لخّصلي

Early in fetal development, primitivegerm cells in the ovaries differentiate into oogonia.In spermatogenesis, four functional sperm develop from each primary spermatocyte.The diploid(46 chromosomes) primary oocytes replicatetheir DNAand begin the first meiotic division, but the processstops in prophaseand the cells remain in this suspendedstate until puberty.Many of the primary oocytes degenerate before birth, but even with this decline, the two ovaries together contain approximately 700,000 oocytes at birth.This is the lifetime supply, and no more will develop.If fertilizationoccurs, meiosis II continues,again this is an unequal division with all of the cytoplasm going to the ovum, which has 23 single-stranded chromosome.The large cellundergoes an unequal division so that nearly all the cytoplasm, organelles, and half the chromosomes go to one cell, which becomes a secondary oocyte.If fertilization does not occur, the second meiotic division is never completedand the secondary oocyte degenerates. These divide rapidly to form thousands of cells, still called oogonia,which have a full complementof 46 (23 pairs) chromosomes.This is quite different than the male in which spermatogonia and primary spermatocytescontinue to be produced throughout the reproductive lifetime.

Early in fetal development,
primitivegerm cells in the
ovaries differentiate into
oogonia. These divide rapidly
to form thousands of cells, still
called oogonia,which have a
full complementof 46 (23
pairs) chromosomes. Oogonia
then enter a growth phase,
enlarge, and become primary
oocytes.
The diploid(46 chromosomes) primary oocytes replicatetheir DNAand begin the first meiotic division, but
the processstops in prophaseand the cells remain in this suspendedstate until puberty.Many of the primary
oocytes degenerate before birth, but even with this decline, the two ovaries together contain approximately
700,000 oocytes at birth.This is the lifetime supply, and no more will develop. This is quite different than the
male in which spermatogonia and primary spermatocytescontinue to be produced throughout the reproductive
lifetime. By puberty the number of primary oocytes has further declined to about 400,000.
Beginning at puberty, under the influence of folliclestimulating hormone(FSH), several primary
oocytes start to grow again each month. One of the primary oocytes seems to outgrow the others and it resumes
meiosis I. The other cells degenerate. The large cellundergoes an unequal division so that nearly all the
cytoplasm, organelles, and half the chromosomes go to one cell, which becomes a secondary oocyte. The
remaining half of the chromosomes go to a smaller cell called the first polar body. The secondary oocyte
begins the second meiotic division, but the process stops in metaphase. At this point ovulationoccurs. If
fertilizationoccurs, meiosis II continues,again this is an unequal division with all of the cytoplasm going to the
ovum, which has 23 single-stranded chromosome. The smaller cell from this division is a second polar body.
The first polar body also usually divides in meiosis I to produce two even smaller polarbodies. If fertilization
does not occur, the second meiotic division is never completedand the secondary oocyte degenerates. Here
again there are obvious differences between the male and female. In spermatogenesis, four functional sperm
develop from each primary spermatocyte. In oogenesis, only one functional fertilizable cell develops from a
primary oocyte. The other three cells are polar bodies and they degenerate.