How is polyspermy prevented?
The acrosome reaction: When sperms bind to the zona pellucida, the contents of their acrosome are relased which digest the zona pellucida.
Penetration of the egg membrane: The acrosome reaction exposes an area of the sperm which has special proteins that allow it to bind to the egg. The very first sperm that is able to do this fertilizes the egg as the sperm nucleus enters the ovum.
The cortical reaction: The penetration of the egg activates it.
The cortical granules empty out their contents.
In mamals, the cortical granule enzymes digest the sperms' binding proteins so no other sperm can bind.
The enzymes also make the zona pellucida harden.
How does the placenta allow exchange of materials
The basic functional unit of the placenta is the placental villus
Mother's blood flows in the intervillous space around the capillary
Fetal blood flows in the capillaries close to the villus's surface.
The placental barrier is selectively permeable and separates both bloods.
What are the role of hormones during parturition?
The fetus signals to the placenta to stop progesterone secretion so oxytocin is released.
Oxytocin stimulates the myometrium to contract
Stretch receptors detect contractions and tell the pituitary gland to increase oxytocin secretion
This is a positive feedback system. It results in more vigorous contractions, causing more oxytocin to be released. It increases the contractions slowly and they have minimum intensity.
That's all for this post! Well done!
Drafted by Venetia (Biology)