Fertilisation PDF

Maturation of Spermatozoa Capacitation of Sperm Sperm Produced in seminiferous tubules Morphologically Mature, Functionally Immature Non-Motile and Not Capable of Fertilisation Capacitation of sperm Biochemical Changes to Sperm - Functionally Maturity Takes Place within the epididymis, and U...

Maturation of Spermatozoa Capacitation of Sperm Sperm Produced in seminiferous tubules Morphologically Mature, Functionally Immature Non-Motile and Not Capable of Fertilisation Capacitation of sperm Biochemical Changes to Sperm - Functionally Maturity Takes Place within the epididymis, and Uterus and Oviduct Evidence to demonstrate sperm capacitation from early IVF experiments Experiment One No fertilization occurred when sperm isolated from the testes were added to an oocyte Experiment Two No fertilization occurred when ejaculated sperm were added to an oocyte Experiment Three Fertilization occurred when ejaculated sperm were added to uterine fluid and then added to an oocyte Suggesting alteration in the sperm within the spidymis and female tract See notes on previous slide Capacitation Sperm Cell Membrane Changes (Notes for previous slide) Bicarbonate Channel opens Influx of HCO3- into the sperm Increase in pH within the sperm pH Dependent loss of Cholesterol from the membrane of the sperm Activation of a pH dependent soluble adenylate cylase (SACY) Increased cAMP and PKA activity This causes ZP (Zona Pellucida receptors) to be expressed on the cell surface of the sperm These ZP receptors are the Egg Binding Proteins Fluctuations in intracellular Calcium Increased protein phosphorylation via cAMP Absolute Requirement for Capacitation Changes in Sperm Due to Capacitation Increased Metabolism Fructose for Energy Flagellum Increase Rate of Beating Increased Sperm Motility Activation of Acrosomal Enzymes Pro-Acrosin Changes to Acrosin Sperm Transport Fertilisation - Tubal Ampulla Sperm - Deposited in Vagina and Travel to Tubal Ampulla Flagellar Movement Factors That Increase Uterine Contractility Ovarian Steroids Influence Sperm Transport High Oestrogen of Late Follicular Phase Cervical Mucus alkaline, Less viscous PGs in Seminal Fluid Oxytocin Ovulated Oocyte Zona Pellucida Polar Body Cumulus Cells Oocyte Zona Pellucida (ZP) • A glycoprotein matrix surrounding the mammalian oocyte • Important role in • Species-specific sperm–egg binding • Acrosome reaction in the ZP-bound spermatozoa • Prevention of polyspermy • Protection of the embryo prior to implantation Zona Pellucida Glycoproteins • Human ZP matrix is composed of 4 glycoproteins • ZP glycoprotein -1 (ZP1) • ZP glycoprotein -2 (ZP2) • ZP glycoprotein -3 (ZP3) • ZP glycoprotein -4 (ZP4) Mutations in the genes encoding human ZP glycoproteins are one of the causative factors leading to abnormal ZP matrix and infertility in women Zona Pellucida Glycoproteins • ZP3 and ZP4 • Heterocomplexes of ZP3 and ZP4 are responsible for spermegg binding and induction of acrosomal reaction in the sperm • ZP 2 • May be involved in the species specificity of Sperm Binding • ZP1 • Cross links other ZP proteins to ensure stable structure of ZP • Not Essential for Fertilisation Sperm to Egg binding – Acrosomal Reaction Reaction in Sperm to Egg binding – Acrosomal Reaction (Notes for previous slide) Glycan-binding molecules (receptors) on sperm bind to ZP glycans (ligands) Progesterone binds to receptors on ZP Calcium-mediated signal transduction cascade causes release of acrosomal contents via exocytosis (acrosomal reaction) Sustained increased Calcium causes the release of hydrolytic and proteolytic acrosomal enzymes to facilitate sperm fusion with the oocyte. Any structural or functional acrosomal abnormality could impair sperm fusion, and ultimately result in infertility Reaction in Egg to sperm binding –Cortical Reaction • Cortical Granules specialized Golgi-derived membrane enclosed • Subjacent to the plasma membrane of mature unfertilized oocyte • Cortical granules are released by exocytosis in response to stimulation by the fertilizing sperm. • Process begins at the site of egg sperm rapidly spreads to entire surface of the egg • Sperm Block Reaction in Egg to sperm binding –Cortical Reaction Prevention of polyspermy Decreased Effective Block to Polyspermy Age of Ova ◦ Time of Insemination Relative to Ovulation ◦ Aged Oocyte Increases Chance of Polyspermy Heating of Ova ◦ Increased Metabolism and Lifespan of Egg ◦ Fever or Environmental Effect on Body Temperature Excessive Sperm Numbers ◦ At Site of Fertilisation ◦ Sperm Barriers Reduce Numbers  Cervix, Uterus, Utero-Tubular Junction Triploidy (in vitro) There are 3 pronuclei within this one zygote. In the laboratory, such embryos are discarded. In vivo, such embryos almost always abort spontaneously. Advanced Fertility Center of Chicago http://www.advancedfertility.com/ Egg Activation Role of PLC-zeta Egg Activation Cont’d Egg Activation Role of PLCzeta Pronuclei Fusion • Haploid sperm nucleus enters Egg • Becomes the sperm pronucleus • Sperm pronucleus swells, migrates towards the egg pronucleus Pronuclei Fusion • When in close proximity the pronuclear envelopes vesiculate • Nuclear membranes break up to form a circle of small vesicles • Surround the chromatin of each nucleus • Chromatin from each pronucleus intermixes • Form the diploid zygote nucleus • Nuclear envelope reforms around the zygote nucleus • Embryonic development begins The Nuclei Fuse Together What happens now? • Development of the zygote, the study of which is known as embryology or developmental biology. • The zygote undergoes a series of mitotic cell divisions called cleavage • The stages of development are: Fertilized ovum (zygote)  2-cell stage  4-cell stage  8-cell stage  Morula  Blastula  Early Gastrula  Late Gastrula Early Embryonic Development