oogenesis and ovarian cycle

OOGENESIS
• Oogenesis (ovogenesis) is the sequence of events by which oogonia are transformed into mature oocytes
• This maturation process begins before birth and is completed after puberty
• Oogenesis continues to menopause, which is permanent cessation of the menses (bleeding associated with the menstrual cycles)
 2 maturation processes of oocytes are involved;
 Prenatal maturation
 Postnatal maturation
Prenatal Maturation of Oocytes
• Primordial germ cells (46, 2N) migrate from the wall of the yolk sac and arrive in the ovary at 5th week and differentiate into oogonia (46, 2N), which populate the ovary through mitotic division.
• A majority of oogonia undergo mitotic division to form primary oocytes
(46, 2N)
• by the 5th month of prenatal development, the total number of germ cells in the ovary reaches its maximum, which is estimated at 7 million

 Note: All primary oocytes are formed by the 5th month of fetal life
• At this time, cell death begins, and many oogonia as well as primary oocytes become atretic
• By the 7th month, the majority of oogonia have degenerated except for a few near the surface
 Note: No oogonia are present at birth
• All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a single layer of flattened, follicular epithelial cells
• The primary oocytes enclosed by this layer of cells constitutes a primordial follicle
• Primary oocytes begin the first meiotic division before birth, but completion of prophase 1 does not occur until puberty
• The follicular cells surrounding the primary oocyte are believed to secrete a substance, oocyte maturation inhibitor (OMI) which keeps the meiotic process of the oocyte arrested

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Postnatal Maturation of Oocytes
Beginning during puberty, usually one follicle matures each month and ovulation occurs, except when oral contraceptives are used The total number of primary oocytes at birth is estimated to vary from 600,000 to 800,000