The oocyte, also known as the egg, is the female gamete or reproductive cell. In humans, it is produced and stored in the ovaries until it is released during ovulation, which typically occurs once a month during the menstrual cycle.
The oocyte undergoes a process called meiosis, which involves two divisions, to produce a haploid cell with half the number of chromosomes as the parent cell. The oocyte is arrested at an early stage of the first meiotic division as a primary oocyte within the ovary. During each menstrual cycle, pituitary gonadotropin stimulates the completion of the first meiotic division, resulting in the formation of a secondary oocyte and a polar body. The secondary oocyte is then arrested at the metaphase of the second meiotic division until it is fertilized by a sperm.
Early oocytes are also classified as immature, either germinal vesicle (GV) or metaphase I (MI) stage. The breakdown of the germinal vesicle indicates a resumption of meiosis and the extrusion of the first polar body indicates completion of the first meiotic division in human oocytes.
The released oocyte is surrounded by a thick specialized extracellular matrix, called the zona pellucida, which protects the oocyte and also plays a role in fertilization. The zona pellucida is covered in layers of cells, called the granulosa layer, which provide support and nutrients to the developing oocyte.
In assisted reproductive technology, such as Aashakiran IVF, the oocytes are retrieved from the ovaries and fertilized in vitro with sperm to create embryos. The development of a primordial follicle containing an oocyte to a preovulatory follicle takes over 120 days in adult human females, and careful monitoring and hormonal stimulation may be used to increase the number of oocytes available for retrieval during IVF.