FIGURE 11.

The extracellular matrix components and their specialization in the basal lamina. A: the extracellular matrix of peripheral tissue contains the fibrous proteins collagen, laminin, and fibronectin, which are embedded in proteoglycans and hyaluronan. Proteoglycans and hyaluronan both contain glycosaminoglycans (GAGs), which have a large capacity for binding water and Na⁺. Insert: proteoglycans contain a core protein, which is the binding target of numerous repeating disaccharide chains. Hyaluronan is also composed of a repeating disaccharide chain but differs with respect to its lack of core proteins. Hyaluronan interacts with proteoglycans but not with collagen, laminin, and fibronectin. Most of the extracellular matrix proteins interact with multiple classes of membrane receptors, including integrins and CD44. These receptors are coupled via linker proteins on the cytosolic site with the actin cytoskeleton. B: the basal lamina consists of a repetitive, highly organized interweaving of collagen IV, laminin, fibronectin, and proteoglycans. The basal lamina is permeable to soluble proteins such as albumin, and the presence of gaps between the endothelial cells allows free passage of an ultrafiltrate of plasma into the tissue, driven by hydrostatic pressure (blue arrows). The basal lamina induces polarized expression of channels, transporters, and ion pumps in the basolateral membrane (e.g., exocrine glands).