The Common Vein Copyright 2010
Introduction
The uterus undergoes many functional changes in pregnancy in the processes of implantation, gestation and childbirth.
At the time of ovulation, the endometrium enters its secretory phase. As the sperm move through the uterus, enzymes produced by the endometrial glands catalyze the initial steps of capicitation. Capacitation is an enzymatic reaction that allows sperm to move through the cumulus oophorus and zona pellucida of the ovum. Through their role in capacitation, the endometrial enzymes are necessary components in the process of fertilization. Enzymes from the endometrial glands are also responsible for digestion of the zona pellucida of the fertilized embryo. When the embryo enters the uterus after fertilization, it remains surrounded by the mucopolysaccharide of the zona pellucida. The enzymes form the endometrial glands digest away the mucopolysaccharide, allowing the trophoblastic cells to invade the endometrium and implantation to occur. In the days after implantation, the interstitium and glandular secretions of the endometrium provide nutrition for the developing embryo until the fetal-maternal circulation develops.
During gestation, as the myometrium hypertrophies, it also becomes more contractile. In the first trimester, the uterus begins having infrequent uncoordinated contractions. These contractions are mild and usually go unnoticed. As pregnancy progresses, the contractions increase in intensity and frequency as pregnancy progresses. These contractions become more regular towards the end of the third trimester and become rhythmic as labor begins.
As labor begins, there is an increase in the number of oxytocin receptors on the myometrial smooth muscle cells. Release of the hormone oxytocin from the posterior pituitary stimulates the strong rhythmic contractions of the myometrium. There is also an increase in the number of gap junctions connecting the smooth muscle cells. As a result, the contraction of the myometrial smooth muscle cells becomes more coordinated. In the cervix, increased prostaglandin levels lead to increased expression of enzymes called matrix metalloproteinases. These enzymes break down cross-linking bonds between collagen fibers. Simultaneously, cervical hyaluronic acid content increases. These processes cause the softening, effacement and dilation of the cervix. Once the cervix is dilated, rhythmic myometrial contractions move the fetus through the birth canal. The ligaments of the pelvis relax to allow widening of the pelvic outlet, allowing the fetus to pass. After the delivery of the neonate and separation of the placenta, constriction of the uterine vasculature prevents maternal blood loss. Contraction of the myometrium also compresses the arterioles that fed the placenta to prevent blood loss.