Mucus

Mucus, a semi-transparent, viscous, sticky mass; when acted upon by dilute acetic acid, alcohol, or ether, M. precipitates in the form of threads and grains. In water, M. swells. It is easily soluble in caustic alkalis. In excess acetic acid and in digestive juices, M. is not soluble. The described properties of M. depend on the fact that its mass is formed by mucin (see Mucin). True M., i.e., consisting of mucin, is found in the adult human body only in the epithelial tissue of mucous membranes as a product of its secretion. Chemically, the mucin that constitutes M. is not the same everywhere; for example, salivary mucin contains 23.5% glucosamine, while tracheal mucin contains 35.0%. During embryonic development, M. is also present in connective tissue as paraplastic substances, and by the time of birth, M. remains only in the umbilical cord, forming the mass of "Wharton's jelly." In the adult, M. can reappear in connective tissue only under pathological conditions as a result of mucoid degeneration. Besides true M., consisting of mucin, substances with similar physical properties are found in the body, depending on their content of pseudomucin or mucoid substances, which are similar to mucin but do not precipitate with acetic acid. Substances of this kind, simulating true M., are found in the vitreous body of the eye, joint fluid, ovarian cysts, tendons, cartilage, bone, heart valves, and arterial walls. For detecting M. in tissues, there are a number of staining methods. Non-specific stains are basic aniline dyes that give a metachromatic reaction (M. appears red). These include: polychrome methylene blue, toluidine blue, thionin, safranin, cresyl violet. Selectively stain: mucicarmine in red and mucihematin in blue. A good method, especially for detecting small droplets of M. in cells of the gastrointestinal tract, is Best's carmine staining. The morphogenesis of M. in epithelial tissue, especially in goblet cells, has been studied in considerable detail. Initially, small grains appear in the cytoplasm of cells, which still stain with acid dyes, in the formation of which chondriosomes apparently participate. Then these grains turn into true mucin droplets with all the reactions of mucin. Subsequently, the mucin droplets swell and, breaking the cell wall, spread over the surface of the mucous membrane. This mechanism of M. formation is regarded as epithelial secretion, and under certain conditions (inflammation), any covering epithelium can begin to secrete M. The secretion of M. is, as it were, a facultative property of the epithelium. The basis for the formation of M. in connective tissue in adults under pathological conditions is based on other poorly studied mechanisms (see Mucoid degeneration). The mechanism of M. formation in fetal connective tissue is also not clarified. Forming and located on the surface of mucous membranes, M. prevents their damage and gives them a smooth, slippery, shiny appearance, which apparently is the physiological significance of mucus. Methods for staining mucus. Among the methods for staining M., two groups are distinguished: one group of stains giving metachromasia, and another that stains M. selectively. Of the metachromatic staining methods, thionin staining can be recommended, with the best results obtained when fixing in a concentrated aqueous solution of mercuric chloride without impurities and without subsequent treatment with iodine: 1) fixation in concentrated aqueous solution of mercuric chloride for 2-8 hours; 2) without washing, it is possible to harden and dehydrate in absolute alcohol as soon as possible; 3) embedding in paraffin; 4) deparaffinized sections kept in concentrated aqueous solution of mercuric chloride for 1/2 minute; 5) rinse in alcohol; 6) stain for 5-15 minutes in a solution of thionin (2 drops of hot saturated aqueous solution of thionin diluted in 5 cm³ of water); 7) rinse in 90% alcohol; 8) rapid dehydration in absolute alcohol; 9) clearing in Minox mixture (1 part clove oil, 5 parts thyme oil) or xylene; 10) balsam. M. and M.-containing tissues are stained in a red-violet tone, other tissues in various shades of blue. Good results with formalin fixation, but better with alcohol fixation, are obtained with polychrome blue staining according to Unna: 1) embedding in paraffin; 2) 10 minutes in polychrome blue; 3) rinse in acidified acetic acid water; 4) hold for 1/4-1/2 minutes in 10% solution of potassium bichromate; 5) dry with filter paper; 6) differentiate for 1/4-1/2 minutes in a mixture of 50.0 aniline oil and 0.5 hydrochloric acid; 7) dehydrate in alcohol, xylene, balsam. Nuclei turn blue; M., cartilage, amyloid-red. Mucicarmine and Best's carmine stain M. selectively (see Carmine).

N. Kraevsky.