3.12: Cells are connected and communicate with each other.

Cells reach out and connect; no telephone lines required.

So far in this chapter we have examined the cell as a free-living and independent entity. The majority of cells in any multicellular organism, however, are connected to other cells. Cells adhere to other cells in a variety of ways, involving numerous types of protein and glycoprotein adhesion molecules. We examine three primary types of connections between animal cells: (1) tight junctions, (2) desmosomes, and (3) gap junctions (FIGURE 3-26).

Figure 3.26: Cell connections: tight junctions, desmosomes, and gap junctions.

Tight junctions form continuous, water-tight seals around cells and also anchor cells in place. Much like the caulking around a tub or sink that keeps water from leaking into the surrounding walls, tight junctions prevent fluid flow between cells. Tight junctions are particularly important in the small intestine, where digestion occurs. Cells lining the small intestine absorb nutrients from the watery fluid moving through your gut. If the fluid—and the resident bacteria—inside the intestine were to leak between the cells and into your body cavity, you would not be able to extract sufficient energy and nutrients from your food, and the bacteria would make you sick. The tight junctions instead force fluid to pass into the cells that line the intestine, where the nutrients can be used.

Desmosomes are like spot welds or rivets that fasten cells together into strong sheets. They occur at irregular intervals and function like fastened Velcro: they hold cells together but are not water-tight, allowing fluid to pass around them. Desmosomes and other similar junctions are found in much of the tissue that lines the cavities of animals’ bodies. They also are found in muscle tissue, holding fibers together. Genetic disorders that reduce cells’ ability to form desmosome proteins or lead to destruction of desmosomes by the immune system result in the formation of blisters, where layers of skin separate from each other.

Finally, gap junctions are pores surrounded by special proteins that form open channels between two cells (see Figure 3-26). Functioning like secret passageways, these junctions are large enough for salts, sugars, amino acids, and the chemicals that carry electrical signals to pass through, but are too small for the passage of organelles or very large molecules such as proteins and nucleic acids. Gap junctions are an important mechanism for cell-to-cell communication. In the heart, for example, the electrical signal telling muscle cells to contract is passed from cell to cell through gap junctions. Gap junctions are also important in allowing a cell to recognize that it has bumped up against another cell; chemicals flowing from one cell to the next can signal the body to stop producing cells of a particular type.

Compared with normal cells, cancer cells have fewer gap junctions. And research suggests that the resulting reduction in intercellular communication among cancer cells may be important in the formation of masses of cells, called tumors, and the spread of cancer cells throughout the body. (Interestingly, treatments that cause an increase in the number of gap junctions tend to reduce tumor growth.)