Oogenesis is the process by which the female reproductive cell, the ovum or egg cell, is formed and matures. This specialized type of cell division results in a cell containing half the genetic material necessary for creating a new organism. The creation of a fully developed ovum is fundamental to sexual reproduction, as it carries the female’s genetic contribution and contains the stored nutrients to sustain early development.
The Timing and Location of Oogenesis
The timeline of oogenesis is exceptionally long, beginning years before birth and potentially concluding decades later. The process starts within the developing ovaries of a female fetus, where primordial germ cells multiply rapidly through mitosis, creating a large pool of diploid cells called oogonia. This phase of cell proliferation is largely complete by the time the female is born, setting a fixed number of potential egg cells for her entire lifespan.
By the end of the fetal period, the oogonia transition into primary oocytes. These cells immediately begin the first stage of meiosis, the division process that reduces the chromosome number. This meiotic division is not completed; it arrests in prophase I, entering a prolonged resting phase called the dictyate stage. These primary oocytes reside within small sacs of cells known as primordial follicles, located in the cortex of the ovaries.
The arrested state is maintained until the onset of puberty, where hormonal signals periodically stimulate a small number of follicles to mature. Over the reproductive years, only a few hundred of the original pool of hundreds of thousands of primary oocytes will complete the next stage of development. The process continues cyclically, with one primary oocyte typically completing division each menstrual cycle, until the reserve is depleted at menopause.
The Cellular Stages of Ovum Development
The earliest stage involves the mitotic division of the diploid oogonia within the fetal ovary. These cells divide multiple times to build the stock of future germ cells. They then differentiate into primary oocytes, which are also diploid, containing 46 chromosomes, each consisting of two chromatids.
The completion of Meiosis I is triggered by hormonal surges just before ovulation. This division is unusual because the cell material is split unevenly, a process known as unequal cytokinesis. The outcome is two very different cells: a large secondary oocyte and a much smaller first polar body.
The secondary oocyte receives nearly all of the original cell’s cytoplasm and stored nutrients, which are needed to support a potential embryo. Both the secondary oocyte and the first polar body are now haploid, containing 23 chromosomes that still consist of duplicated chromatids. The first polar body is discarded genetic material and typically degenerates.
Immediately following its formation, the secondary oocyte begins Meiosis II but quickly halts again at metaphase II. It is in this arrested state that the cell is released from the ovary during ovulation. The completion of Meiosis II will only occur if a sperm cell successfully penetrates the secondary oocyte.
If fertilization occurs, the secondary oocyte completes the final division, which again involves unequal cytokinesis. This results in the formation of the mature ovum and a second polar body. The mature ovum is a fully haploid cell with 23 single chromosomes, ready to fuse its nucleus with that of the sperm. The second polar body, like the first, is a small cell containing excess chromosomes that eventually disintegrates.
The Mature Ovum and Fertilization
The mature ovum is one of the largest cells in the human body, measuring approximately 120 micrometers in diameter. This size results from the unequal cytokinesis throughout meiosis, which ensures the ovum retains a large volume of cytoplasm rich in proteins and messenger RNA. These stored cytoplasmic components provide the initial nourishment and regulatory molecules for the developing embryo before it can implant in the uterus.
The ovum is encased in a thick, transparent layer of glycoproteins called the zona pellucida. This layer is surrounded by follicular cells known as the corona radiata, which help nourish the ovum and must be penetrated by a sperm for fertilization. The zona pellucida also prevents multiple sperm from fertilizing the egg, a process called polyspermy.
Oogenesis is completed only upon the successful fusion of the sperm and the ovum. Once the mature ovum is formed, its nucleus combines with the sperm’s nucleus, restoring the full diploid number of chromosomes and forming the zygote. If the secondary oocyte is not fertilized within about 24 hours of ovulation, it degenerates, and the final meiotic division never occurs.