7 - Gametes and fertilisation Flashcards
What is the structure of sperm cells?
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.
sperm nucleus
contains 23 chromosomes (haploid)
acrosome function
hydrolytic enzyme for digesting the zona pellucida
sperm mitochondria
provides the tail with ATP for movement
What happens in fertilisation?
- 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.
sea urchin fertilization steps
- 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
acrosomal reaction
The discharge of hydrolytic enzymes from the acrosome, a vesicle in the tip of a sperm, when the sperm approaches or contacts an egg.
cortical reaction
- 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
Conspecific fertilisation
egg fertilised by sperm of same species
What is the fast block to polyspermy?
- 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.
What is the slow block to polyspermy?
- 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.
Hyaluronidase
digests polysaccharides that hold cells together
zona pellucida
A thick, transpartent coating rich in glycoproteins that surrounds an oocyte.
corona radiata
The layer of granulosa cells that surround an oocyte after is has been ovulated.
first polar body
One result of the first meiotic division in oogenesis, it has received no cytoplasm and has no future as a gamete.
acrosomal vesicle
Specialized secretory vesicle in the sperm that carries enzymes used to digest the outer coverings of eggs.
mammalian sperm-egg interaction
- 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
fertilization and oocyte activation
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
Pronucleus formation begins
The sperm is absorbed into the cytoplasm, and the female pronucleus develops
Spindle formation and cleavage preparation
the male pronucleus develops and spindle fibers appear in preparation for the first cleavage division
What happens in the first cleavage division in human mitosis?
- Each pronucleus enters mitosis.
- Nuclear membrane breaks down.
- Chromosomes attach to common spindle.
Cleavage begins
The first cleavage division nears completion about 30 hours afterfertilization.
