Oogenesis is the biological process that forms the female reproductive cell, or gamete, known as the ovum or egg cell. This process is the counterpart to spermatogenesis in males, but it follows a distinct and highly regulated timeline. Oogenesis produces a large, haploid cell carrying the genetic material necessary for the potential creation of a new organism. A defining characteristic is that it begins long before birth and produces a finite supply of potential gametes, unlike the continuous production seen in males.
The Beginning: Oogenesis Before Birth
The initial stages of egg cell formation occur exclusively within the developing female fetus. This process begins with primordial germ cells, which migrate to the developing ovaries and rapidly multiply through repeated rounds of mitosis. These proliferating cells are called oogonia, and they build the initial reservoir of germ cells.
By the middle of gestation, the number of oogonia peaks, reaching approximately seven million. Shortly thereafter, most of these cells undergo a programmed process of cell death, or atresia, and the remaining oogonia differentiate into primary oocytes. These primary oocytes then initiate the first meiotic division, Meiosis I, which reduces the number of chromosomes by half.
However, this meiotic division does not complete. Instead, all primary oocytes become suspended in a resting phase called Prophase I. This first arrest can last for decades, from the fetal stage until the individual reaches reproductive maturity. By the time of birth, the total number of primary oocytes has reduced to about one to two million, and no new oocytes are created after this point.
Throughout childhood, the number of primary oocytes continues to diminish through atresia, leaving a pool of approximately 400,000 potential egg cells at the onset of puberty. This long-term arrest of the primary oocyte contrasts with the continuous nature of sperm production. The cell remains dormant, awaiting the specific hormonal signals that will prompt its monthly reawakening.
Monthly Release: Resumption of Meiosis I
The process of oogenesis resumes only after puberty, driven by the cyclical release of hormones from the pituitary gland, specifically the Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH). In each menstrual cycle, a cohort of primary oocytes within their ovarian follicles is stimulated to grow and develop, though typically only one dominant oocyte will complete the next stage. The surge of LH is the primary trigger that prompts the selected primary oocyte to break its long-term arrest and complete Meiosis I.
The completion of Meiosis I results in the creation of two daughter cells that are significantly unequal in size. One cell retains almost all of the parent cell’s cytoplasm, organelles, and stored nutrients, becoming the large, haploid secondary oocyte. The second cell receives a full set of chromosomes but very little cytoplasm, forming the small, non-functional first polar body.
This unequal distribution of cellular material, known as unequal cytokinesis, ensures the resulting secondary oocyte is robust and nutrient-rich to support the early stages of embryonic development if fertilization occurs. The newly formed secondary oocyte immediately begins the second meiotic division, Meiosis II, but quickly halts again. This second checkpoint arrests the secondary oocyte in Metaphase II, the state in which it is released from the ovary during ovulation.
The secondary oocyte is then swept into the fallopian tube, where it remains viable for a short window. If the secondary oocyte is not penetrated by a sperm cell, it will degenerate, never completing the process of oogenesis. The first polar body, having shed half of the chromosomes with minimal cytoplasmic loss, typically also degenerates.
The Final Step: Completion of Meiosis II
The final step in oogenesis, the completion of Meiosis II, is entirely dependent on an external stimulus: the penetration of the secondary oocyte by a sperm cell. This act of fertilization provides the molecular trigger that releases the secondary oocyte from its Metaphase II arrest. The sperm’s entry initiates a cascade of intracellular events, including a rise in calcium ions, which signals the cell to complete the final division.
Similar to the first meiotic division, Meiosis II is also an unequal cytoplasmic division. The secondary oocyte divides into two cells, again allocating the vast majority of its cytoplasm to one cell. This large cell is the mature ovum, which now contains a full complement of chromosomes from both the sperm and the egg after the fusion of their nuclei.
The smaller cell formed during this final division is the second polar body, which, like the first, contains a haploid set of chromosomes but very little cytoplasm and will soon degenerate. In some cases, the first polar body may also divide to form two additional polar bodies. Thus, one primary oocyte ultimately yields a single functional ovum and up to three non-functional polar bodies.