1. Gametogenesis
Mona A. Elsafadi

2. Gametogenesis a process of formation and development of specialized generative cells (gametes)
Involves chromosomes and cytoplasm of the gametes.
Prepares sex cells for fertilization.
Chromosomes number is reduced by half.
The shape of the cells is altered.
Mona A. Elsafadi

3. Meiosis
is a process of reduction division in which the number of chromosomes per cell is cut in half and results in the formation of gametes.
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4. Meiosis Involves two meiotic divisions Takes place in germ cells only.
Produces haploid gametes ( sperms and oocytes).
Occurs during gametogenesis
Mona A. Elsafadi

5. Meiosis
Homologous chromosomes: pairs of chromosomes, one inherited from each parent and are similar in shape and size and have identical gene loci.
X and Y chromosomes are not homologues but they have homologous segments at the tips of their short arms. They pair at these regions only.
Mona A. Elsafadi

6. Meiosis
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7. Meiosis I has three unique features:
Synapsis: a process of forming complexes of homologous chromosomes following chromosome replication.
Homologous recombination (crossing over): the genetic exchange process that occurs between paired homologous chromosomes, so the sister chromatids in meiosis II are not always identical.
Reduction division: at the end of meiosis each cell contains only half the original complement of chromosomes.
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8. Second Meiotic Division
Has no DNA replication.
The haploid number of chromosomes is retained.
Each daughter cells has the reduced haploid number of chromosomes with single chromatid chromosomes.
Similar to an ordinary mitosis except that the chromosome number of the cell entering the second meiotic division is haploid.
Mona A. Elsafadi

9. Importance of meiosis:
Provides constancy of chromosome number by reducing it from diploid to haploid.
Allows random assortment of maternal and paternal chromosomes between the gametes.
Relocates segments of maternal and paternal chromosomes by crossing over of chromosome segments.
Mona A. Elsafadi

10. Spermatogenesis
The sequence of events by which spermatogonia are transformed into spermatozoa (mature sperms)
Mona A. Elsafadi

11. Spermatogenesis
Purpose: to produce mature sperms which are able to fertilize the oocytes to produce the zygotes.
Location:
Seminiferous tubules of the testes
Epididymis
Duration: about 64 days in human
Timing: begins at puberty and continues until death.
Mona A. Elsafadi

12. Mona A. Elsafadi

13. Stages of spermatogenesis
Spermatocytogenesis: at puberty spermatogonia proliferate by mitosis to yield primary spermatocytes.
Meiosis:
Meiosis I: formation of secondary spermatocytes (1N)
Meiosis II: formation of spermatids (IN)
Spermiogenesis: maturation process by which spermatids are transformed into mature sperms and supported by Sertoli cells.
Mona A. Elsafadi

14. Spermiogenesis
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15. Sertoli Cells Lining the seminiferous tubules.
Support and nurture the germ cells.
Involved in the regulation of spermatogenesis.
Mona A. Elsafadi

16. Epididymis
Elongated coiled duct along the posterior border of the testis
Continuous with the vas deferens which transports the sperms to the urethra.
Where sperms are stored and become functionally mature after transporting passively form seminiferous tubules.
Mona A. Elsafadi

17. Mature Sperm
Is a free-swimming, actively motile cell consisting of a head, a neck and a tail
Mona A. Elsafadi

18. Sperm’s Head Forms most of the bulk of the sperm.
Contains haploid nucleus.
Contains Acrosome:
Covers the anterior two-third of the nucleus
Caplike saccular organelle containing several enzymes, which facilitate sperm penetration of the corona radiata and zona pellucida during fertilization.
Mona A. Elsafadi

19. Sperm’s Tail Provides the motility of the sperm.
Consists of three segments:
Middle piece
Principal piece
End piece
The middle piece contains mitochondria, which provides ATP for activity.
Mona A. Elsafadi

20. Genes and molecular Factors
Bcl-2 family proteins involved in:
germ cells maturation
germ cells survival at different stages.
Y chromosome:
Essential for normal spermatogenesis
Microdeletions result in defective spermatogenesis and infertility
Mona A. Elsafadi

21. Oogenesis
Is the sequence of events by which oogonia are transformed into mature oocytes.
Mona A. Elsafadi

22. Oogenesis Timing: No. of oocytes: begins before birth
Is completed after puberty
Continues to menopause
No. of oocytes:
2 millions primary oocytes at birth
40,000 remain at adolescence
400 become secondary oocytes and ovulated
Mona A. Elsafadi

23. Oogenesis
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24. Prenatal Maturation
During early fetal life, oogonia proliferate by mitosis.
Before birth, oogonia enlarge to form primary oocytes.
Primordial follicle: primary oocyte surrounded by a single layer of flattened epithelial cells
Primary follicle: enlarged primary oocyte surrounded by zona pellucida and a single layer of columnar follicular epithelial cells
Secondary follicle: primary follicle with more than one layer of follicular cells
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25. Oocyte Maturation
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26. Prenatal Maturation First meiotic division
Begins before birth
Completed after adolescence (11-19 yrs)
Oocyte maturation inhibitor:
Secreted by primary oocytes
maintains the meiotic arrest
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27. Postnatal Maturation The primary oocytes remain dormant until puberty.
Shortly before ovulation, primary oocyte increases in size and complete first meiotic division to form secondary oocyte with first polar body.
At ovulation, secondary oocyte begins second meiotic division but arrested at metaphase.
If fertilization occurs, second meiotic division is resumed and produces second polar body.
Mona A. Elsafadi

28. Postnatal Maturation Begins during puberty
One follicle matures and ovulates each month
Long duration of first meiotic arrest may cause:
High frequency of meiotic errors (non-disjunction)
Primary oocytes to be vulnerable to environmental agents (radiation)
Mona A. Elsafadi

29. Comparison of Gametes No primary oocytes form after birth
Division of cytoplasm is unequal
Massive immotile cells
Surrounded by zona pellucida and a layer of follicular cells.
Only one kind of secondary oocyte: 23,X
Continuous production of primary spermatocytes
Division of cytoplasm is equal
Microscopic motile cells
Sparse cytoplasm and have heads covered with acrosomes and tails.
Two kinds of normal sperm: 23,X and 23,Y
Mona A. Elsafadi

30. Thanks
Mona A. Elsafadi