2. 2
Introduction
• Mitosis occurs in somatic cells to give 2 identical daughter cells.
• Meiosis occurs only in germ cells to generate male and female
gametes, sperms and Oocytes respectively.
•Differences between Mitosis and Meiosis :
1. Duplication of DNA and division of Centromere occurs in
two separate cell cycles in Meiosis.
2. Crossover between nonsister chromatids to interchange
chromatid fragments (genes) occurs in Meiosis.
•Sex chromosome components:
•The pair of sex chromosomes is XX in female and XY in male.
•The difference in sex chromosome complement forms the basis
of primary sex determination.
3. Gametogenesis
•Gametogenesis is the process of formation of mature germ cells
(gametes).
• By the end of the 5th week, migrated primordial germ cells (PGCs)
arrive to the developing Gonads (Ovary / Testis) and form the primitive
gametes (primitive ova or sperm).
•In preparation for fertilization, germ cells increase their number by
Mitosis and undergo gametogenesis. Gametogenesis includes,
Meiosis to reduce number of chromosomes (1n) and cytoplasmic
changes to complete their maturation.
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5. •Oogenesis-1
The process of formation of mature ovum ready for fertilization.
This maturation process begins during fetal life and completed after puberty.
• Main function of the ovary to
produce mature Ovum ready for
fertilization.
•Oogenesis occurs in the cortex of
ovary which, contains Oocytes at
various stages of development.
• Oogenesis is a repetitive monthly
ovarian cycle during the females
reproductive period.
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6. Oogenesis-2
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• By end of 5wk:
The arrived PGCs to the developing ovary undergo several mitotic
divisions and differentiate into (primitive ovum) Oogonia (2n) cells.
By 5th month, the number of Oogonia reach about 7 million.
7. Oogenesis-3
During fetal life: Mitotic divisions of oogonia produce 1ry oocytes
that arrested in a prolong Prophase Meiosis I until puberty.
Each 1ry Oocyte becomes surrounded by a one layer of flat epithelial
cells to form an ovarian follicle called the Primordial follicle.
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Many1ry Oocytes will be degenerated
during fetal life.
The ovary of the newborn contain
about 2 million of 1ry Oocytes but,
most degenerate during childhood
and about 40,000 remain to puberty.
Of the remaining, only about 400 will
mature and ovulated during the female
reproductive age (400 ÷ 12 = 33 years).
8. Oogenesis- 4
At puberty:
Each month about 20 ovarian follicles
begin to develop but usually only one
reaches maturity. When the 2ry follicle
is mature a surge (flow) of LH induces
the preovulatory growth phase.
Meiosis I is completed, resulting in
formation of 2ry Oocyte and 1st polar
body.
At Ovulation:
The 2ry Oocyte enter Meiosis II but
arrested in Metaphase II and
only completed it in Fertilization.
Ovulated ovum enter into uterine tube
in anticipation to be fertilized and stay
viable only ~ 24 hours.
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Dr. Maher Hadidi
9. REMEMBER:
•Gametogenesis begins in males at puberty while in females it
begins during fetal life.
•Thus, the time required to produce a mature ovum may extended
to 40-45 years; for this reason, chromosomal abnormalities are
attributed to the Female rather to the male.
•During childhood the ovary is inactive therefore, degenerations and
infections happens during this period
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Dr. Maher Hadidi
