25.2
Oogenesis
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 f…
In humans mature eggs are generated through the process of oogenesis.
Oogenesis commences during embryonic development in the forming female ovaries. Here diploid precursor cells individually termed oogonium enter mitosis, and divide to form primary oocytes. Each of which is surrounded by supporting cells.
Such oocytes begin the first stage of meiosis. Meiosis I, which proceeds until maternal and paternal chromosomes exchange information. At this point meiosis halts due to support cell derived inhibitory factors.
The process only restarts during puberty. Here menstrual cycle signals cause a primary oocyte to enlarge and reinter Meiosis I, producing two haploid cells of unequal sizes. The larger cytoplasm rich cell is termed a secondary oocyte, while the smaller first polar body eventually dissolves.
The secondary oocyte briefly enters the second phase of meiosis, meiosis II. But this process again stops. Ovulation then releases this cell and those surrounding it from the ovary. And the complex migrates into a fallopian tube where fertilization occurs if sperm are present.
Meiosis resumes post fertilization and ends with a secondary oocyte dividing forming an undersized second polar body which similarly dissipates, and a larger ovum. The latter is the haploid mature egg in which distinct paternal and maternal nuclei are transiently visible.
Thus oogenesis only yields a single mature egg cell for every primary oocyte that successfully completes the process.
View the full transcript and gain access to JoVE Core videos
Q1: When does oogenesis begin in human females?
Oogenesis commences during embryonic development in the forming female ovaries. Diploid precursor cells called oogonia enter mitosis and divide to form primary oocytes, each surrounded by supporting cells. These primary oocytes then begin meiosis I, which halts until puberty when menstrual cycle signals restart the process.
Q2: Why does oogenesis produce unequal cell divisions?
Oogenesis produces asymmetric divisions to ensure the mature egg receives sufficient cytoplasm and nutrients for early development. During meiosis I, a large secondary oocyte containing most cytoplasm forms alongside a smaller first polar body that dissolves. This unequal distribution repeats in meiosis II, guaranteeing the ovum has adequate resources to support embryonic growth after fertilization.
Q3: What happens to meiosis during oogenesis after ovulation?
After ovulation, the secondary oocyte enters the fallopian tube where fertilization may occur. Meiosis II remains arrested until sperm fertilizes the egg. Upon fertilization, meiosis II resumes and completes, producing a second polar body that dissolves and a mature haploid ovum with distinct paternal and maternal nuclei.
Q4: How does maternal age affect oogenesis and chromosome segregation?
Increased maternal age elevates the risk of chromosome segregation errors during meiosis I or II, potentially causing aneuploidy—an abnormal chromosome number. Proposed explanations include degradation of the meiotic spindle apparatus over time or accumulation of abnormal cells in ovaries. Women 35 and older are typically offered prenatal testing to detect chromosomal abnormalities.
Q5: How does endometriosis impact oocyte quality?
Endometriosis, where uterine lining accumulates outside the uterus, may compromise oocyte quality. Oocytes from women with endometriosis show defects in the meiotic spindle apparatus or reduced fertilization rates during in vitro fertilization. Scientists hypothesize poor oocyte quality results from increased immune-associated proteins or altered hormone levels in affected patients.
Q6: What environmental factors can negatively affect oogenesis?
Environmental exposures including tobacco use, hormone supplementation, and bisphenol-A from plastics have been suggested to adversely affect oogenesis and meiosis. These factors may increase the risk of aneuploidy and compromise oocyte quality. Research continues to clarify the mechanisms by which environmental stressors influence the oogenesis process.
Q7: Why does oogenesis yield only one mature egg per primary oocyte?
Oogenesis produces a single mature ovum because meiotic divisions are asymmetric. Meiosis I generates a large secondary oocyte and a small first polar body; meiosis II produces a large ovum and a second polar body. The smaller polar bodies dissolve, ensuring only one egg cell with sufficient cytoplasm and nutrients completes development after cleavage and blastulation.