Figure 19–49 Organization of fibronectin into fibrils at the cell surface. this fluorescence micrograph shows the front end of a migrating mouse fibroblast. Extracellular fibronectin is stained green and intracellular actin filaments are stained red. the fibronectin is initially present as small dotlike aggregates near the leading edge of the cell. It accumulates at focal adhesions (sites of anchorage of actin filaments, discussed later) and becomes organized into fibrils parallel to the actin filaments. Integrin molecules spanning the cell membrane link the fibronectin outside the cell to the actin filaments inside it (see Figure 19–55). tension exerted on the fibronectin molecules through this linkage is thought to stretch them, exposing binding sites that promote fibril formation. (Courtesy of roumen pankov and Kenneth Yamada.) and Schwann cells (which wrap around peripheral nerve cell axons to form myelin). The basal lamina thus separates these cells and epithelia from the underlying or surrounding connective tissue and forms the mechanical connection between them. In other locations, such as the kidney glomerulus, a basal lamina lies between two cell sheets and functions as a selective filter (Figure 19–50). Basal laminae have more than simple structural and filtering roles, however. They are able to determine cell polarity; influence cell metabolism; organize the proteins in adjacent plasma membranes; promote cell survival, proliferation, or differentiation; and serve as highways for cell migration.
The mechanical role is nevertheless essential. In the skin, for example, the epithelial outer layer—the epidermis—depends on the strength of the basal lamina to keep it attached to the underlying connective tissue—the dermis. In people with genetic defects in certain basal lamina proteins or in a special type of collagen that anchors the basal lamina to the underlying connective tissue, the epidermis becomes detached from the dermis. This causes a blistering disease called junctional epidermolysis bullosa, a severe and sometimes lethal condition.
Laminin and type IV Collagen are Major Components of the Basal Lamina
The basal lamina is synthesized by the cells on each side of it: the epithelial cells contribute one set of basal lamina components, while cells of the underlying bed of connective tissue (called the stroma, Greek for “bedding”) contribute another set (Figure 19–51). Although the precise composition of the mature basal lamina varies from tissue to tissue and even from region to region in the same lamina, it
Figure 19–50 Three ways in which basal laminae are organized. Basal laminae (yellow) surround certain cells (such as skeletal muscle cells), underlie epithelia, and are interposed between two cell sheets (as in the kidney glomerulus). Note that, in the kidney glomerulus, both cell sheets have gaps in them, and the basal lamina has a filtering as well as a supportive function, helping to determine which molecules will pass into the urine from the blood. the filtration also depends on other protein-based structures, called slit diaphragms, that span the intercellular gaps in the epithelial sheet.
Figure 19–51 The basal lamina in the cornea of a chick embryo. In this scanning electron micrograph, some of the epithelial cells have been removed to expose the upper surface of the matlike basal lamina. a network of collagen fibrils in the underlying connective tissue interacts with the lower face of the lamina. (Courtesy of robert trelstad.) typically contains the glycoproteins laminin, type IV collagen, and nidogen, along with the proteoglycan perlecan. Other common basal lamina components are fibronectin and type XVIII collagen (an atypical member of the collagen family, forming the core protein of a proteoglycan).