The process of oogenesis:
Introduction
Oogenesis is the process of female gamete (egg/ovum) development in sexually reproducing organisms. It generates haploid eggs through meiosis that fuse with sperm during fertilization to propagate genetic diversity. In mammals, oogenesis is initiated in the embryonic ovaries but ovulation and release of mature eggs do not occur until sexual maturity after puberty.
Early Stages of Development
Inside the ovary, female germ cells undergo mitotic proliferation to form primary oocytes surrounded by somatic follicle cells. By birth, this proliferation has formed millions of immature primary oocytes, each with a full complement of duplicated chromosomes (diploid 2n cells).
Around puberty, the ovarian and menstrual cycle begins. It prepares the uterine lining for potential embryo implantation while stimulating follicle and egg maturation each cycle. One primary oocyte will progress through meiotic cell division during every ovulation cycle.
Meiosis I
Unlike spermatogenesis which continuously forms haploid sperm, oocyte meiosis begins but is arrested part way resulting in a secondary oocyte containing duplicated copies of half the original chromosomes. The cell divides unevenly in Meiosis I, extruding one small polar body containing a duplicate genome.
Prophase I arrest occurs until hormone signals are sufficient to resume meiosis before ovulation. This months-long pause allows ample time for mRNA production and protein synthesis, stockpiling vital supplies required to kickstart embryogenesis upon fertilization.
Meiosis II
Upon sufficient luteinizing hormone signaling at ovulation, Meiosis II proceeds to completion. This second round of cell division segregates sister chromatids into the ovulated secondary oocyte and a second polar body. The polar body disintegrates while one ovum containing a single copy of each chromosome (1n) travels toward the uterus.
Fertilization & Embryo Formation
If mating occurs after ovulation, sperm will fertilize the ovulated egg. The genetic material combines, restoring the diploid genome. Now called a zygote, cell division and differentiation initiate embryo development. The stockpiled proteins and mRNA drive rapid cell proliferation, generating a multicellular embryo as the woman’s cycle continues preparing uterine implantation before menstruation would otherwise occur.
Concluding Summary
In brief, oogenesis entails follicle development of primary oocytes, the onset of meiosis, prophase I arrest until ovulation years later, completion of meiosis generating a secondary oocyte and polar bodies, ovulation releasing a reduced chromosome ovum, fusion with sperm during fertilization, and ultimately zygote formation to propagate genetic diversity in offspring. The prolonged intermission unique to oocytes provides reserves driving human embryogenesis if conception occurs. Together with the synchronized uterine cycle changes, these distinctive features of human oogenesis support reproduction.