturing, the follicle is turned into a corpus luteum. Development of oocytes in ovarian follicles In a larger perspective, the whole folliculogenesis from primordial to preovulatory follicle is located in the stage of meiosis I of ootidogenesis in oogenesis. Embryonic development in males and females