Fertilisation

Fertilisation In all species the egg cells release a specific signal, usually a polypeptide, which acts as a signal to the sperm cell to fertilise the egg and not other cells. The signals appear to be species specific Other important events include the restoration of the diploid number and mechanisms that prevent polyspermy.

The acrosome reaction The cumulus is a thick loose grouping of cells in a gelatinous matrix. The sperm cell must penetrate this mass to reach the zona pellucida, a glycoprotein matrix surrounding the egg plasma membrane. Contact between the zona pellucida and proteins in the sperm cells membrane trigger the acrosome reaction.

The acrosome reaction The acrosome vesicle fuses with the sperm plasma membrane and releases protease enzymes that digest a path through the zona pellucida by softening the layers of cells surrounding the oocyte. A fine needle-like filament develops at the tip of the sperm which pierces the softened membrane of the ovum.

The cortical reaction The cortical vesicle fuses with the plasma membrane releasing enzymes that destroy the sperm binding proteins on the zona pellucida. This prevents polyspermy. The release of Ca2+ also activates meiosis and prepares the cell for completion of reduction division, MII and cell division.

Implantation

The blastocyst After fertilisation, the zygote divides (cleavage) mitotically until a hollow ball of cells, the blastocyst, is produced. It takes three days to reach the uterus and a further three or four days to become implanted in the lining of the uterus.

Trophoblast The outer layer of cells of the blastocyst, called the trophoblast, develops into the embryonic membranes, the chorion and the amnion.

Chorion The chorion develops villi (chorionic villi) which grow into the surrounding uterine tissue from which they absorb nutrients. The villi increase the surface area for the absorption of nutrients from the wall of the uterus.

Chorion The chorion also secretes human chorionic gonadotrophin (hCG), which prevents the breakdown of the corpus luteum. It is hCG that is used as the basis of most pregnancy tests.

Placenta The villi form part of the placenta, which is connected to the foetus by the umbilical cord. The amnion develops as a membrane around the foetus and encloses the amniotic fluid, a watery liquid which protects the foetus by cushioning it from physical damage.