Hundreds of millions of sperm cells are deposited in the vagina during sexual intercourse. A sperm cell can survive in the woman’s body for up to six days, but it can only fertilize the oocyte in the 12 to 24 hours after ovulation.
The woman’s body helps sperm reach an oocyte. A process called capacitation in the woman’s body chemically activates sperm. The oocyte also secretes a chemical that attracts sperm. Sperm are also assisted by contractions of the female’s muscles, by their tails moving, and by upwardly moving mucus propelled by waving, hairlike fringes on cells of the female reproductive tract. Still, only 200 or so sperm near the oocyte.
When a sperm contracts the ring of follicle cells guarding a secondary oocyte, its tip bursts, releasing enzymes that bore through a protective layer of glycoprotein called the zona pellucida. Fertilization, or conception, begins when the outer membranes of the sperm and secondary oocyte meet. The encounter is dramatic. A wave of electricity spreads physical and chemical changes across the entire oocyte surface-changes that keep other sperm out. If more than one sperm entered an oocyte, the resulting cell would have too much genetic material to develop normally. However, when two sperm fertilize two oocytes, fraternal twins result.
Within 12 hours of the sperm’s penetration, the nuclear membrane of the ovum disappears, and the two sets of chromosomes, called pronuclei, approach one another. Fertilization completes when the two genetic packages meet, forming the genetic instructions for a new individual.
The fertilized ovum is called a zygote. For the first two weeks of prenatal development, the structure is called a preimplantation embryo, or preembryo for short.
Cleavage and implantation
About a day after fertilization, the zygote divides mitotically, beginning a period of rapid cell division called cleavage. The resulting early cells are called blastomeres. When they have divided to form a solid ball of 16 or more cells, the preembryo is called a morula.
During cleavage, organelles and molecules from the secondary oocyte’s cytoplasm still control cellular activity, but some of the preembryo’s genes begin to function. The ball of cells hollows out, and its center fills with fluid as it becomes a blastocyst. Some of the cells form a clump called the inner cell mass. These cells will eventually form the embryo.
A week after conception, the blastocyst begins to nestle into the rich lining of the woman’s uterus. This event, called implantation, takes about a week. As it starts, the outermost cells of the preembryo, called the trophoblast, secrete human chorionic gonadotropin, a hormone which prevents menstruation. HCG detected in a woman’s urine or blood indicates pregnancy.