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Fertilization By Augustine U Agu MBBS, MSc, PhD 09/13/2024 1 Learning objectives Meaning of fertilization ? Movement of spermatozoa from vagina to ampulla Meaning of Capacitation Acrosome reaction Phases of fe...

Fertilization By Augustine U Agu MBBS, MSc, PhD 09/13/2024 1 Learning objectives Meaning of fertilization ? Movement of spermatozoa from vagina to ampulla Meaning of Capacitation Acrosome reaction Phases of fertilization Results of fertilization 09/13/2024 2 What is fertilization? It is the process by which male & female gametes fuse. Occurs in the ampullary region of the uterine tube. 09/13/2024 3 Movement of spermatozoa from vagina to ampulla About 1% of sperm deposited in the vagina enter the cervix. Movement of sperm from the cervix to the oviduct is achieved mainly by their own propulsion, but may be assisted by movements of fluids created by uterine cilia. The trip requires a minimum of 2-7 hrs. After reaching the isthmus, sperm become less motile & cease their migration. At ovulation, sperm again become motile, may be by the chemo-attractants produced by cumulus cells 09/13/2024 4 Sperms are not able to fertilize the oocyte immediately upon arrival in the female genital tract but must undergo: – Capacitation & acrosome reaction 09/13/2024 5 Capacitation Is the process that allows the acrosome reaction to occur, so that sperm can penetrate the zona pellucida of the oocyte Lasts approximately 7 hours. Much of it occurs in the uterine tube It entails epithelial interactions b/w the sperm & mucosal surface of the tube. It involves removal of the glycoprotein coat & seminal plasma proteins of the plasma membrane that overlies the acrosomal region of the spermatozoa. Only capacitated sperm can pass through the corona cells & undergo the acrosome reaction. 09/13/2024 6 Acrosome reaction Is the exocytosis of the acrosome, or the fusion of the acrosomal membrane with the sperm's plasma membrane Is induced by zona proteins. It culminates in the release of enzymes needed to penetrate the zona pellucida, such as acrosin & trypsin-like substances 09/13/2024 7 The phases of fertilization Phase 1: penetration of the corona radiata Out of the 200 to 300 million spermatozoa deposited in the female genital tract, only 300 to 500 reach the site of fertilization. Only one of these fertilizes the egg. It is thought that the others aid the fertilizing sperm in penetrating the barriers protecting the female gamete. Capacitated sperm pass freely through corona cells Sperm pass through the corona radiata barrier 09/13/2024 8 PHASE 2: PENETRATION OF THE ZONA PELLUCIDA The zona facilitates & maintains sperm binding & induces the acrosome reaction. The binding & acrosome reaction are mediated by zona protein (ligand ZP3). Release of acrosomal enzymes One or more spermatozoa penetrate (acrosin) allows sperm to penetrate the the zona pellucida zona, thereby coming in contact with the plasma membrane of the oocyte Permeability of the zona pellucida changes when the head of the sperm comes in contact with the oocyte surface. 09/13/2024 9 The contact results in release of lysosomal enzymes from cortical granules lining the plasma membrane of the oocyte. These enzymes alter properties of the zona pellucida (zona reaction) to prevent sperm penetration & inactivate species-specific receptor sites for spermatozoa on the zona surface. A spermn has penetrated the oocyte, Other spermatozoa are embedded which has finished its 2nd meiotic division in the zona pellucida, but only one was able to penetrate the oocyte 09/13/2024 10 Phase 3: fusion of the oocyte & sperm cell membranes The initial adhesion of sperm to the oocyte is mediated in part by the interaction of integrins on the oocyte & their ligands, disintegrins, on sperm. After adhesion, the plasma membranes of the sperm & egg fuse One spermatozoon penetrates the The fusion is accomplished b/w the oocyte membrane while losing its own plasma membrane oocyte membrane & the membrane that covers the posterior region of the sperm head 09/13/2024 11 Both the head & tail of the spermatozoon enter the cytoplasm of the oocyte, but the plasma membrane is left behind on the oocyte surface. As soon as the spermatozoon has entered the oocyte, the egg responds in 3 ways: Cortical and zona reactions. – The oocyte membrane becomes impenetrable to other spermatozoa – The zona pellucida alters its structure & composition to prevent sperm binding & penetration, thus prevent polyspermy 09/13/2024 12 Resumption of the 2nd meiotic division. – The oocyte finishes its 2nd meiotic division immediately after entry of the spermatozoon. – One of the daughter cells, becomes 2nd polar body & the other the definitive oocyte. – Its chromosomes (22+X) arrange themselves in a vesicular nucleus called female pronucleus Metabolic activation of the egg – The activating factor is probably carried by the spermatozoon. 09/13/2024 13 The spermatozoon, moves forward until it lies close to the female pronucleus. Its nucleus becomes swollen & forms the male pronucleus Morphologically, the male and female pronuclei are indistinguishable, & eventually, they come into close contact & lose their nuclear envelopes 09/13/2024 14 During growth of male & female pronuclei (both haploid), each pronucleus must replicate its DNA. The 23 maternal & 23 paternal (double) chromosomes split longitudinally at the centromere, & sister chromatids move to opposite poles, providing each cell of the zygote with the normal diploid number of chromosomes & DNA 09/13/2024 15 Results of fertilization Restoration of the diploid number of chromosomes – half from the father & ½ from the mother. Hence, the zygote contains a new combination of chromosomes different from both parents. Determination of the sex of the new individual. – An X-carrying sperm produces a female (XX) embryo, & a Y-carrying sperm produces a male (XY) embryo. thus, sex of the embryo is determined at fertilization. Initiation of cleavage. – Without fertilization, the oocyte usually degenerates 24 hours after ovulation. A. Oocyte immediately after ovulation. B. A sperm has penetrated the oocyte. Heads of several sperm are stuck in the zona pellucida. C. Male & female pronuclei. D and E. Chromosomes become arranged on the spindle, split longitudinally, & move to opposite poles. F. Two-cell stage 09/13/2024 16 Thanks for listening 09/13/2024 17

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