Oogenesis is the process through which the female gamete, the egg cell or ovum, is created and matured within the ovaries. This sequence of cell divisions is fundamental to sexual reproduction in humans. Unlike the continuous production of sperm in males, oogenesis begins before birth and is completed only after fertilization, marking it as a unique biological pathway. The stages of oogenesis ensure the resulting ovum is equipped with the necessary resources for early embryonic development.
Stage One: Formation of Primary Oocytes in the Fetus
The initial phase of oogenesis occurs during the prenatal development of the female fetus. Primordial germ cells within the developing ovaries multiply through mitotic divisions, giving rise to precursor cells called oogonia. This multiplication establishes the entire lifetime supply of potential egg cells before birth.
Each oogonium then undergoes a growth phase, enlarging in size and accumulating cellular material. These cells enter the first stage of meiosis, a cell division that reduces the chromosome number by half, and are termed primary oocytes.
The primary oocytes do not complete this first meiotic division; instead, they become suspended in the Prophase I stage of meiosis. This prolonged arrest is maintained throughout fetal life, childhood, and until the onset of puberty. A female is born with a finite, non-renewable population of primary oocytes, typically numbering around 400,000.
Stage Two: Resumption of Meiosis at Puberty
The primary oocytes remain dormant for many years, sometimes decades, arrested in Prophase I. This period of stasis ends with the hormonal changes that mark the start of puberty. The resumption of meiosis is triggered monthly by a surge in pituitary gonadotropins, specifically Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH).
In each ovarian cycle, a small cohort of primary oocytes is stimulated, though typically only one will fully mature and be released. The primary oocyte completes its first meiotic division just before ovulation, yielding two daughter cells of unequal size: one large secondary oocyte and one very small first polar body.
The secondary oocyte contains nearly all the cytoplasm and cellular organelles. The smaller cell, consisting mainly of extruded nuclear material, is called the first polar body. The secondary oocyte is now haploid, meaning it contains half the number of chromosomes.
Stage Three: Completion of Maturation Upon Fertilization
The newly formed secondary oocyte quickly begins the second meiotic division but immediately halts its progress in Metaphase II. This arrested cell structure is released from the ovary during ovulation and travels down the fallopian tube.
The completion of oogenesis is conditional on fertilization. If the secondary oocyte is not penetrated by a sperm cell, it will degenerate and be shed during menstruation, never completing Meiosis II. The Metaphase II arrest is maintained until a sperm successfully penetrates the oocyte’s outer layers.
Upon penetration, the secondary oocyte completes its second meiotic division. This final division is unequal, resulting in two cells. The larger cell is the mature ovum, whose nucleus fuses with the sperm nucleus to form a diploid zygote. The smaller cell produced is the second polar body, which, like the first, is essentially a small packet of discarded genetic material.
The Unequal Division and Finite Supply of Oocytes
The unequal division of cytoplasm during both Meiosis I and Meiosis II is central to oogenesis. The purpose of forming the small, non-functional polar bodies is to ensure the single mature ovum retains the maximum volume of cytoplasm and stored nutrients. This large cytoplasmic volume is a reserve of energy, organelles, and molecular machinery required to sustain the early stages of embryonic development before implantation and the establishment of placental support.
The female reproductive lifespan is constrained by the finite number of oocytes created before birth. Throughout the years, 99.9% of all primary oocytes undergo a natural process of degeneration called atresia. This continuous depletion starts in the fetus and continues until menopause, ensuring that only approximately 400 oocytes ever reach full maturation and ovulation. The loss through atresia contributes significantly to the decline in reproductive capacity with age.