Lecture 3: Gametogenesis PDF

 Cell division There are 2 types of cell division Meiosis Mitosis  One parent cell divides into 4 daughter cells....

 Cell division There are 2 types of cell division Meiosis Mitosis  One parent cell divides into 4 daughter cells.  One parent cell divides into 2 daughter cells. wy  Each daughter cell has half the number of chromosomes in the parent cell.  All cells have the same number of chromosomes.  Each daughter cell has half the amount of DNA present in the parent cell.  All cells have the same amount of DNA.  Occurs in germ cells for reproduction  Occurs in autosomal cells for growth or tissue repair. da en H R. D DR.Hendawy “Anatomist" Page 1  Abnormal meiosis (Nondisjunction)  Non separation of a pair of homologous chromosomes during meiosis wy  May occur in the autosomes or sex chromosomes.  Usually occurs in the ova due to advanced maternal age.  One cell receives 24 chromosomes and the other 22 chromosomes. da  Fertilization of an abnormal gamete with a normal gamete of 23 chromosomes results in:  An individual of trisomy (47 chromosomes) eg: en  Down syndrome (trisomy 21)  Klinfelter syndrome (44+XXY) H  Superfemale syndrome (44+XXX).  An individual of monosomy (45 chromosomes) eg:  Turner syndrome (44+XO). R. D DR.Hendawy “Anatomist" Page 2  Gametogenesis Definition: process of formation of gametes (sperm or ovum). Site: gonads (testis & ovary). Aim: wy  Preparation of the germ cells for fertilization.  Reduction of the number of chromosomes from diploid to haploid. Spermatogenesis Definition: transformation of a spermatogonium into sperm. da Site: seminiferous tubules of the testis. Time: from puberty and continue to old age. Duration: about 2 months en Steps: A. Spermato-cyto-gnesis  Spermatogonia undergo mitosis to form daughter spermatogonia (46 S.chromosome, 2n DNA) H  Daughter spermatogonia replicate & differentiate into primary spermatocytes R. (46 D.chromosome, 4n DNA).  1ry spermatocytes undergo 1st meiosis to give two 2ry spermatocytes (23 D.chromosome, 2n DNA). D  2ry spermatocytes undergo 2nd meiosis to give two spermatids (23 S.chromosome, 1n DNA). DR.Hendawy “Anatomist" Page 3  B. Spermiogenesis Definition: morphological changes of spermatid to become mature sperm. Steps: wy 1. The nucleus forms most of the head. 2. Golgi apparatus surrounds the anterior 2/3 of the nucleus to form the acrosome 3. The mitochondria and the 2 centrioles move to the opposite pole. da The centriole elongates to form tail while the mitochondria forms a sheath that surrounds the proximal part of the tail 4. Shedding of most of the cytoplasm. en 5. Now it is called Mature sperm which consists of:  Head:  Contains the haploid number of chromosomes (22+X/ Y, n). H  Its anterior 2/3 are covered by acrosome.  Neck: contains centriole. R.  Tail: consists of:  Middle piece: contains mitochondrial sheath surrounding the proximal part of the tail.  Principle piece: longest part that provides sperm motility. D  End piece. DR.Hendawy “Anatomist" Page 4  Seminal fluid Source:  Sperms: formed in the testis.  Fluid: from the prostate, seminal vesicle & bulbourethral glands. wy Characters:  Sperms:  Number: 200 – 600 million.  Motility: more than 80 % are motile.  Fluid: da  Volume: 3-5 cc / ejaculation.  Color: white.  Odor: seminiferous odor.  Reaction: alkaline en Function:  It contain fructose that helps in sperm nutrition  It is essential for sperms motility.  It neutralizes the vaginal acidity Abnormality:  In number: H  Azo-spermia: absent sperms. R.  Oligo-spermia: sperm count less than 20 million/ml.  In motility: Necrospermia: dead sperms.  In shape: normally less than 10 %.  Giant sperms: very big sperms. D  Dwarf sperms: very small sperms.  Sperm with 2 heads.  Sperm with 2 tails. DR.Hendawy “Anatomist" Page 5  Oogenesis Definition: process by which oogonium is transformed into mature ovum Site: cortex of the ovary. Time: from intra-uterine life to menopause. wy Steps: A. Before birth  Oogonia undergo mitosis giving daughter oogonia (46 S.chromosome, 2n DNA)  Daughter oogonia replicate & differentiate into 1ry oocytes (46 D.chromosome, 4n DNA) da  1ry oocytes enter 1st meiosis but become arrested in prophase till puberty.  The female is born with ovaries containing about 2 million 1ry oocytes  Only 40,000 1ry oocytes are present at puberty. en B. After puberty  Every month, about 5-15 1ry oocytes complete 1st meiosis giving : One 2ry oocytes (23 D.chromosome, 2n DNA) & one 1st polar body H  Both 2ry oocytes & 1st polar body enter the 2nd meiosis and arrested at metaphase till fertilization. R.  If fertilization occurs, 2nd oocyte completes the 2nd meiosis giving mature ovum (23S.chromosome, 1n DNA) & 2nd polar body D  If fertilization doesn’t occur, both 2ry oocyte & 1st polar body degenerate N.B: Only about 480 2ry oocytes develop during the reproductive life of the female. DR.Hendawy “Anatomist" Page 6