DEV2011 - W3: Fertilisation to gastrulation

When do sperm gain full motility?

Sperm don’t gain full motility until they reach the uterus.

Enzyme secreted by uterine wall:

• generate influx of Ca2+ → increase sperm motility (make sperm hyperactive)

• alters plasma membrane of sperm head for binding to egg

What is the acrosome?

Explain the acrosome reaction.

• Acrosome derived from Golgi

• contains digestive enzymes (including hyaluronidase and acrosin)

  • These enzymes are needed to break through the layer of cumulus cells that surround the egg

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Acrosome reaction:

• sperm binds to zona pellucida (at surface of egg)

• acrosome content released to help sperm head break through zona pellucida

• plasma membrane of sperm and egg are fused - allowing sperm nucleus and centriole to enter egg. Sperm mitochondria in most cases cannot enter (mitochondria is maternally derived)

What is the polyspermy block?

Fast block and slow block

Fast block

• egg membrane undergoes rapid depolarisation (sodium influx) to stop any more sperm from binding to the egg

Slow block

• occurs after fast block

• release of calcium into egg → stimulates cortical granule response (slow block)

• cortical granules (just under the surface of the egg) fuse, and release their contents into the space between the cell membrane and the vitelline envelope (zona pellucida)

• results in hardening of the vitelline envelope (egg surface) and permanently blocks further sperm from binding

Pre-implantation development: What is unification and cleavage?

Unification

• pronuclei unify right after fertilisation

(Sperm’s centriole duplicates and forms critically important first mitotic spindle - allowing chromosomes to align for cell division)

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Cleavage stages:

• First cleavage produces 2 cells (blastomeres)

  • At this point, the embryo is increasing in size - this means it is increasing in cell number (NOT ACTUAL SIZE!!) because of the hard shell around the egg

• ~day 5 blastocyst formation (with trophectoderm and forming of inner cell mass)

When does the egg (oocyte) complete meiosis II (since it stalls at metaphase II)?

Egg completes meiosis 2 as fertilisation occurs - and second polar body is produced (which is important later on)