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Cells and extracellular material together comprise the tissues that make up animal organs. In all tissues, cells are the basic structural and functional units, the smallest living parts of the body. Animal cells are enclosed by cell membranes and are eukaryotic, each with a distinct, membrane-enclosed nucleus surrounded by cytoplasm, fluid containing a system of membranous organelles, nonmembranous molecular assemblies, and a cytoskeleton. In contrast, the smaller prokaryotic cells of bacteria typically have a cell wall and lack nuclei and membranous cytoplasmic structures.


The average adult human body consists of nearly 40 trillion cells, according to the best available estimate. These cells exist as hundreds of histologically distinct cell types, all derived from the zygote, and the single cell formed by the merger of a spermatozoon with an oocyte at fertilization. The first zygotic cellular divisions produce cells called blastomeres, and as part of the early embryo’s inner cell mass blastomeres give rise to all tissue types of the fetus. Explanted to tissue culture cells of the inner cell mass are called embryonic stem cells. Most cells of the fetus undergo a specialization process called differentiation in which they predominantly express sets of genes that mediate specific cytoplasmic activities, becoming efficiently organized in tissues with specialized functions and usually changing their shape accordingly. For example, muscle cell precursors elongate into long, fiber-like cells containing large arrays of actin and myosin. All animal cells contain actin filaments and myosins, but muscle cells are specialized for using these proteins to convert chemical energy into forceful contractions.

Major cellular functions performed by specialized cells in the body are listed in Table 2–1. It is important to understand that the functions listed there can be performed by most cells of the body; specialized cells have greatly expanded their capacity for one or more of these functions during differentiation. Changes in cells’ microenvironments under normal and pathologic conditions can cause the same cell type to have variable features and activities. Cells that appear similar structurally often have different families of receptors for signaling molecules such as hormones and extracellular matrix (ECM) components, causing them to behave differently. For example, because of their diverse arrays of receptors, breast fibroblasts and uterine smooth muscle cells are exceptionally sensitive to female sex hormones, while most other fibroblasts and smooth muscle cells are insensitive.

TABLE 2–1Differentiated cells typically specialize in one activity.

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