Biology · Ch 2 — Human Reproduction
Oogenesis
Oogenesis
Oogenesis is the process by which the female germ cells develop into a mature ovum, and it differs from spermatogenesis in several fundamental ways: it begins and largely completes its early stages before birth rather than at puberty, it is a discontinuous process with long meiotic arrests rather than a continuous production line, and each round of meiosis in oogenesis normally yields only a single functional gamete rather than four.
During foetal development, primordial germ cells in the developing ovary multiply by mitosis to form oogonia, and by around the fifth month of foetal life, all the oogonia that will ever be formed have already entered meiosis I and enlarged into primary oocytes. Crucially, these primary oocytes do not complete meiosis I at this stage; instead, each one arrests in the prophase stage of meiosis I, surrounded by a single layer of flattened follicular cells forming a primordial follicle (see section 2.5), and remains arrested in this state for anywhere from a little over a decade to more than four decades — from before the girl herself is born, through her own birth and childhood, until that particular follicle is recruited for further growth in a menstrual cycle after puberty.
In each menstrual cycle after puberty, a batch of primordial follicles begins growing (as described in section 2.5), but the primary oocyte within each does not resume meiosis until shortly before ovulation, triggered by the mid-cycle LH surge. At that point, the primary oocyte within the maturing Graafian follicle completes meiosis I, but does so unequally: almost all the cytoplasm is retained by one daughter cell, now called the secondary oocyte, while the other daughter cell, receiving very little cytoplasm, becomes a small, non-functional first polar body. The secondary oocyte immediately enters meiosis II but again arrests, this time at metaphase II, and it is this metaphase-II-arrested secondary oocyte, still surrounded by its corona radiata and zona pellucida, that is actually released from the ovary at ovulation. …
What this figure shows. A flow diagram tracing oogenesis from an oogonium (diploid germ cell) present in the foetal ovary, which enters meiosis I and enlarges into a primary oocyte that then arrests in prophase I of meiosis, remaining in this arrested state from before birth until puberty. The diagram then shows that, in a given menstrual cycle, one primary oocyte resumes and completes meiosis I shortly before ovulation, producing two unequal haploid cells — a large secondary oocyte, which receives almost all the cytoplasm, and a small, non-functional first polar body. The secondary oocyte is shown entering meiosis II but arresting again, this time at metaphase II, and it is this arrested secondary oocyte that is actually released at ovulation. A final branch of the diagram shows that meiosis II is completed only if a sperm penetrates the secondary oocyte, at which point it divides unequally once more into a mature ovum …