Gamete Transport and Fertilization PDF

Gamete Transport and FERTILIZATION  Fertilization fFertilization in mammals occurs in the oviduct.  The ova is viable 12-24 hours of after ovulation  Sperms is viable up to 72 hours inside the female’s body Mechanisms of Fertilization Stages of Fertilization  Inter...

Gamete Transport and FERTILIZATION  Fertilization fFertilization in mammals occurs in the oviduct.  The ova is viable 12-24 hours of after ovulation  Sperms is viable up to 72 hours inside the female’s body Mechanisms of Fertilization Stages of Fertilization  Internal Alter sperm motility and structure (CAPICITATION) Sperm binds to receptor (ZP3) – (ACROSOMAL REACTION) Release of Ca from SER of Oocyte- (SLOW BLOCK TO POLYSPERMY), (FORMATION OF OVUM)  Fertilization 1% of sperm deposited in the vagina enters cervix (movement of sperm due to muscular contraction of the uterus and uterine tube) Movement occur from 30 min to 2- 3 days Upon reaching isthmus sperm is less motile and cease their migration  Capacitation set of physiological changes by which the sperm becomes competent to fertilize the egg Screen images of capacitated sperm cells (http://www.embryology.ch/anglais/dbefruchtung/weg03.html)  Capacitation Period of conditioning the female repro tract Involves epithelial interaction of the sperm and mucosal surface Glycoprotein coat and seminal plasma proteins are removed from plasma membrane that overlies acrosome of sperm  when sperm pass from uterus to the oviduct swim at higher velocities and greater force with hyaluronidase penetrate the extracellular matrix of cumulus cells  ability to sense temperature difference and preferentially swim from cooler to warmer sites capacitated sensing a thermal gradient of 2ºC  sensing mechanism of sperm that is attracted to the molecules secreted by the oocyte and cumulus cells In humans, progesterone has been shown to bind to a receptor that activates Ca2+ channels in the cell membrane of the sperm tail, leading to sperm hyperactivity Recognition of zona pellucida Acrosome reaction Traversing the zona pellucida 1. Hyaluronidase : assist in penetration of the corona radiata barrier 2. Trypsin -like substances : for the digestion of the zona pellucida 3. Acrosin : help the sperm to cross the zona pellucida 4. Progesterone (present in follicular fluid) seems to stimulate the acrosome reaction  Acrosome Reaction After binding to the ZP (induced by Zona proteins) Starts with the release of enzymes (Acrosin and Trypsin like substance) Phase1 Penetration to corona radiata Phase2 Penetration to ZP Pase3 Fusion of oocyte and sperm membrane  Phases of Fertilization Phase1  From millions to 300-500 sperm reach site of fertilization  Capacitated sperm pass thru corona cells Phase2  The Zona is a glycoprotein shell facilitates and maintains binding and induces acrosome reaction  Binding and acrosome reaction are mediated by ligand ZP3  Phases of Fertilization (cont…) Release of acrosin allows sperm to penetrate Zona, thereby contact with oocyte membrane Permeability to Zona changes when head of the sperm comes in contact with oocyte surface results in release of lysosomal enzymes from cortical granules lining the oocyte plasma membrane (ZONA REACTION) Only 1 seems to penetrate even other sperms have been embedded in the ZP  Phases of Fertilization P3 Initial adhesion of sperm to oocyte is mediated by interaction of integrins on oocyte and their ligands, disintegrins, on sperm Plasma membrane of gametes fused As plasma membranes covering the acrosomal head cap disappears during acrosome reaction Then actual fusion is accomplished between their membranes  Phases of Fertilization cont…  Gamete Fusion fusion of the sperm plasma membrane with the plasma membrane of the egg – forming a fertilization cone Gamete Fusion Docking & Invagination of the sperm Spermatozoon in perivitelline space Post-acrosomal membrane docking into the oocyte cell membrane Invagination of sperm cell into the egg cell http://www.embryology.ch/anglais/dbefruchtung/eindring01.html  Docking & Invagination of the sperm Polyspermy block Hardening of the zona pellucida Entry of spermatozoon Termination of the 2nd meiosis of oocyte Preparation for gamete fusion  Polyspermy Prevention fast block to polyspermy is achieved by changing the electric potential of the egg plasma membrane  Polyspermy Prevention Slow Block in mammals, the cortical granule reaction does not create a fertilization envelope enzymes modify the zona pellucida sperm receptors such that they can no longer bind sperm (zona reaction) Another slow block to polyspermy involves the Juno protein  Regulation of Sperm Entry permeability of ZP changes the sperm head and cortical granules release lysosomal enzymes N-acetyl glucosaminidase cleave to N- acetylglucosamine from ZP3 carbohydrate to which sperm bind enzymes alter properties of ZP (zona reaction) to prevent sperm penetration inactivate species-specific receptor sites mice with IZUM (immunoglobulin like protein) can bind and penetrate ZP but cannot fuse with cell mem  Fusion of oocyte and sperm initial adhesion is mediated by interaction of – integrins of egg – ligands and disintegrins (fertilin) of sperm cortical and zona reactions – oocyte membrane impenetrable – ZP alters structure and composition resumption of second meiotic division – oocyte finishes second meiosis – 2 daughter cells: second polar body & definitive oocyte  Fusion of Genetic Material mammalian male pronucleus enlarges while the oocyte nucleus completes its second meiotic division both nuclei and indistinguishable and lose nuclear envelopes the centrosome (new centriole) accompanying the male pronucleus produces its asters and contacts the female pronucleus each pronucleus migrates toward the other, replicating its DNA as it travels  Fusion of Genetic Material nuclear envelope breaks down glutathione in egg reduces disulfide bonds and uncoil sperm chromatins sperm brings pronucleus, mitochondria, centriole mitochondria are degraded into the egg cytoplasm by dilution and destruction chromatin condenses into chromosomes that orient themselves on a common mitotic spindle Fusion of Genetic Material Fusion of Genetic Material  presence of Ca2+ is important in activating the development of embryo Calcium release activates a series of metabolic reactions – (respiration and protein synthesis) increase substantially Cytosolic pH becomes slightly alkaline  Summary The male nucleus enters the egg cytoplasm and becomes the male pronucleus As a result of the sperm fusing with the egg plasmalemma, the oocyte nucleus, which is at that division giving rise to another polar body. Following the second meiotic division, the nucleus of the ovum becomes the female pronucleus The haploid male and female pronuclei move toward one and other, meet, and fuse to form the diploid The zygote will now proceed to undergo cleavage After fertilization…  Results of Fertilization restoration of diploid number of chromosomes determination of the sex of the new individual initiation of cleavage Zygote development 24-36 hours after fertilization. Thank you