The basal lamina is the structural attachment site for overlying epithelial cells and underlying connective tissue.
Former descriptions of basal lamina were based on the investigation of specimens routinely prepared for electron microscopy. The examination of the site of epithelial base-
FIGURE 5.25 • Tracheal basement membrane. Photomicrograph of an H&E–stained section of the pseudostratified ciliated epithelium of the trachea. The basement membrane appears as a thick homogeneous layer immediately below the epithelium. It is actually a part of the connective tissue and is composed largely of densely packed collagen fibrils. 450.
ment membranes with the EM reveals a discrete layer of electron-dense matrix material 40to 60-nm thick between the epithelium and the adjacent connective tissue (Fig. 5.28) called the basal lamina or, sometimes, lamina densa. When observed at high resolution, this layer exhibits a network of fine, 3to 4-nm filaments composed of laminins, a type IV collagen molecule, and various associated proteoglycans and glycoproteins. Between the basal lamina and the cell is a relatively clear or electron-lucent area, the lamina lucida (also about 40 nm wide). The area outlined by the lamina lucida contains extracellular portions of CAMs, mainly fbronectin and laminin receptors. These receptors are members of the integrin family of transmembrane proteins.
With the development of new EM preparation techniques, the lamina lucida appears to be an artifact of fixation; in the living state, the basal lamina is composed of a single layer of the lamina densa.
If the tissue specimen for EM is fixed using low-temperature, high-pressure freezing (HPF) methods (without chemical fixatives), it retains much more of the tissue than specimens routinely fixed with glutaraldehyde. EM examination of such specimens reveals that the basal lamina is composed only of the lamina densa. No lamina lucida is detected. The lamina lucida may thus be an artifact of chemical fixation that appears as the epithelial cells shrink away from a high concentration of macromolecules deposited next to the basal domain of the epithelial cells. It probably results from the rapid dehydration that occurs during tissue processing for electron microscopy. Other structures visible with traditional electron microscopy are also not visible when tissues are prepared by the HPF method (Fig. 5.29).
FIGURE 5.26 • Photomicrographs showing serial sections of intestinal glands of the colon. The glands in this specimen have been cross-sectioned and appear as round profiles. a. This specimen was stained with H&E. Note that neither the basement membrane nor the mucin that is located within the goblet cells is stained. 550. b. This section was stained by the PAS method. It reveals the basement membrane as a thin, magenta layer (arrows) between the base of the epithelial cells of the glands and the adjacent connective tissue. The mucin within the goblet cells is also PAS positive. 550.
FIGURE 5.27 • Smooth muscle external lamina. This photomicrograph is stained by the PAS method and counterstained with hematoxylin (pale nuclei). The muscle cells have been cut in cross section and appear as polygonal profiles because of the presence of PAS-positive basement membrane material surrounding each cell. The cytoplasm is not stained. As the plane of section passes through each smooth muscle cell, it may or may not pass through the portion of the cell that includes the nucleus. Therefore, in some of the polygonal profiles, nuclei can be seen; in other profiles, no nuclei are seen. 850.
The basal lamina in nonepithelial cells is referred to as the external lamina.