mucous membranes

mucous membranes are the moist linings of the orifices and internal parts of the body that are in continuity with the external surface. They cover, protect, and provide secretory and absorptive functions in the channels and extended pockets of the outside world that are incorporated in the body. This applies to: the whole of the alimentary tract from the mouth to the anus; the respiratory tract from the nose through the larynx, trachea, and bronchial tree leading to the microscopic millions of ‘blind ends’ at the lung alveoli; the urogenital tract — vulva, vagina, uterus, and Fallopian tubes in the female, urethra and bladder in both sexes reaching to the kidneys via the ureters, and the vas deferens and tubules reaching into each testis in the male. The linings of all of these are epithelia and most are known as mucous membranes.

Although all these linings are moist, this is by no means everywhere related to the presence of mucus. Actual mucus-secreting cells are scattered among other cells of many mucous membranes, particularly in the intestines and the upper part of the respiratory tract. They are known as ‘goblet cells’ because of the shape of the globule of mucus which may be seen under the microscope inside the cells or discharging through a disrupted cell membrane.

The nature of the cells forming a particular mucous membrane (or mucosa) reflects the specialized function at that site. All these functions are related in some way to interaction between the internal and external environments of the body: nutrition, gas exchange, excretion, or the intrusions and extrusions required for reproduction.

The lining layers are of varying depth. In the areas which are closest to the transition from the skin — in the mouth, anus, and vagina — there are layers of thin cells, like those of the skin, but without the thickened protective outermost layer. In most other sites there is a single layer which may consist of tall ‘columnar’ cells, flat ‘squamous’ cells, or intermediate ‘cuboidal’ cells — again according to function. Many mucous membranes have glands whose ducts dip from the surface to clusters of secretory cells in the deeper layer of tissue (submucosa).

In the alimentary tract, from the mouth through to the end of the small intestine, the glands of the mucosa produce enzymes and other chemical substances necessary for digestion. In the intestines, although the lining is a single-layered sheet of cells, it is thrown into folds, and also has frond-like protrusions (villi), which enormously extend the surface area available for absorption, particularly in the small intestine; here also, the surface of each cell has thousands of microvilli. There are goblet cells scattered throughout, but they become more dense in the large intestine, where lubrication by mucus becomes more necessary as the faeces become more solid.

In the respiratory tract, including the nose and pharynx, and the eustachian tubes that connect it to the middle ears, and down all the branching airways of the lungs as far as the small bronchioles, the cardinal feature of the cells of the mucous membrane is that they are ciliated. The beating movement of the cilia helps to shift upwards and outwards any foreign particles which adhere to a layer of mucus secreted from interspersed goblet cells. (In the finest tubes of the bronchial tree the cells become progressively flatter, until in the alveoli they form the thinnest lining of any epithelium anywhere in the body, facilitating diffusion of gases — no longer called a mucous membrane.)

In parts of the genital tracts also, the lining cells are ciliated, assisting movement of an ovum down the Fallopian tube, or movement of sperm along the tubules of the epididymis, from testis to vas deferens. There are glands in the mucous membranes of the Fallopian tubes, uterus, vagina, and vulva, whose secretions all facilitate the reproductive process, from coitus through fertilization to pregnancy; it is the mucosa of the uterus (the endometrium) which thickens and grows new glands monthly in anticipation of possible pregnancy, and which is shed if this doesn't happen, or develops further if it does. In the male, the glands of the mucosa of the genital tract secrete substances which provide an appropriate environment for sperm on their journey from the testes, and the components of the seminal fluid which accompanies them to their potential destination in the female tract.

In the urinary tract, the mucous membranes that line the urethra, bladder, and ureters are several cells thick, allowing, especially in the bladder, for expansion; the particular protection required here is against the acidity of the urine. (These linings are in continuity with that of the ‘pelvis’ of the kidney and in turn the ducts and tubules leading to the thin membranes at the glomeruli, which filter the blood.)

Sheila Jennett

See also alimentary system; epithelium; lungs.