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.)
See also alimentary system; epithelium; lungs.
Mucous membranes play a key role in sexual activities. These membranes line body canals that open to the outside environment, including the mouth, lips, nose, the inside of the eyelids, the lungs and windpipe, the digestive tract, the vagina, the inside of the vulva, the glans and foreskin of the penis, the urethra, and the rectum. Mucous membranes are comprised of epithelial tissue, or the cells that layer to form a protective covering. Skin has several layers of epithelial cells. Mucous membranes are made up of three layers of tissue: a layer of epithelial cells, which also contains blood vessels, lymph vessels, and goblet cells that secret mucus; a layer of lamina, or connective tissue; and a layer of muscle cells. The presence of blood vessels makes mucous membranes pink in color. Some mucous membranes also contain pigments that hide the blood vessels.
Mucous membranes protect the surfaces of the canals and organs they cover. The mucus secreted by the goblet cells is a thick fluid that shields surfaces and organs, lubricates passages, traps foreign particles, and absorbs water-soluble substances such as salts. Not all mucous membranes secrete mucus—the name of the membrane refers to where the membranes are located rather than to their secretion of mucus.
The color of mucous membranes, especially in the eyelid, reflects the number of red blood cells flowing through membrane blood vessels. For this reason, the mucous membranes of the eyelid provide some indication of relative health. If the mucous membranes are pale, this indicates a decreased number of red blood cells, which may indicate a number of problems such as shock, bleeding, or poisoning. Yellow membranes indicate jaundice, which may be caused by liver diseases such as hepatitis.
Not only are mucous membranes sensitive to the touch, they respond to touch and other stimuli by producing mucus. In addition, sexual stimulation in other regions such as the lips and nipples increases the flow of mucus in areas such as the genitals, which are lined with mucous membranes. Increased mucus makes touching more pleasurable by providing a layer of protection against irritation and making passage over surfaces easier. Males who have been circumcised have had a portion of this mucous membrane removed and may experience more irritation in the less protected glans. Increased mucus in the vulva and vagina not only indicates sexual arousal, but also facilitates penetration and the movement of sperm. Postmenopausal women often experience a decrease in mucus secretions, which makes sexual intercourse more difficult and uncomfortable. Commercial lubricants can be used to substitute for the decrease in mucus. Because the mucous membranes are sensitive and absorb water-soluble chemicals with which they come into contact, it is necessary to choose such lubricants with care to avoid damaging the membranes.
Marieb, Elaine N., and Katja Hoehn. 2006. Human Anatomy and Physiology. 7th edition. San Francisco: Pearson/Benjamin Cummings.
Marieb, Elaine N. 2006. Essentials of Human Anatomy and Physiology. 8th edition. San Francisco: Pearson/Benjamin Cummings.
Netter, Frank H. 2006. Atlas of Human Anatomy. 4th edition. New York: Saunders/Elsevier.