Oogenesis occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo.
Oogenesis Complete in three Phases
A. Multiplication Phase
B. Growth Phase
C. Maturation Phase
1. Primordial germ cell undergoes mitosis to produce Oogonia (occurs Pre-natally). Oogonia are diploid (2n) stem cells that divide mitotically to produce millions of germ cells. Even before birth, most of these germ cells degenerate in a process known as atresia.
2. A few oogonia, however, develop into larger cells called primary oocytes (2n).
3. Each Primary Oocyte starts to undergo meiosis I replicating their DNA, but they are arrested at the first Meiotic Prophase. During this arrested stage of development, each primary oocyte is surrounded by a single layer of flat follicular cells, and the entire structure is called a primordial follicle. The ovarian cortex surrounding the primordial follicles consists of collagen fibers and fibroblast-like stromal cells. At birth, approximately 200,000 to 2,000,000 primary oocytes remain in each ovary. Of these, about 40,000 are still present at puberty, and around 400 will mature and ovulate during a woman’s reproductive lifetime. The remainder of the primary oocytes undergo atresia.
4. Each month after puberty until menopause, gonadotropins (FSH and LH) secreted by the anterior pituitary further stimulate the development of several primordial follicles, although only one will typically reach the maturity needed for ovulation. A few primordial follicles start to grow, developing into primary follicles. Each primary follicle consists of a primary oocyte that is surrounded in a later stage of development by several layers of cuboidal and low columnar cells called granulosa cells. The outermost granulosa cells rest on a basement membrane. As the primary follicle grows, it forms a clear glycoprotein layer called the zona pellucida between the primary oocyte and the granulosa cells. In addition, stromal cells surrounding the basement membrane begin to form an organized layer called the theca folliculi.
5. With continuing maturation, a primary follicle develops into a secondary follicle. In a secondary follicle, the theca differentiates into two layers: (1) the theca interna, a highly vascularized internal layer of cuboidal secretory cells that secrete estrogens and (2) the theca externa, an outer layer of stromal cells and collagen fibers. In addition, the granulosa cells begin to secrete follicular fluid, which builds up in a cavity called the antrum in the center of the secondary follicle. The innermost layer of granulosa cells becomes firmly attached to the zona pellucida and is now called the corona radiate.
6. The secondary follicle eventually becomes larger, turning into a mature (graafian) follicle. While in this follicle, and just before ovulation, the diploid primary oocyte completes meiosis I, producing two haploid (n) cells of unequal size—each with 23 chromosomes. The smaller cell produced by meiosis I, called the first polar body, is essentially a packet of discarded nuclear material. The larger cell, known as the secondary oocyte, receives most of the cytoplasm. Once a secondary oocyte is formed, it begins meiosis II but then stops in metaphase.
7. The mature (graafian) follicle soon ruptures and releases its secondary oocyte, a process known as ovulation. At ovulation, the secondary oocyte is expelled into the pelvic cavity together with the first polar body and corona radiata. Normally these cells are swept into the uterine tube. If fertilization does not occur, the cells degenerate. If sperm are present in the uterine tube and one penetrates the secondary oocyte, however, meiosis II resumes. The secondary oocyte splits into two haploid cells, again of unequal size. The larger cell is the ovum, or mature egg; the smaller one is the second polar body. The nuclei of the sperm cell and the ovum then unite, forming a diploid zygote. If the first polar body undergoes another division to produce two polar bodies, then the primary oocyte ultimately gives rise to three haploid polar bodies, which all degenerate, and a single haploid ovum. Thus, one primary oocyte gives rise to a single gamete (an ovum).