7 - Gametes and fertilisation

fuse with egg. • Centriole allows mitotic spindle formation. Neck: • Mitochondria are arranged in a spiral shape. • Generate energy to allow cell to move. Tail: • Flagellum allows cell to be motile.

contains 23 chromosomes (haploid)

hydrolytic enzyme for digesting the zona pellucida

provides the tail with ATP for movement

• A haploid sperm fuses with a dormant haploid egg, making a cell with the normal diploid number of chromosomes, known as a zygote. • Egg can be activated artificially without fertilisation by increasing calcium pathways.

• sperm contacts jelly coat = exocytosis from acrosome • hydrolytic enzymes releases, make hole in jelly coat • surface proteins bind to receptors • contact and fusion of sperm and egg membranes • cortical granules fuse with plasma membrane • entry of sperm nucleus

The discharge of hydrolytic enzymes from the acrosome, a vesicle in the tip of a sperm, when the sperm approaches or contacts an egg.

• calcium ion released after penetration of sperm into egg • calcium ions depolarize the membrane of the ovum • prevents fertilization of ovum by multiple sperm cells • acts as slow block to polyspermy

egg fertilised by sperm of same species

• There is fusion of sperm & egg cell plasma membranes. • Channels in egg plasma membrane open. • This allows a Na⁺ influx from seawater. • Na⁺ influx causes a wave of depolarisation to sweep across the cell, much like the depolarization that occurs in muscle and nerve cells. • This change to a positive membrane potential prevents any other sperm from touching the ovum.

• First sperm to reach the oocyte fuses with the plasmalemma, triggering Ca²⁺ release from vesicles within the oocyte and exocytosis of proteases from the cortical granules. • This cortical reaction spreads like a wave across the surface of the oocyte, and the proteases convert the zona pellucida to the impermeable perivitelline barrier, preventing polyspermy. • Leading to the formation of a fertilisation membrane. • Depolarisation will lead to a release of calcium from the endoplasmic reticulum, calcium concentration in cytoplasm will increase.

digests polysaccharides that hold cells together

A thick, transpartent coating rich in glycoproteins that surrounds an oocyte.

The layer of granulosa cells that surround an oocyte after is has been ovulated.

One result of the first meiotic division in oogenesis, it has received no cytoplasm and has no future as a gamete.

Specialized secretory vesicle in the sperm that carries enzymes used to digest the outer coverings of eggs.

• zona pellucida ZP3 binds to Beta 1-4- galactosyltransferase on sperm • acrosomal contents released • acrosin and beta-N-acetylglucosaminidase digest zona pellucida glycoproteins • fertilin on sperm head binds integrin-like protein and CD9 • membrane fusion • Fast block to polyspermy (depolarisation) • Slow block to polyspermy

Acrosomal enzymes from multiple sperm create gaps in the corona radiata. A single sperm then makes contact with the oocyte membrane, and membrane fusion occurs, triggering oocyte activation and completion of meiosis

The sperm is absorbed into the cytoplasm, and the female pronucleus develops

the male pronucleus develops and spindle fibers appear in preparation for the first cleavage division

• Each pronucleus enters mitosis. • Nuclear membrane breaks down. • Chromosomes attach to common spindle.

The first cleavage division nears completion about 30 hours afterfertilization.