Oogenesis is the biological process responsible for creating the female gamete, the ovum or egg cell. This sequence of cell divisions and maturation is unique because it takes place over decades, beginning long before a female is born and only concluding if fertilization occurs. The primary location for this entire formation and maturation process is within the female reproductive organ, the ovary.
The Anatomical Location of Oogenesis
The entire process of oogenesis is contained within the ovaries, a pair of almond-shaped organs situated in the pelvic cavity. Histologically, the ovary is divided into two main regions: the inner medulla and the outer cortex. The inner medulla contains the blood vessels, lymphatic vessels, and nerves that supply the organ.
Oogenesis occurs almost exclusively in the outer cortex, a dense layer of connective tissue. This cortex houses the developing oocytes inside specialized structures known as ovarian follicles. An ovarian follicle is the functional unit of the ovary, consisting of an immature egg cell, the oocyte, surrounded by protective and hormone-producing follicular cells, also called granulosa cells. The follicle supports the oocyte’s growth and survival while secreting hormones like estrogen that regulate the reproductive cycle. These follicles progress through various stages—primordial, primary, secondary, and tertiary (Graafian)—as the oocyte within them matures.
The Initial Stages: Oogenesis Before Birth
The initial stages of oogenesis occur entirely during fetal development. Primordial germ cells migrate to the developing ovary and differentiate into immature germ cells called oogonia. These oogonia then multiply rapidly through millions of rounds of mitosis. This period of multiplication lasts until approximately the seventh month of gestation, establishing the total number of potential egg cells, peaking at an estimated seven million primary oocytes.
Following this proliferative phase, the oogonia stop dividing and develop into primary oocytes. Each primary oocyte initiates the first meiotic division (Meiosis I) to reduce the chromosome number by half. Crucially, the process of Meiosis I is halted at a specific stage known as prophase I. The primary oocytes, surrounded by follicular cells to form a primordial follicle, then enter a prolonged resting state called the dictyotene. A female is born with this fixed supply of primary oocytes, which number between one and two million, and this pool continuously declines until menopause.
Resumption and Completion of Oogenesis
The suspended state of the primary oocytes lasts throughout childhood and is only broken starting at puberty. At the start of each menstrual cycle, hormonal signals, primarily a surge of luteinizing hormone (LH), trigger a select group of primordial follicles to resume development. Only one, or rarely a few, of these follicles will continue to mature fully.
The dominant primary oocyte completes its long-arrested Meiosis I just before ovulation, resulting in two cells of unequal size. The larger cell is the haploid secondary oocyte, and the much smaller cell is the first polar body, which is discarded genetic material. The secondary oocyte immediately begins the second meiotic division (Meiosis II) but arrests a second time at metaphase II.
The secondary oocyte is then released from the ovary during ovulation, still arrested in Meiosis II. This final meiotic division will only be completed if the secondary oocyte is penetrated by a sperm during fertilization. Completion of Meiosis II produces a mature ovum and a second polar body is cast off. If fertilization does not occur, the secondary oocyte degenerates within about 24 hours.