uterus: Definition and Much More from Answers.com

The uterus has three major functions: to prepare a bed for a fertilized ovum, to nourish the developing embryo during pregnancy, and to expel the fetus. Shaped like an upside-down pear, and tilted forwards, it lies within the pelvis and is held in place, along with its two extensions, the Fallopian tubes, by ligaments and folds of the peritoneum. The cervix forms the lower third, connected by a narrow isthmus to the main muscular body of the uterus. The non-pregnant uterus weighs 45-60 g and is 7-8 cm long but its weight increases more than ten-fold by the end of pregnancy.

The main bulk of the uterus is made up of smooth muscle fibres known as the myometrium. The inner layer of muscle fibres is arranged in a circular pattern and the outer layer in a longitudinal pattern with a middle layer of interlacing oblique fibres. Inside the muscle is a cellular lining with a glandular (secretory) surface — the endometrium. While the myometrium is responsible for contractions of the uterus (obviously particularly important during labour), it is the endometrium which develops in every cycle to prepare for an embryo, and which is shed during menstruation. Thus during reproductive years the uterus is a highly dynamic organ, its functions being controlled by oestrogen and progesterone secreted by the ovaries and by other hormones associated with pregnancy and labour.

At the time of puberty, when oestrogen secretion from the ovaries begins to rise, there is an increase in both the size of the uterus and the blood flow which it receives. When menstrual cycles are established both the myometrium and the endometrium show cyclical, hormone-dependent changes in structure and function. These change again after implantation of an embryo, during pregnancy, and at delivery.

In the first half of the menstrual cycle the uterus prepares to receive and transport sperm from the cervix to the oviducts (Fallopian tubes). Under the influence of oestrogen secreted by the ovaries, the myometrium becomes more excitable and begins to contract sporadically. Meanwhile the cells under the surface of the endometrium begin to proliferate (hence the term proliferative phase of the uterine cycle) and those on the surface grow projections into the cavity of the uterus and invade the deeper layer underneath. Thus, from a relatively smooth surface at the beginning of each cycle, the endometrium not only thickens but becomes a highly indented structure, with the epithelial glands secreting a watery fluid. Spiral arteries grow up into the projections.

Towards mid-cycle, as ovulation approaches, the uterus is primed to bind progesterone. Thus, in the second half of the menstrual cycle, when progesterone production by the ovaries is high, the progestogenic or secretory phase of the uterine cycle begins. Progesterone stimulates the glandular cells of the lining to produce a thick secretion rich in proteins, sugars, and amino acids, and the whole endometrium thickens. The spiral arteries become fully developed and show rhythmic dilatations and constrictions. Progesterone also causes an increase in the size of the smooth muscle cells of the myometrium, but, in contrast to oestrogen, progesterone reduces excitability and so contractions are quietened. So the uterus prepares itself for an embryo, with an endometrium about 5 mm thick and well supplied with blood. It is important to note that these actions of progesterone will only occur if the uterus has been primed with oestrogen during the first half of the cycle.

If fertilization does not occur, the corpus luteum begins to degenerate and its hormone secretions begin to wane. The uterus loses its hormonal support, blood flow to the endometrial tissue is reduced, and consequently this lining layer dies. However, there is some bleeding from the spiral arteries into the disintegrating endometrium, and thus blood and dead cells are shed through the cervix and vagina. At the end of menstruation the endometrium is only about 0.5 mm thick — the change in thickness has been ten-fold.

The cervix also shows cyclical changes with each menstrual cycle. In the first half of the cycle, under the influence of oestrogen, the tissue becomes more vascular, the muscle relaxes and the lining becomes more secretory. In the second half of the cycle when progesterone production is elevated secretion is reduced and the tissue becomes firmer. However, the most important changes seen in the cervix are in the composition and properties of mucus secreted by its lining. Tests on cervical mucus are important since a hostile, impenetrable mucus can reduce fertility. As ovulation approaches the water and salt content of the mucus increase and it becomes less viscous, allowing for easier penetration of sperm. If mucus is taken from a cervical smear at this stage of the cycle and allowed to dry on a glass slide, a characteristic fern-leaf pattern of crystallization occurs, known as ferning. One can also draw this mucus out into long threads — a property known as spinnbarkheit. In contrast, mucus obtained in the second half of the cycle is thick, and strands of mucus cannot be stretched far before they break — a low spinnbarkheit. Thus the ability of sperm to penetrate cervical mucus is high at ovulation but low during the luteal phase when progesterone secretion is elevated. Indeed, the effects of progesterone on cervical mucus are such that low-dose progestogenic contraceptives given throughout the cycle can suppress sperm penetration through the cervix even at the time of ovulation when oestrogen levels are high.

In pregnancy the increase in size of the uterus is impressive: its walls remain thick despite the distension, because of the dramatic growth of its muscle fibres. The size and number of the blood vessels which supply it increase, carrying a twenty-fold increase in blood flow by full term. After delivery it shrinks rapidly, although taking some weeks to return to its previous size.