In 1907, American embryologist H. V. Wilson discovered that if he pressed a live sponge through fine cloth he could break up the sponge into individual cells. Then if he swirled the cells together, they would coalesce back into a group resembling a sponge. If he swirled the cells from sponges of two different species together, he observed that the cells sorted themselves out—that is, cells from one species of sponge associated only with cells from that same species (Fig. 10.12a). Fifty years later, German-born embryologist Johannes Holtfreter observed that if he took neuronal cells and skin cells from an amphibian embryo and treated them the same way that Wilson had treated sponge cells, the embryonic cells would sort themselves according to tissue type (Fig. 10.12b).

Cells are able to sort themselves because of the presence of various proteins on their surface called cell adhesion molecules that attach cells to one another or to the extracellular matrix. While a number of cell adhesion molecules are now known, the cadherins (calcium-dependent adherence proteins) are especially important in the adhesion of cells to other cells. There are many different kinds of cadherin, and a given cadherin may bind only to another cadherin of the same type. This property explains Holtfreter’s observations of the cells from amphibian embryos. E-cadherin (for “epidermal cadherin”) is present on the surface of embryonic epidermal cells, and N-cadherin (for “neural cadherin”) is present on neuronal cells. The epidermal cells adhered to one another through E-cadherin, and the neuronal cells adhered to each other through N-cadherin.

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Cadherins are transmembrane proteins (Chapter 5). The extracellular domain of a cadherin molecule binds to the extracellular domain of a cadherin of the same type on an adjacent cell. The cytoplasmic portion of the protein is linked to the cytoskeleton, including microfilaments and intermediate filaments (Fig. 10.13a). This arrangement provides structural continuity from the cytoskeleton of one cell to the cytoskeleton of another, increasing the strength of tissues and organs.

As well as being stably connected to other cells, cells also attach to proteins of the extracellular matrix. Cell adhesion molecules that enable cells to adhere to the extracellular matrix are called integrins. Like cadherins, integrins are transmembrane proteins, and their cytoplasmic domain is linked to microfilaments or intermediate filaments (Fig. 10.13b). Also like the cadherins, integrins are of many different types, each binding to a specific extracellular matrix protein. Integrins are present on the surface of virtually every animal cell. In addition to their role in adhesion, integrins also act as receptors that communicate information about the extracellular matrix to the interior of the cell (section 10.4)