Fertilization

The union of a sperm and an ovum. In natural fertilization, the sperm and ovum unite in the fallopian tube of the woman following sexual intercourse. A single sperm penetrates the ovum by releasing enzymes that can dissolve the outer layers of the ovum. Once inside, the sperm’s nucleus fuses with that of the ovum, and its empty body shell and tail drop off. Then, the newly fertilized ovum, called a zygote, forms an outer layer that is impenetrable to other sperm. The zygote undergoes repeated cell divisions as it passes down the fallopian tube to the uterus, where it implants and will eventually grow into an embryo. Fertilization may also occur as a result of semen being artificially introduced into the cervix (see artificial insemination) or may take place in a laboratory (see in vitro fertilization).