In human women, oogenesis produces one mature egg cell or ovum for every precursor cell that enters meiosis. This process differs in two unique ways from the equivalent procedure of spermatogenesis in males. First, meiotic divisions during oogenesis are asymmetric, meaning that a large oocyte (containing most of the cytoplasm) and minor polar body are produced as a result of meiosis I, and again following meiosis II. Since only oocytes will go on to form embryos if fertilized, this unequal distribution of cell contents ensures that there are enough cytoplasm and nutrients to nourish the early stages of development. Second, during oogenesis, meiosis “arrests” at two distinct points: once during embryonic growth and a second time during puberty. In mammals, oocytes are suspended in prophase I until sexual maturation, at which point meiosis I continues under hormonal influence until an egg precursor cell is released into a fallopian tube. At ovulation, the precursor exits the ovary and, only if fertilization occurs, is stimulated to complete meiosis II and form a complete egg.
Oogenesis, Age, and Other Factors
Defects during oogenesis can result in severe consequences. In particular, problems with chromosome segregation during either meiosis I or meiosis II may lead to an embryo being aneuploid, meaning that it contains an abnormal number of chromosomes. Increased age elevates a woman’s risk of having a child with certain types of aneuploidy, such as Patau syndrome—characterized by central nervous system abnormalities, developmental delays and infant mortality—which is caused by an extra copy of chromosome 13. Several explanations for this “age effect” have been proposed, which include the degradation over time of the meiotic spindle apparatus (responsible for splitting chromosomes during division), or the gradual accumulation of abnormal cells in ovaries. As a result, women who are 35 years and older are typically offered prenatal testing, such as blood tests, nuchal translucency screening by ultrasound, chorionic villus sampling, or amniocentesis, which can determine whether a fetus carries any chromosomal abnormalities.
Aside from a woman’s age, other researchers are looking at how certain diseases can influence oogenesis and oocyte quality. One such condition gaining interest is endometriosis, during which the blood-rich lining that typically collects in a woman’s uterus before menstruation accumulates elsewhere in the body, like in ovarian cysts, along the large intestine or upon the lining of the abdominal cavity. Interestingly, oocytes collected from women with endometriosis undergoing in vitro fertilization may show defects in the meiotic spindle apparatus or decreases in fertilization rates. Research on this disease is ongoing, but some scientists have hypothesized that such poor oocyte quality may be the result of increased immune-associated proteins or altered hormone levels in these patients.
Finally, other work has been performed to determine the effect of environmental factors on oogenesis, and their relation to aneuploidy. Chewing tobacco, hormone use (especially in older women) and even exposure to bisphenol-A, a component of many plastics, have all been suggested to affect oogenesis adversely and the process of meiosis therein.