Fertilization

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  • 1. Fertilization
  • 2. • “Verily WE created insan from mixture of germinal drop (fertilized egg) in order to try him: so We gave him (the gifts) of Hearing and Sight”. • (Surah 76, Ad-Dahr or Al-Insan, Ayat 2, Para 29)
  • 3. • Fertilization is the union of mature male and female gametes to form a viable zygote capable of producing a new human being. • • Fertilization occurs in the ampullary part of uterine tube.
  • 4. • Any male can fertilize any female. But this is forbidden in Islam. • They will marry. Without marriage it is zina (adultry). • Remember zina is one of the kbair (the big sins). The nikah will be in front of the society. • The guardian of the lady will hand over her to the gentleman. • The man will accept her in front of the society and they will become husband and wife.
  • 5. • So the legal fertilization accepted in Islam, will be the union of a mature gamete (sperm) of husband with a mature gamete (secondary oocyte) of wife in the ampullary part of uterine tube.
  • 6. Oocyte transport • At the time of ovulation, the fimbriae, of the fallopian tube, move over the ovary and sweep the ovulatory mass containing secondary oocyte, into the infundibulum (infundibular part of uterine tube).
  • 7. • The ovulatory mass is then guided into the ampulla through the movements of cilia, and by muscular contraction of uterine tube wall. • It has been estimated that the ovulatory mass takes about 25 minutes to reach the site of fertilization.
  • 8. Sperm transport • Usually 200 to 500 million sperms are deposited on the cervix and in the posterior fornix of the vagina at intercourse. • • So 200 to 500 million sperms are for one secondary oocyte. • See the importance of one female mature gamete.
  • 9. • The sperms pass by movements of their tails through the cervical canal • but • The passage of sperms through the uterus and uterine tubes appears to be assisted by muscular contraction of the walls of these organs.
  • 10. • The prostaglandins present in the seminal plasma may stimulate uterine motility at the time of intercourse and assist in the movement of sperms through the uterus and the uterine tubes to the site of fertilization.
  • 11. • It is not known how long it takes sperms to reach the fertilization site but the time of transport is probably short. • Only 300 to 500 sperms reach the fertilization site.
  • 12. • The secondary oocyte is usually fertilized within 12 hours after ovulation. • Observations have shown that in vitro the unfertilized human secondary oocyte dies within 12 to 24 hours.
  • 13. • Most sperms probably do not survive for more than 24 hours in the female genital tract. • Many sperms are stored in the crypts, glands and even in the cervical mucus. • This storage of sperms ensures a constant realize of sperms and increases the chances of fertilization. • So some sperms may be able to fertilize a secondary oocyte even after 24 hours.
  • 14. • It is also known that not all parts of the ejaculate are equally potent in the fertilization process. • In the first portion of ejaculate are spermatozoa, epididymal fluid, and the secretions from the prostate gland.
  • 15. • In the last portions of the ejaculate are the secretions of the seminal vesicles. • Thus spermatozoa in the initial portion of the ejaculate have better motility and survival than those in the later portion.
  • 16. • It is generally believed that fertilization cannot be delayed until the oocyte reaches the uterus, since it becomes overripe and undergoes dissolution and degeneration in the uterus.
  • 17. Conditioning of sperms • The sperms in the female genital tract, pass through two processes before fertilization • 1. Capacitation • 2. Acrosome reaction
  • 18. • Capacitation is an activation process that takes about seven hours. • The glycoprotein coat and plasma proteins are removed from the plasma membrane over the acrosome.
  • 19. • Capacitation starts in the uterus and continues in the uterine tubes. • The follicular fluid, released at ovulation, enhances the process. • Look at the reception given to sperms. • Completion of capacitation permits the acrosome reaction to occur.
  • 20. • Acrosome reaction occurs during the passage of sperm through the corona radiata. • The outer membrane of the acrosome fuses at many places with the overlying cell membrane of the sperm head. • The fused membranes then rupture, producing multiple perforations through which the enzymes in the acrosome escape and ooze out.
  • 21. • Progesterone seems to stimulate the acrosome reaction (Austin, 1975). • It is present in large amounts in follicular fluid released at ovulation and between the follicular cells of corona radiate.
  • 22. • During the acrosome reaction following substances (enzymes) are released: • 1. Hyaluronidase, needed to assist in penetration of corona radiata cells • 2. Trypsin-like substances, needed for the digestion of zona pellucida • 3. Acrosin or Zona lysine is also needed to help the sperm to cross zona pellucida.
  • 23. Penetration through Corona Radiata • Of the 200 to 500 million sperms deposited in the female genital tract only 400 reach the site of fertilization. • Only one of these sperms is marked for fertilization. • ALLAH Himself knows the secrets of this super selection of genetic material.
  • 24. • The other sperms help the fertilizing sperm in penetrating the corona radiata, the first barrier protecting the female gamete. • Probably the tails of the sperms help to disperse the corona radiata cells. • The enzymes released from the acrosome, principally hyaluronidase, also help in the dispersal of corona radiata cells.
  • 25. Penetration of Zona Pellucida • The enzyme acrosin causes lysis of zona pellucida. • Trypsin like substances digest zona pellucida. • Once the sperm touches zona pellucida, it becomes firmly attached to it and penetrates it rapidly.
  • 26. • As one sperm passes through zona pellucida, the permeability of zona pellucida changes and the zona pellucida becomes impermeable to others sperms. • This is called zona reaction. • It is believed that granules released from the secondary oocyte, which contain lysosomal enzymes, produce this zona reaction.
  • 27. Penetration of cell membrane • After passing through zona pellucida, the sperm becomes attached to the cell membrane of secondary oocyte. • Since the plasma membrane covering the acrosome has already disappeared during acrosome reaction, the remaining membrane that covers the posterior region of the sperm head, fuses with the oocyte plasma membrane.
  • 28. Formation of Zygote • The head and tail of the sperm enter the cytoplasm of the oocyte, leaving the sperm’s plasma membrane attached to the plasma membrane of secondary oocyte.
  • 29. • The secondary oocyte now completes its second meiotic division, forming a mature ovum and the second polar body. • The nucleus of the ovum is known as the female pronucleus. • Once within the cytoplasm of the ovum the tail of the sperm rapidly degenerates, and its head enlarges to form the male pronucleus.
  • 30. • The male and female pronuclei approach each other in the center of the ovum, where they come into contact and loose their nuclear envelopes. •
  • 31. • The maternal and paternal chromosomes intermingle at equatorial plate of metaphase. • Now the cell is called zygote. • It immediately completes its first mitotic division and the zygote reaches two-cell-stage.
  • 32. • So now we can say that fertilization is the sequence of events that begins with contact between a sperm and a secondary oocyte and ends with the intermingling of the maternal and paternal chromosomes.
  • 33. Consequences of Fertilization • Restoration of diploid number of chromosomes. Fusion of the two (male and female) germ cells produces a zygote, a diploid cell with 46 chromosomes. • • Restoration of diploid amount of DNA.
  • 34. • Meiosis in the male and female germ cells allows independent assortment of maternal and paternal chromosomes. • Crossing over reallocates and shuffles the genes and there is a recombination of genetic material.
  • 35. • The zygote contains a new combination of chromosomes that is different from those of parents. • Through this mechanism ALLAH forms the bases of biparental inheritance and results in variation of the human species.
  • 36. • Zona pellucida and the cell membrane of secondary oocyte become impermeable to other sperms thereby preventing the polyspermy. • Fertilization initiates the development by stimulating the zygote to undergo a series of rapid mitotic cells divisions called cleavage.
  • 37. • Fertilization by an X-bearing sperm produces an XX zygote which will develop into a female, whereas fertilization by a sperm with Y chromosome produces an XY zygote, which will develop into a male. • Hence it is the father whose gamete determines the sex of their offspring.