Human Reproduction - Fertilization and Implantation

Fertilization is the process of fusion of a haploid male gamete (spermatozoon, $n$) with a haploid female gamete (ovum, $n$) to form a diploid zygote ($2n$). It typically occurs in the ampullary-isthmic junction of the Fallopian tube.

Capacitation: The functional maturation of sperm in the female reproductive tract, involving the removal of membrane cholesterol and proteins to increase motility and prepare for the acrosomal reaction.

Acrosomal Reaction: Upon contact with the zona pellucida, the acrosome releases enzymes like hyaluronidase and acrosin (zona lysin) to dissolve the egg's protective layers.

Prevention of Polyspermy: The entry of one sperm induces a cortical reaction (depolarization of the membrane and release of cortical granules) that hardens the zona pellucida, ensuring only one sperm fertilizes the ovum.

Cleavage: A series of rapid mitotic divisions where the zygote transforms into a $2, 4, 8, 16$ daughter cell stage called blastomeres. The solid ball of $8-16$ blastomeres is known as the Morula.

Blastocyst Formation: The morula continues to divide and transforms into a blastocyst, characterized by an outer layer called the trophoblast and an Inner Cell Mass ($ICM$).

Implantation: The process where the blastocyst becomes embedded in the endometrium of the uterus, usually occurring about $7$ days after fertilization. The trophoblast layer differentiates into the chorion to form the placenta.

Sex Determination: In humans, the sex of the fetus is determined by the father. The female produces only $X$ type gametes ($22 + X$), while the male produces $50\%$ $X$ ($22 + X$) and $50\%$ $Y$ ($22 + Y$) gametes.