Gametogenesis (Histology & Cell Biology) PDF

Ajman University College of Dentistry Histology & Cell Biology BDS105 Dr. Al-Moutassem Billah Khair Gametogenesis  Gametogenesis The process of formation and development of gametes ( germ cells ) in reproductive organs Primordial germ cells (2n)...

Ajman University College of Dentistry Histology & Cell Biology BDS105 Dr. Al-Moutassem Billah Khair Gametogenesis  Gametogenesis The process of formation and development of gametes ( germ cells ) in reproductive organs Primordial germ cells (2n) ♂ Spermatogonia ( 2n ) Oogonia ( 2n ) ♀ Spermatogenesis Oogenesis 4 Sperms (1n ) 1 Ovum ( 1n ) 3 Polar bodies Fertilization Zygote (2n) Embryo  Spermatogenesis Primordial germ cells ( 2n )  At the time of the birth, germ cells in male can be recognized in the sex cords of the testis as large, pale cells surrounded by supporting cells (epithelial cells), which become Sertoli cells.  Shortly before puberty, the sex cords become the seminiferous tubules. At the same time primordial germ cells give rise to:  Spermatogonia ( 2n ) Type A Type B Mitosis   Continuous reserve of stem cells (2n) Mitosis   On completion of  the last division Type B Spermatogenesis Type B ( 2n )  Primary spermatocytes ( 2n ) Enter prolonged prophase ( 22 days ) Rapid completion of the 1st meiosis  Secondary spermatocytes ( 2n DNA ) (23 double-structured chromosomes)  2nd meiosis  Spermatids ( 1n ) ( 1n DNA, 23 chromosomes )  Spermatogenesis Spermatids ( 1n ) ( 1n DNA, 23 chromosomes )  Spermiogenesis: The series of changes resulting in the transformation of spermatids into spermatozoa: *formation of the acrosome *condensation of the nucleus *formation of neck, middle piece & tail *shedding of most of the cytoplasm  Spermatozoa ( 1n ) S p e r m a t o g e n e s I s Oogenesis Prenatal maturation Primordial germ cells ( 2n ) ( 3rd week of development )  Gonads( 4th –5th week )  Differentiation of cells  Oogonia ( 2n )  Clusters ( end of the 3rd month ) ( surrounded by layer of flat epithelial cells )  most of oogonia divide by mitosis   Oogenesis most of oogonia divide by mitosis   increase in number some differentiate into of oogonia primary oocytes ( 2n ). [ 5th month ≈ 7X106 ] Immediately, they enter prophase of 1st meiosis.   5th –7th month cell death take place and the most of oogonia have degenerated except for a few near the surface. All surviving primary oocytes have entered the 1st meiosis & most of them are surrounded by a layer of flat epithelial cells.  Primordial follicles ( 4n ) Oogenesis Postnatal maturation Near the time of birth, all primary oocytes (4n) have started prophase of the 1st meiosis (diplotene) and remain in it & do not finish their division before puberty is reached, because of OMI OMI (a substance secreted by follicular cells that stops the progress of meiosis). [ 7X105 – 2X106 cells at birth ]  With the onset of puberty primary oocytes increase in size and flat follicular cells cuboidal cells granulosa cells [ 4X105 cells at the beginning of puberty ]  Primary follicle Granulosa cells & Oocyte Layer of glycoproteins ( Zona pellucida ) Oogenesis Primary follicle Granulosa cells & Oocyte Layer of glycoproteins ( Zona pellucida )  changes in epithelial cells  Secondary follicle  At maturity follicle becomes graafian follicle  maturation of Primary oocyte one reaches full maturity every 4 weeks (Ovarian cycle) [ 500 primary oocytes will be ovulated in the reproductive lifetime of the female individual ] Oogenesis Primary oocyte resumes its 1st meiosis  2 daughter cells of unequal size, but each with 23 double structured chromosomes (4n)   Secondary oocyte 1st Polar body (2n DNA, 1n chromo.) (2n DNA, 1n chromo.) (most of cytoplasm) (none cytoplasm)   2nd meiosis without DNA replication   Secondary oocyte (in metaphase, 2nd meiosis)   Fertilization   Fertilized ovum (1n+1n ♂) 2 polar bodies (1n) & polar body (1n) each with 23 single chromosomes Oogenesis Oogenesis Oogenesis