When sperm enters the female reproductive tract, it is not yet mature. It goes over capacitation, which means that it matures when traveling towards the egg. But how does it find the egg? There might be some kind of chemoattractants the egg releases. When the sperm binds these attractants (one way of species specificity, the sperm has receptors only for the own species eggs proteins) it gets hyperactivated. Its internal pH concentration increases (due internal messenger pathways) which induces the usage of ATP which makes the flagella spin faster (activates dynein). When the sperm finds the egg, it first encounters the cumulus cells (somatic cells) that surround the egg and nourish it.
The sperm releases hyaluronidase enzyme that degrades the hyaluronic acids and lets the sperm penetrate to the next layer of the zona pellucida. Zona pellucida has three different proteins, ZP1, ZP2 and ZP3. The ZP3 binds to mature sperm and causes acrosomal vesicle of the sperm to fuse and release its enzymes. These enzymes help the sperm penetrate the zona pellucida layer (yet another way of species specificity). ZP2 binds to sperm that has undergone acrosomal release and ZP1 is only a stucture protein.
After this the egg membrane elongates and engulfs the sperm with the help of microvilli. The sperm and egg membranes fuse and this triggers the fast block of polyspermy. Fusion initiates the release of Ca2+ ions which causes the egg cell membrane to depolarize. This prevents sperm from binding and initiates also the slow block of polyspermy; the exocytosis of cortical granules (also known as zona reaction). These cortical granules have different sets of enzymes that harden the zona pellucida so that sperm cannot penetrate it. The Ca2+ also activates the egg metabolism (Maturation Promoting Factor is degraded and the egg completes meiosis).Now the egg has two pronuclei which fuse together and meiosis completes.
The formed zygote enters to the next phase, cleavage stage.
No comments:
Post a Comment