Oocytes

In the fetal ovary, all oocytes begin as oogonia. These proliferate via mitosis. They then enter meiosis I, becoming primary oocytesImmature egg cells halted in development until puberty.. These primary oocytes arrest in prophaseThe first stage of mitosis where chromatin condenses into visible chromosomes. I and remain dormant until puberty. At this stage, the ovary contains primordial folliclesThe earliest stage of follicle development.. Each follicle consists of a primary oocyte. It is surrounded by a single layer of flattened follicular (granulosa) cellsThe basic structural and functional units of life.. These primordial folliclesStructures in the ovaries that contain developing oocytes. are found in the ovarian cortex. They can be seen histologically as small, simple structures near the tunica albugineaA fibrous layer surrounding the ovary.. These are the only type of oocyte present from birth until puberty.

Beginning at puberty, a cohort of primary oocytes resumes developmentThe process of growth and differentiation. during each menstrual cycleThe monthly cycle of changes in the female reproductive system.. This process occurs under the influence of follicle-stimulating hormone (FSH). The surrounding follicular cellsCells surrounding the oocyte that support its development. become cuboidal and proliferate into multiple layers, forming a primary follicleAn early stage of follicle development.. As the follicle matures, the oocyte stays in prophase I but enlarges. It becomes surrounded by the zona pellucidaA glycoprotein layer surrounding the oocyte., which is a glycoprotein-rich extracellular layer. The granulosa cellsCells in the follicle that support the oocyte and produce estrogen. continue to multiply and are supported by an outer layer of thecaA layer of cells surrounding the follicle that produces hormones. cells.

As development proceeds, the follicle forms a fluid-filled cavity called the antrum, designating it a secondary follicleA developing follicle with multiple layers of cells.. The oocyte is still a primary oocyte, but the follicle is preparing for ovulationThe release of a mature oocyte from the ovary.. The mature or Graafian follicle is the largest and most structurally distinct. The oocyte is eccentrically placed in a mound of granulosa cells called the cumulus oophorus. It is surrounded directly by the corona radiataThe layer of follicular cells that surrounds a developing oocyte.. Just before ovulation, the oocyte completes meiosis I to become a secondary oocyteThe egg cell released during ovulation. and is arrested in metaphaseA stage of mitosis where chromosomes align at the metaphase plate. II. This is the oocyte that is ovulated and visible within the mature follicle.

If fertilization occurs, the secondary oocyte quickly completes meiosis II, forming a mature ovumA mature egg cell and a second polar bodyA small, non-functional cell produced during oogenesis.. However, if fertilization does not occur, the oocyte degenerates without completing meiosis. Histologically, once the oocyte is released, the empty follicle transforms into the corpus luteum, which no longer contains an oocyte. If pregnancy does not occur, it degenerates into the corpus albicansA scar-like structure that forms when the corpus luteum degenerates., a fibrous scar.