Oogenesis | How egg cells are produced |
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3 سال پیش
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Oogenesis begins in the female
Oogenesis begins in the female embryo with the production of oogonia from primordial germ cells. The oogonia divide by mitosis to form cells that begin meiosis, but stop the process at prophase I before birth.
These developmentally arrested cells, which are primary oocytes, each reside within a small follicle, a cavity lined with protective cells. To the best of our current knowledge, women are born with all the primary oocytes they will ever have.
Beginning at puberty, follicle-stimulating hormone (FSH) periodically stimulates a small group of follicles to resume growth and development. Typically, only one follicle fully matures each month, with its primary oocyte com pleting meiosis I.
The second meiotic division begins, but stops at metaphase. Thus arrested in meiosis II, the secondary oocyte is released at ovulation, when its follicle breaks open. Only if a sperm penetrates the oocyte does meiosis II resume. Each of the two meiotic divisions involves unequal cytokinesis, with the smaller cells becoming polar bodies that eventually degenerate (the first polar body may or may not divide again). As a result, the functional product of complete oogenesis is a single mature egg containing a sperm head. Fertilization is defined strictly as the fusion of the haploid nuclei of the sperm and secondary oocyte, although the term is often used loosely to mean the entry of the sperm head into the egg.
These developmentally arrested cells, which are primary oocytes, each reside within a small follicle, a cavity lined with protective cells. To the best of our current knowledge, women are born with all the primary oocytes they will ever have.
Beginning at puberty, follicle-stimulating hormone (FSH) periodically stimulates a small group of follicles to resume growth and development. Typically, only one follicle fully matures each month, with its primary oocyte com pleting meiosis I.
The second meiotic division begins, but stops at metaphase. Thus arrested in meiosis II, the secondary oocyte is released at ovulation, when its follicle breaks open. Only if a sperm penetrates the oocyte does meiosis II resume. Each of the two meiotic divisions involves unequal cytokinesis, with the smaller cells becoming polar bodies that eventually degenerate (the first polar body may or may not divide again). As a result, the functional product of complete oogenesis is a single mature egg containing a sperm head. Fertilization is defined strictly as the fusion of the haploid nuclei of the sperm and secondary oocyte, although the term is often used loosely to mean the entry of the sperm head into the egg.
3 سال پیش
در تاریخ 1400/11/26 منتشر شده
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