Fertilisation PDF

Preparing for Fertilisation Fertilisation = fusion of egg and sperm and occurs in the ampulla of the oviduct Ca is required for acrosome reaction, cortical granule exocytosis, cell cycle To occur, it requires 2 mature fertile gametes: Egg: Oocyte maturation (nuclear and cytoplasm...

Preparing for Fertilisation Fertilisation = fusion of egg and sperm and occurs in the ampulla of the oviduct Ca is required for acrosome reaction, cortical granule exocytosis, cell cycle To occur, it requires 2 mature fertile gametes: Egg: Oocyte maturation (nuclear and cytoplasmic) Arrest at metaphase II until ovulation After ovulation, oocyte resumes meiosis II - transitions are controlled by CDK1 checkpoints Sperm: Acquire functionality and maturity in epididymis but do not have ability to fertilise egg Undergo capacitation in female reproductive tract (not necessary to actually be in tract, is time-dependent) and are able to fertilise within 1-2 hours Penetration of Zona Pellucida Capacitation: Allows for penetration of zona pellucida Capacitation Penetration of Zona Pellucida Sperm tail becomes fully functional and mobile Hyper-activated motility - increased force generated Allows for acrosome reaction Acrosome reaction: Exocytotic event - fusion of plasma and acrosomal membranes --. Release of hydrolytic enzymes that dissolve zona pellucida Stimulated by binding to zona pellucida Activated by increase in intracellular Ca Zona Pellucida Consists of 3 glycoproteins (ZP1, 2, 3) which are synthesised by oocytes as they grow ZP3 - bind acrosome intact sperm → induces acrosome reaction by increasing intracellular Ca2+ inside sperm ZP2 - binds acrosome reacted sperm Fusion of Oocyte and Sperm Membranes Proteins necessary for egg-sperm fusion Egg protein Cd9 - transmembrane protein involved in cell adhesion and can promote fusion in other cells Sperm protein Izumo  Egg Activation Egg activation involves 2 major events: cortical granule exocytosis and resumption of cell cycle Both triggered by Ca2+ release ○ Fertilisation stimulates Ca2+ oscillations ○ Necessary and sufficient for egg activation (no Ca no activation, present will cause activation) All mitochondria from mother All centromeres from father Putting sperm directly into egg can cause fertilisation - basis of IVF Cortical Granule Exocytosis Resumption of Cell Cycle Wave of rapid depolarisation of zona pellucida 1. Sperm head containing DNA inserted into cytoplasm of oocyte (increase in Ca2+) 2. Increase in intracellular Ca triggered by phospholipase C zeta Exocytosis of cortical granules → cleavage of (PLC zeta) from sperm ZP2 and modification of ZP3 3. Activation of egg and resumption of meiosis II → 2 haploid Polyspermy block as no other sperm can cells (ovum and polar body) attach or undergo acrosome reaction 4. Before fusing, pronuclei of oocyte and sperm duplicate DNA 5. Microtubules pull pronuclei together and membranes dissolve → zygote 6. Mitosis continues