Fertilization

Fertilization in humans joins egg and sperm cells, potentially yielding a new individual.

Prior to fertilization, a haploid egg, surrounded by an inner protein coat termed the zona pellucida, and an outer matrix of supportive cumulus cells, exits an ovary and migrates into a fallopian tube.

Concurrently, haploid sperm must be present in the genital tract, and partly due to uterine contractions, are similarly driven into the fallopian tubes. Once there, they enter a maturation process called capacitation, which involves egg-fusing proteins being transported to the sperm head.

Here, sperm also begin to demonstrate increased tail or flagellum movements. This propels the cells towards the egg, which they locate by sensing chemical signals, like progesterone, exuded by the cumulus egg complex.

Upon arriving, sperm undergo the acrosome reaction, during which a vesicle harboring digestive enzymes in a sperm's tip, the acrosome, expels its contents into the extracellular space, enabling the cells to burrow through the zona pellucida.

Once the first sperm fully penetrates the protein coat, its, and the egg cell's membrane coalesce. This alters the egg, and the zona pellucida hardens, which blocks other cells from entering.

Fertilization ends with the sperm releasing its contents into the egg. The resulting diploid structure, now containing both parents' genetic material, will eventually generate a novel individual.