The alveolar basal lamina, which defines the border between the original interstitium and the air space, can be conveniendy and specifically visualized for light microscopy by immunohistochemistry using antibodies directed to its constituent proteins, such as laminin and type IV collagen. It can also be identified by electron microscopy. Cells actively synthesizing connective-tissue proteins can be identified by several complementary techniques.
At electron microscopy, synthetically active fibroblasts have abundant rough endoplasmic reticulum. In our laboratory, we use immunohistochemistry with antibodies directed to the propeptides of type I collagen to obtain more direct evidence of collagen synthesis. Because the propeptides are cleaved from the collagen molecule when it is secreted from the fibroblast, extracellular fibrils of processed collagen are not recognized. Only intracellular collagen stains. Conveniendy, although collagen is constitutively synthesized by adult lung, the levels are sufficiendy low that normal lung tissue is unstained, and the antibody stains only fibroblasts activated to increase their synthesis by disease. A third approach to localizing synthetically active fibroblasts is the detection of mRNi for connective tissue proteins by in situ hybridization.