Gametogenesis PDF

At the end of this lecture the students should be able to: Describe the steps of oogenesis and spermatogenesis.  M www.A 4 IN ly ix if ya zygot Is are derived from primordial ▪Gametes germ cells (PGCs) that...

At the end of this lecture the students should be able to: Describe the steps of oogenesis and spermatogenesis.  M www.A 4 IN ly ix if ya zygot Is are derived from primordial ▪Gametes germ cells (PGCs) that y are formed in the epiblast during the 2nd week and that move to the wall of the yolk sac. 7  Ej from the yolk sac in't4th week these cells begin to migrate ▪During the toward the developing gonads, where they arrive by the end of the 5th 4 organ week. jj6W Testes ovary ▪Mitotic divisions increase their number during their migration and also when they arrive in the gonad. a N 8 81Gt'd Ill I kidney ▪ In general, gametogenesis has the same general sequence. ▪ N refers to the amount of DNA and n refers to the haploid number of chromosomes. ed Mitosis to produce Germ cell (2n) HB 4 27 1N 2ns 46 more germ cells N 46 5 II 20 92 Primary gametocyte (2n) 3k Onset of meiosis I 2N (meiosis I prophase) 888 y Complete meiosis I 2N 2N Secondary gametocyte (1n) 92 ar 00 Meiosis II 23 OOI.at 1N 1N 23 1N 1N Gametes (1n) eplication 23 23 23 Spermatids ovum  My 23 41 23 P I meiosis I 23 meisis  10 Prenatal maturation 5 th week ▪In female, once primordial germ cells arrive gonads they differentiate into oogonia. L germce ▪They undergo number of mitotic divisions to form clusters of oogonia that are surrounded by flat epithelial cells (follicular cells) at the end of 00 3rd month of pregnancy. Prenatal maturation ▪By the end of 5th month each ovary contain 7 millions of oogonia, but we some of them arrest their cell division in prophase of meiosis I and form primary oocytes. 2N 2n  12 Prenatal maturation IN 2n By the 7th month, majority of oogonia are degenerated except few of them near surface of the ovary which are differentiated into primary oocytes. IN 2n Each of the surviving primary oocytes that have entered 1st meiotic division is surrounded by a layer of flat epithelial cells to form primordial follicles.  13 oddly Postnatal maturation: Primary oocytes remain in prophase I and do not finish their 1st meiotic division before puberty is reached, apparently because of oocyte maturation inhibitor (OMI) that secreted from the follicular cells. Must w 6116 jgb154B'd o wit x 66,5  14 Postnatal maturation: ▪Number of primary oocytes at birth is 700,000 - 2 million. ▪During childhood is about 400,000 are present. ▪By beginning of puberty only less than 500 will be ovulated in the whole reproductive lifetime. Postnatal maturation ▪ With the onset of puberty, 15-20 primordial follicles begin to mature with each ovarian cycle. ▪ One primordial follicle continues as growing primary, secondary and Graafian follicles. Y ou mature follivels ▪ Rupture of Graafian follicle releases ovum which is taken by the fallopian tube fimbria to be ready for fertilization. oud a First I poly  000 Oogenesis (Postnatal maturation) Cont. ▪As soon as the follicle is mature, the primary oocyte completes its 1st meiotic division, leading to formation of two daughter unequal cells. 20 ON th tu an zo IN pm chromose Oogenesis (Postnatal maturation) Cont. 9186k ▪One cell is the secondary oocyte, receives most of the cytoplasm; A ▪the other is the 1st polar body, receives very little amount of the cytoplasm and is located between zona pellucida and cell membrane of the secondary oocyte. 2 2h23 23 Oogenesis (Postnatal maturation) Cont. ▪Each secondary oocyte or 1st Polar body has 23 chromosomes (1n) and double O amount of DNA (2N). e i Oogenesis (Postnatal maturation) Cont. IN ▪ The secondary oocyte enters the 2nd meiotic division without DNA replication. ▪ Then, the secondary oocyte is arrested in metaphase of meiosis II approximately 3 hours before ovulation. Oogenesis (Postnatal maturation) Cont. ▪ After ovulation, 2nd meiotic division is completed only if the oocyte is fertilized, otherwise, the oocyte is degenerated within 24 hours. ▪ The first polar body also enters the 2nd meiotic division without DNA replication. 46 Yr IN 46 2N In 46 In IN 23 27 27 23 Spermatogenesis includes all of the events by which spermatogonia are transformed into spermatozoa.  IN an At the time of birth germ cells in the male can be recognized in the sex cords of the testis as large and pale cells surrounded by supporting cells.  24 I Edit 65 and become the Shortly before puberty, sex cords acquire a lumen seminiferous tubules dand primordial germ cells give rise to spermatogonia. b sperms 95  25 Spermatogonia are of two types fromgermatocell ▪ spermatogonia Type A. 8 ▪ spermatogonia Type B. 9  Oo 26 ▪ In normal progression some of spermatogonia type A divide and d give rise to type B spermatogonia that subsequently undergo mitosis, and give rise to primary spermatocytes. f wks 2N an a J  27 ▪ Primary spermatocytes enter prolonged prophase of 1st meiotic division (22 days). mei us  28 ▪ This is followed by rapid completion of meiosis I and formation of secondary spermatocytes that contain 23 chromosomes (1n) and a double amount of DNA (2N). 2N In 231143g  29 ▪ Secondary spermatocytes begin to enter immediately the 2nd meiotic division to form spermatids. in Gmeiosis t 6 Ir 8  30 ▪ Throughout these series of events, from type A spermatogonia to spermatids, 64461 we cytokinesis is not completed. Thus the progeny of one type A spermatogonia form a cluster of cells that maintain contact throughout the differentiation. b b I just  31 MI In N remain contact with Sertoli cells for support and protection. However spermatids Spermatids (rounded cells) changes into sperms which are elongated motile cells by a process known as spermiogenesis. a large sporating  Pramingandsecondryoocytis sperm Changes in spermiogenesis include: ✓Formation of acrosome that help in penetration of the ovum. 22g Testes get ✓Condensation of the nucleus. epidydmis ✓Formation of neck, middle piece, and tail. ✓Shedding of most of the cytoplasm. I 5 g wit  33 as ▪ Sperms then enter the epididymis for full maturation and acquiring motility. ▪ In human, spermatogenesis require 64 days to be completed. ▪ It begins at puberty and continues throughout life. ▪ Each ejaculation contains about 225 million of sperm. ▪ Only one sperm fertilizes the ovum. Thank You