The creation of reproductive cells, or gametes, is a fundamental process in sexual reproduction that ensures the offspring maintains the correct number of chromosomes. In females, this process is called oogenesis and involves meiosis, a specialized cell division that reduces the chromosome count by half. Without this reduction, the fusion of two cells would double the chromosome number in each generation. Oogenesis involves several steps of division and arrest, ultimately producing a mature egg cell ready for fertilization.
Formation During Meiosis I
The process begins with a primary oocyte, a diploid cell containing two complete sets of chromosomes, one from each parent. This cell starts the first meiotic division, known as Meiosis I, which is a reductional division intended to separate the homologous pairs of chromosomes. The primary oocyte is arrested in development until puberty, when it resumes Meiosis I under hormonal influence just before ovulation. This first division is highly unusual because it is an unequal split of the cell’s contents.
The primary oocyte divides into two cells of dramatically different sizes. The larger cell, called the secondary oocyte, retains nearly all of the cytoplasm, organelles, and nutrients. The much smaller cell, which receives only one set of chromosomes and very little cytoplasm, is the first polar body. This unequal division conserves maximum cellular resources within the future egg cell, ensuring its viability and ability to support early embryonic development. The formation of the secondary oocyte marks the completion of the first stage in chromosome reduction.
Chromosomal Status of the Secondary Oocyte
The secondary oocyte is best described as haploid in terms of its chromosome number. Haploid refers to a cell having one complete set of chromosomes, denoted as ‘n’. Since Meiosis I separated the homologous chromosome pairs, the secondary oocyte has 23 chromosomes, the haploid number for humans. This is a reduction from the 46 chromosomes (2n) found in the starting primary oocyte.
However, each of these 23 chromosomes is still duplicated, consisting of two sister chromatids joined together. While the cell has a haploid number of chromosomes (1n), it contains double the amount of DNA found in a mature gamete (2c DNA content). The secondary oocyte is therefore technically 1n, 2c, which is the state after a reductional division but before the final separation of sister chromatids in Meiosis II.
The secondary oocyte does not complete Meiosis II, but instead arrests its division at a precise point called Metaphase II. The cell remains suspended in this arrested state until a specific external event occurs. This Metaphase II arrest allows the cell to be released from the ovary during ovulation and travel through the fallopian tube, ready to be fertilized.
Completion of Meiosis and Fertilization
The second meiotic division is only triggered if the secondary oocyte is penetrated by a sperm cell, which is the biological signal for fertilization. The entry of the sperm causes a cascade of internal events, including a rise in calcium ions, which breaks the Metaphase II arrest. This signal allows the secondary oocyte to finally complete Meiosis II, an equational division that separates the sister chromatids.
Upon completion of Meiosis II, the cell once again divides unequally to conserve cytoplasm. It produces a large, mature ovum, which now contains a true haploid set of 23 unduplicated chromosomes (1n, 1c). The small cell produced is the second polar body, which is also haploid and quickly disintegrates.
The ovum, now fully mature, is ready for its nucleus to fuse with the haploid nucleus of the sperm. This fusion restores the diploid chromosome number (2n) and a 2c DNA content, forming the zygote, the first cell of the new individual. The existence of the secondary oocyte in its arrested state ensures that the egg is ready to complete the final division immediately upon fertilization.