Immature, diploid female germ cells that undergoe two meiotic divisions to become mature ova or egg cells. Starting off as oogonia through a process of oocytogenesis, they become ova by a process of ootidogenesis. Oocytes are rich in cytoplasm that nourishes the cell during early development. Moreover, they are the recipients of mitochondria from maternal cells, and which go on to control embryonic metabolism and apoptosis. They are two types of oocytes: primary (appear before birth) and secondary (appear after birth). In humans, primary oocytes arise from the process of oocytogenesis during embryogenesis: oogonia in the ovaries divide and undergo the first meiosis resulting in the primary oocyte, but it only does so at the time ovulation. On completion of the first meiosis, the second bout of meiosis starts, but stops at the metaphase of this meiosis until fertilization by the sperma takes place. Every year, millions of oocytes are harvested throughout the world, and cultured in in vitro fertilization clinics as a means offering infertile couples the opportunity of having their own children.
See Apoptosis (or cell death), Diploid, Embryogenesis, Germ plasm, Meiosis (or reduction division), Metaphase, Mitochondria, Oogenia, Spermatogonia, Spermatozoa