|See also oocyte the formation and maturation of ova from undifferentiated cells in the ovary|
oogenesis o·o·gen·e·sis (ō'ə-jěn'ĭ-sĭs)
The formation and the development of the ovum. Also called ovigenesis.
|oogenesis (ō'ə-jěn'ĭ-sĭs) Pronunciation Key
The formation, development, and maturation of an ovum or egg cell.
oogenetic adjective (ō'ə-jə-nět'ĭk)
Our Living Language : The details of the exact nature of oogenesis vary by species, since the females of some species produce thousands of eggs at a time, while in others, females produce relatively few mature eggs. The human female, for example, ovulates only about 400 times during her lifetime. Oogenesis in humans begins in embryonic and fetal development, when diploid germ cells called oogonia divide by mitosis to produce cells called primary oocytes. The primary oocytes of the female fetus enlarge and begin to undergo meiosis (in which the number of chromosomes will be reduced by half). But they are suspended in an early phase of meiosis called prophase until the female reaches puberty. The human female has about 700,000 such primary oocytes at birth. After puberty, one of the oocytes resumes development each month in response to changes initiated by follicle-stimulating hormone. The primary oocyte undergoes the first meiotic division, producing a cell called a secondary oocyte and another called the first polar body. During cytokinesis, most of the cytoplasm of the primary oocyte moves to the secondary oocyte. The first polar body undergoes the second meiotic division and its daughter cells degenerate. The secondary oocyte is released from the ovary during ovulation. If it encounters a spermatozoon and fertilization is initiated, the secondary oocyte undergoes the second meiotic division, producing the ovum and a second polar body that degenerates. The spermatozoon then fertilizes the ovum. If the secondary oocyte does not encounter a spermatozoon, it does not undergo the second meiotic division and simply degenerates.
in the human female reproductive system, growth process in which the primary egg cell (or ovum) becomes a mature ovum. In any one human generation, the egg's development starts before the female that carries it is even born; 8 to 20 weeks after the fetus has started to grow, cells that are to become mature ova have been multiplying, and by the time that the female is born, all of the egg cells that the ovaries will release during the active reproductive years of the female are already present in the ovaries. These cells, known as the primary ova, number around 400,000. The primary ova remain dormant until just prior to ovulation, when an egg is released from the ovary. Some egg cells may not mature for 40 years; others degenerate and never mature.
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