UTERUS: APPEARANCE AND FUNCTION
The uterus is about the size and shape of a small hollow pear with its wide end upwards. Two fine tubes, the Fallopian tubes, branch outwards from the upper part of the uterus towards the ovaries. During reproductive life, a mature egg bursts from an ovary at approximately monthly intervals. In the normal course of events, an egg travels through a Fallopian tube to the uterus. On the way there, conception can occur if the egg meets and fuses with a sperm cell. The resulting embryo may develop into a viable pregnancy if it successfully embeds in the uterus.
The largest part of the uterus, the myometrium, is strong and flexible, being composed of thick bands of muscle and elastic tissue that run in several directions. This muscular web enables the uterus to be either small and tidy, somewhat like a clenched fist or, should conception occur, to stretch and grow into a strong, flexible capsule capable of nourishing and protecting a developing baby. The lower end of the uterus, the cervix, has some specialised functions including the production of lubricative or dense secretions at different times of the menstrual cycle. These secretions help minimise friction during sexual intercourse and seem to have a role in sperm movement through the vagina and cervix. During the Pap or cervical smear test, the cervix is the tissue that is sampled.
In addition to the thick myometrium and the cervix, the other important component of the uterus is its very active inner lining, called the endometrium. This is shed during menstruation or, if circumstances permit and a pregnancy occurs, it provides nutrients to an embryo. Before a girl has her first period (an event referred to as the menarche) and after a woman has her last (menopause), the endometrium usually consists of a thin layer of cells which may grow and be shed slowly without any menstrual bleeding.
The fertile years see a big change with menstruation typically occurring for several days about once every month. In the lead up to the menstrual bleed, row upon row of endometrial cells grow rapidly, influenced by sex hormones from the ovaries (particularly oestrogen and progesterone). Recent research suggests that the shedding process requires substances produced by the endometrium itself (such as the hormone-like prostaglandins and enzymes known as matrix metalloproteinases), together with decreasing levels of sex hormones.
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