Muscle cells, adipocytes, and peripheral nerve supporting cells exhibit an extracellular electron-dense material that resembles the basal lamina of epithelium. This material also corresponds to a PAS-positive staining reaction, as described earlier (see Fig. 5.27). Although the term basement membrane is not ordinarily applied to the extracellular stainable material of these nonepithelial cells in light microscopy, the terms basal lamina or external lamina are typically used at the EM level.
The basal lamina contains molecules that come together to form a sheetlike structure.
Analyses of basal laminae derived from epithelia in many locations (kidney glomeruli, lung, cornea, lens of the eye) indicate that they consist of approximately 50 proteins that can be classified into four groups: collagens, laminins, glycoproteins, and proteoglycans. These proteins are synthesized and secreted by the epithelial cells and other cell types that possess an external lamina.  Collagens. At least three types of collagen species are present in the basal lamina; they represent a fraction of the approximately 28 types of collagen found in the body.
The major component, comprising 50% of all basal used inconsistently in the literature. Some authors use basement membrane when referring to both light and electron microscopic images. Others dispense with the term basement membrane altogether and use basal lam-ina in both light and electron microscopy. Because the term basement membrane originated with light mi-croscopy, it is used in this book only in the context of light microscopic descriptions and only in relation to epithelia. tural content to denote the layer present at the interface of connective tissue with epithelial cells. In this context, the light microscopy term basement membrane actually de-scribes basal lamina and the underlying reticular lamina combined. The term external lamina is used to identify basal lamina when it forms a peripheral cellular invest-ment, as in muscle cells and peripheral nerve supporting cells.
 FOLDER 5.4 Functional Considerations: Basement Membrane and Basal Lamina Terminology
The terms basement membrane and basal lamina are The EM term basal lamina is reserved for the ultrastruc-
FIGURE 5.28 • Electron micrograph of two adjoining epithelial cells with their basal lamina. The micrograph shows only the basal portions of the two cells and parts of their nuclei (N). The intercellular space is partially obscured by lateral interdigitations between the two cells (arrows). The basal lamina (BL) appears as a thin layer that follows the contours of the basal domain of the overlying cell. Below the basal lamina are numerous cross-sectioned collagen (reticular) fibrils. 30,000.
lamina proteins, is type IV collagen. The molecular characteristics and function of type IV collagen in forming a scaffold of basal lamina is described in the next section. The presence of different type IV collagen isoforms provides specificity to the basal lamina associated with different tissues. Two nonfibrillar types of collagens, type XV collagen and type XVIII collagen, are also found in the basal lamina. Type XV collagen plays an important role in stabilizing the structure of the external lamina in skeletal and cardiac muscle cells, whereas type XVIII collagen is mainly present in vascular and epithelial basal laminae and is believed to function in angiogenesis. In addition, type VII collagen forms anchoring fibrils that link the basal lamina to the underlying reticular lamina (described below).