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).
fetal alcohol syndrome A rare condition consisting of a combination of congenital defects that result from the continuous consumption of excessive amounts of alcohol by the mother throughout pregnancy. The affected baby has diminished growth, delayed mental development, a small head, a small brain, and small eyes. He or she may have a cleft palate, a small jaw, heart defects, and joint abnormalities. As a newborn, the baby sucks poorly, sleeps badly, and is irritable as a result of alcohol withdrawal. Almost one-fifth of affected babies die during the first few weeks of life; and many who survive are, to some degree, mentally and physically handicapped.
n. the fusion of a spermatozoon and an ovum. Rapid changes in the membrane of the ovum prevent other spermatozoa from penetrating. Penetration stimulates the completion of meiosis and the formation of the second polar body. Once the male and female pronuclei have fused the zygote starts to divide by cleavage.
Health Encyclopedia