Conception is the initial stage in humans that allow for the establishment of pregnancy. Coitus is the word we use to refer to sexual intercourse resulting in the deposition of sperm in the vagina at the level of the cervix. After sperm has been deposited at the cervix it is transported to the uterus where it fertilises the ovum and implants in the uterine stroma. We call this process conception.
In this article we shall look at conception and some clinical conditions.
Transport of Sperm
Following coitus, a small percentage of the millions of sperm deposited at the cervix will reach the site of fertilisation. In order for conception to occur these sperm must travel to the ampulla in the fallopian tube and undergo a series of steps that will allow it to then penetrate the cell layer surrounding the oocyte and so fertilise it.
Oxytocin stimulates uterine contraction which, along with the sperms’ own propulsive activity, aids in transporting the sperm and helping it travel. In order for sperm to penetrate the oocyte it undergoes capacitation. Capacitation reorganises the sperm cell membrane and results in the tail movement changing from a beat-like action to a thrashing whip-like action to help propel the sperm forward.
The changes that occur in capacitation induced by the removal of the protein coat of sperm exposes acrosome enzymes. Therefore, capacitation is vital to allowing the acrosome reaction to occur – which will allow penetration of the zona pellucida.
The Zona Pellucida
The zona pellucida specific cell surface glycoproteins (ZP3 proteins) interact with the capacitated sperm and allow calcium to enter the spermatozoa, which results in an increase in intracellular cAMP. The acrosome then swells and its outer membrane fuses with the sperm plasma membrane. This leads to the release of enzymes from the acrosome into the space surrounding the head of the sperm.
The inner cell membrane of the acrosome is then exposed and another glycoprotein (ZP2) holds the sperm near the egg. The proteolytic enzymes released from the acrosome then allow for penetration of the zona pellucida by the whip-lashing sperm.
Changes Following Penetration of the Zona Pellucida
Penetration of the zona pellucida allows the sperm and oocyte cell membranes to fuse almost immediately. Calcium enters the oocyte and the sperm cell stop moving. The rise in intracellular calcium results in 3 changes in the oocyte:
- The egg cell membrane depolarises in order to prevent polyspermy – this is the primary block.
- The cortical reaction occurs, which is where cortical granules that lie just beneath the egg cell membrane fuse with the membrane and release their contents into the zona pellucida. This is a secondary block to polyspermy as it hardens the zona.
- The egg will complete the final meiotic division – this forms a polar body that is subsequently released. Following fertilisation the zygote undergoes several changes which will result in the implantation within the wall of the uterus in a successful pregnancy.