Figure 19–2 provides a closer view of epithelial cells to illustrate the major types of cell–cell and cell–matrix junctions that we will discuss in this chapter. The diagram shows the typical arrangement of junctions in a simple columnar epithelium such as the lining of the small intestine of a vertebrate. Here, a single layer of tall cells stands on a basal lamina, with the cells’ uppermost surface, or apex, free and exposed to the extracellular medium. On their sides, or lateral surfaces, the cells make junctions with one another. Two types of anchoring junctions link the cytoskeletons of adjacent cells: adherens junctions are anchorage sites for actin filaments; desmosomes are anchorage sites for intermediate filaments. Two additional types of anchoring junctions link the cytoskeleton of the epithelial cells to the basal lamina: actin-linked cell–matrix junctions anchor actin filaments to the matrix, while hemidesmosomes anchor intermediate filaments to it.
Figure 19–1 Two main ways in which animal cells are bound together. In connective tissue, the main stress-bearing component is the extracellular matrix. In epithelial tissue, it is the cytoskeletons of the cells themselves, linked from cell to cell by adhesive junctions. Cell–matrix attachments bond epithelial tissue to the connective tissue beneath it.
adherens junction connects actin flament bundle in one cell with that in the next cell desmosome connects intermediate flaments in one cell to those in the next cell gap junction allows the passage of small water-soluble molecules from cell to cell
Figure 19–2 A summary of the various cell junctions found in a vertebrate epithelial cell, classified according to their primary functions.
In the most apical portion of the cell, the relative positions of the junctions are the same in nearly all vertebrate epithelia. the tight junction occupies the most apical position, followed by the adherens junction (adhesion belt) and then by a special parallel row of desmosomes; together these form a structure called a junctional complex. Gap junctions and additional desmosomes are less regularly organized. two types of cell-matrix anchoring junctions tether the basal surface of the cell to the basal lamina. the drawing is based on epithelial cells of the small intestine.
Figure 19–3 Transmembrane adhesion proteins link the cytoskeleton to extracellular structures. the external linkage may be either to other cells (cell–cell junctions, mediated typically by cadherins) or to extracellular matrix (cell–matrix junctions, mediated typically by integrins). the internal linkage to the cytoskeleton is generally indirect, via intracellular adaptor proteins, to be discussed later.
Two other types of cell–cell junction are shown in Figure 19–2. Tight junctions hold the cells closely together near the apex, sealing the gap between the cells and thereby preventing molecules from leaking across the epithelium. Near the basal end of the cells are channel-forming junctions, called gap junctions, that create passageways linking the cytoplasms of adjacent cells.
Each of the four major anchoring junction types depends on transmembrane adhesion proteins that span the plasma membrane, with one end linking to the cytoskeleton inside the cell and the other end linking to other structures outside it (Figure 19–3). These cytoskeleton-linked transmembrane proteins fall neatly into two superfamilies, corresponding to the two basic kinds of external attachment. Proteins of the cadherin superfamily chiefly mediate attachment of cell to cell (Movie 19.1). Proteins of the integrin superfamily chiefly mediate attachment of cells to matrix. There is specialization within each family: some cadherins link to actin and form adherens junctions, while others link to intermediate filaments and form desmosomes; likewise, some integrins link to actin and form actinlinked cell–matrix junctions, while others link to intermediate filaments and form hemidesmosomes (Table 19–1).