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Information about Fertilization

Published on February 22, 2009

Author: msaeedshafi



Prof. Dr. Saeed Shafi : FERTILIZATION Prof. Dr. Saeed Shafi Slide 2: Maria, a 19 years old school teacher intends to perform hajj with her family. Her menstrual cycle is due during hajj days. Being her family physician you can advise her to use: LH FSH hCG Diethyl stilbesterol Progesteron Slide 3: One of the followings is responsible for transport of female gamete to the site of fertilization: Uterine contractions Tubal peristalsis Stereocilia of fallopian tube Tubal mucosal enzymes Chemotaxis by sperms Slide 4: The principle mechanism leading to ovulation is:  Raised intrafollicular pressure Contraction of smooth muscles in theca externa Enzymatic digestion of follicular wall Increased FSH production from pituitary gland Completion of first meiotic division of primary oocyte Learning Objectives? : Learning Objectives? What is primordial/primary/secondary and mature follicle? Mechanism of Sperm & Ovum transport ? Why females have a limited reproductive life & more prone to environmental hazards? Why only one sperm fertilizes the ovum? What is Fertilization & where it takes place? Define capacitation, Acrosome & Zona reaction? Slide 6: Fertilization is a complex 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 at metaphase of 1st cleavage of Zygote. Slide 11: SPERMIOGENESIS Slide 13: OVULATION Slide 14: OVULATION OOCYTE TRANSPORT : OOCYTE TRANSPORT At the time of ovulation, the fimbriae move over the ovary and sweep the ovulatory mass containing secondary oocyte, into the infundibulum. The ovulatory mass is then guided into the ampulla through the movements of cilia, and by muscular contraction of uterine tube wall (Tubal Peristalsis). It has been estimated that the ovulatory mass takes about 25 minutes to reach the site of fertilization. SPERM TRANSPORT : SPERM TRANSPORT Usually 200 to 500 million sperms are deposited. Sperms pass by movements of their tails through uterus & tubes; assisted by muscular contraction. Prostaglandins present in the seminal plasma may stimulate uterine motility at the time of intercourse. It is not known how long it takes sperms to reach the fertilization site but the time of transport is short 5-45minutes. Oocyte & sperm transport : Oocyte & sperm transport Sperm tail motility Vesiculase in seminal secretions form vaginal plug & prevents backflow of semen into vagina. At ovulation , cervical mucous become less viscid. Uterine contractions due to PG in semen Acid pH of vagina decreases sperm motility Alkaline pH of uterus increases sperm motility Chemotaxis by ovum Secondary Oocyte move by Tubal peristalsis Contents of semen : Contents of semen Sperms < 10% Seminal secretions – 60% Prostatic secretions – 30% Bulbo-uretbral glands – 10% Normal 100 M/ml of ejaculate (2 – 3 ml) < 50 M total or 20 M/ml of semen show sterility CONDITIONING OF THE SPERMS : CONDITIONING OF THE SPERMS The sperms in the female genital tract, before fertilization undergo Capacitation Acrosome reaction Capacitation : Capacitation Starts in uterus& continues in to tubes. Follicular fluid enhances the process. It takes about 7 hours. Glycoproteins are removed from plasma membrane over acrosome. Capacitated sperms show no morphological change, but more active Completion of capacitation permits acrosome reaction to occur. Acrosome reaction : Acrosome reaction occurs during passage of sperm through corona radiata. Outer membrane of the acrosome fuses at many places with overlying cell membrane of sperm head. fused membranes then rupture, producing multiple perforations through which the enzymes (Hyaluronidaze) in the acrosome escape and ooze out. Slide 22: Hyaluronidase: needed to assist in penetration of the corona radiata barrier; Trypsin-like substances: needed for the digestion of the zona pellucida; Acrosin: also needed to help the sperm cross the zona pellucida. Progesterone (present in follicular fluid) seems to stimulate the acrosome reaction. ACROSOME REACTION Phases of fertilization : contd. Phases of fertilization Passage of sperm through corona radiata Hyaluronidase from acrosome Sperm tail Tubal mucosal enzymes Penetration of zona pellucida facilitated by Acrosin Neurominidase Esterases Zona reaction Lysosomal enzymes of cortical granules Slide 24: contd. Fusion of plasma membranes of secondary oocyte and sperm Completion of 2nd meiotic division of oocyte and formation of female pronucleus Formation of male pronucleus. Ootid Fusion of pronuclei to form Zygote Slide 25: FERTILIZATION Slide 26: The secondary oocyte is usually fertilized within 12 hours after ovulation. IVF unfertilized Secondary oocyte dies within 12 - 24 hours. FERTILIZATION : FERTILIZATION Penetration of the Corona Radiata Penetration of Zona Pellucida Penetration of Oocyte Cell Membrane and Formation of Zygote Corona Radiata : Corona Radiata 200 to 500 million sperms deposited only 200 to 500 reach site of fertilization. Only one of these sperms is marked for fertilization. ALLAH Himself knows the secrets of this superselection of genetic material. Zona Pellucida : Zona Pellucida Acrosin causes lysis of the zona pellucida, thereby forming a path for the sperm to follow. Once sperm touches zona pellucida, becomes firmly attached to it and penetrates it rapidly. ZONA REACTION : ZONA REACTION As one sperm passes through zona pellucida, the permeability of zona pellucida changes and 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. Oocyte Cell Membrane : Oocyte Cell Membrane After passing through zona pellucida, sperm becomes attached to cell membrane of secondary oocyte. Slide 32: The head and tail of the sperm enter the cytoplasm of oocyte, leaving the sperm’s plasma membrane attached to the plasma membrane of oocyte. Slide 33: The secondary oocyte completes the 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 sperm degenerates & its head enlarges to form male pronucleus. Ootid & Zygote : Ootid & Zygote The male and female pronuclei come into contact and loose their nuclear envelopes. The maternal and paternal chromosomes intermingle at equatorial plate of metaphase of the first mitotic division of the zygote. Slide 35: Restoration of diploid chromosomes Species variation Shuffling of genes. The zygote contains a new combination of chromosomes, different from the parents. ALLAH forms the bases of biparental inheritance and results in variation of the human species. Results of Fertilization ART : ART What is ART, IVF, ICSI,ZIFT and GIFT Preselection of embryo sex Primary sex ratio Cryopreservation of embryo / zygote Surrogate mothers? Slide 37: Cleavage Series of mitotic cell divisions of zygote to form embryonic cells (blastomeres) Implantation Process during which blastocyst attach to endometrium Slide 38: Blastocyst Morula (mulberry) A ball of 12 – 32 blastomeres Slide 39: Oocyte Sperm Zygote Fertilization Cleavage Morula Blastocyst Implantation Summary? : Summary?

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