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The wealth of diverse specializations to be found among the cells of a higher animal is far greater than any procaryote can show. In a vertebrate more than 200 distinct cell types are plainly distinguishable, and many of these types of cells certainly include, under a single name, a large number of more subtly different varieties. Panel 1-3 (pp. 36-37) shows a small selection. In this profusion of specialized behaviors one can see displayed, in a single organism, the astonishing versatility of the eucaryotic cell. Much of our current knowledge of the general properties of eucaryotic cells has depended on the study of such specialized types of cells, because they demonstrate exceptionally well particular features on which all cells depend in some measure. Each feature and each organelle of the prototype that we have outlined in Panel 1-1 (pp. 18-19) is developed to an unusual degree or revealed with special clarity in one cell type or another. To take one arbitrary example, consider the neuromuscular junction, where just three types of cells are involved: a muscle cell, a nerve cell, and a Schwann cell. Each has a very different role (Figure 1-37):
1. The muscle cell has made contraction its specialty. Its cytoplasm is packed with organized arrays of protein filaments, including vast numbers of actin filaments. There are also many mitochondria interspersed among the protein filaments, supplying ATP as fuel for the contractile apparatus.
2. The nerve cell stimulates the muscle to contract, conveying an excitatory signal to the muscle from the brain or spinal cord. The nerve cell is therefore extraordinarily elongated: its main body, containing the nucleus, may lie a meter or more from the junction with the muscle. The cytoskeleton is consequently well developed so as to maintain the unusual shape of the cell and to transport materials efficiently from one end of the cell to the other. The most crucial specialization of the nerve cell, however, is its plasma membrane, which contains proteins that act as ion pumps and ion channels, causing a movement of ions that is equivalent to a flow of electricity. Whereas all cells contain such pumps and channels in their plasma membranes, the nerve cell has exploited them in such a way that a pulse of electricity can propagate in a fraction of a second from one end of the cell to the other, conveying a signal for action.
3. Lastly, Schwann cells are specialists in the mass production of plasma membrane, which they wrap around the elongated portion of the nerve cell, laying down layer upon layer of membrane like a roll of tape, to form a myelin sheath that serves as insulation.