Figure 310

Three types of specialized membrane junctions: (a) desmosome, (b) tight junction, and (c) gap junction. (d) Electron micrograph of two intestinal epithelial cells joined by a tight junction near the luminal surface and a desmosome below the tight junction.

Electron micrograph from M. Farquhar and G.E. Palade, J. Cell. Biol., 17:375-412 (1963).

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Cell Structure CHAPTER THREE

Cell Structure CHAPTER THREE

surfaces of two adjacent plasma membranes are joined together so that there is no extracellular space between them. Unlike the desmosome, which is limited to a disk-shaped area of the membrane, the tight junction occurs in a band around the entire circumference of the cell.

Most epithelial cells are joined by tight junctions. For example, epithelial cells cover the inner surface of the intestinal tract, where they come in contact with the digestion products in the cavity of the tract. During absorption, the products of digestion move across the epithelium and enter the blood. This transfer could take place theoretically by movement either through the extracellular space between the epithelial cells or through the epithelial cells themselves. For many substances, however, movement through the extracellular space is blocked by the tight junctions, and organic nutrients are required to pass through the cells, rather than between them. In this way, the selective barrier properties of the plasma membrane can control the types and amounts of absorbed substances. The ability of tight junctions to impede molecular movement between cells is not absolute. Ions and water can move through these junctions with varying degrees of ease in different epithelium. Figure 3-10d shows both a tight junction and a desmosome near the luminal border between two epithelial cells.

A third type of junction, the gap junction, consists of protein channels linking the cytosols of adjacent cells (Figure 3-10c). In the region of the gap junction, the two opposing plasma membranes come within 2 to 4 nm of each other, which allows specific proteins from the two membranes to join, forming small, protein-lined channels linking the two cells. The small diameter of these channels (about 1.5 nm) limits what can pass between the cytosols of the connected cells to small molecules and ions, such as sodium and potassium, and excludes the exchange of large proteins. A variety of cell types possess gap junctions, including the muscle cells of the heart and smooth-muscle cells where, as we shall see in Chapter 11, they play a very important role in the transmission of electrical activity between the cells. In other cases, gap junctions coordinate the activities of adjacent cells by allowing chemical messengers to move from one cell to another.

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