The term basement membrane was originally given to a layer of variable thickness at the basal surfaces of epithelia. Although a prominent structure referred to as basement membrane is observed with hematoxylin and eosin (H&cE) stain in a few locations, such as the trachea (Fig. 4.16) and, occasionally, the urinary bladder and ureters, basement membrane requires special staining to be seen in the light microscope. This requirement is due, in part, to its thinness and to the effect of the eosin stain, which makes it indistinguishable from the immediately adjacent connective tissue. In the trachea, the structure that is often described as basement membrane includes not only the true basement membrane but an additional layer of closely spaced and aligned collagen fibrils that belong to the connective tissue.
In contrast to H&E (Fig. 4.17a), the periodic acid—Schiff (PAS) staining technique (Fig. 4.17b) results in a positive reaction at the site of the basement membrane. It appears as a thin, well-defined magenta layer between the epithelium and the connective tissue. The stain reacts with the sugar moieties of proteoglycans, accumulating in sufficient amounts and density to make the basement membrane visible in the light microscope. Techniques involving the reduction of silver salts by the sugars blacken the basement membrane and are also used to demonstrate this structure. Although the basement membrane is classically described as exclusively associated with epithelia, similar PAS-posi-tive and silver-reactive sites can be demonstrated surrounding peripheral nerve supporting cells, adipocytes, and mus-
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