Question: Describe the cellular and molecular events that occur during fertilisation
Approximately 200million spermatozoa are released during ejaculation in intercourse, travelling through the vagina and swimming through the cervix propelled by whip like motions of the flagella. After which muscular contractions of the uterus direct them to the fallopian tubes.
This process usually takes between 30 minutes and 2 hours. Around 200 spermatozoa will reach the oocyte in the fallopian tube. Only one will fertilise.
Capacitation and the acrosomal reaction must take place before fertilisation can begin. Secretions from he uterus wall destabilise the plasma membrane surrounding the head of the seprm resulting in the membrane becoming more fluid which helps prepare the sperm for the events of fertilisation. The sperm become hyperactive as they move through the corona radiata and come into contact with the zonae pellucida. Here, specific receptor proteins called ZP3 triggers the acrosomal reaction during which the enzymatic contents of the acrosome are released. These enzymes digest a path through the zonae pellucida into the perivitelline space and reach the plasma membrane of the secondary oocyte. Here the fusion takes place.
To ensure only one sperm fuses a fast and slow block is released preventing any further ZP3 being released and therefore preventing polyspermy. The slow block is depolarisation and fast block sees
a wave of intracellular calcium being released causing small cortical granules beneath the oocyte membrane to release their contents. Thus rendering ZP3 inactive making the zonae pellucid impermeable.
Upon sperm entering, the oocyte undergoes Meiosis 2 and further develops into a female pronucleus. During this time the sperm develops into male pronucleus. The two pronuclei fuse to form one single diploid nucleus known as a zygote.
