Animals are multicellular communities of individual cells. Lying between and supporting the cells is the extracellular matrix (Figure 1.4) of different types of fiber around which the fluids and solute of the interstitial fluid can easily pass. All the body cells that comprise a single organism share the same set of genetic instructions in their nuclei. Nevertheless, the cells are not all identical. Rather, they form a variety of tissues, groups of cells that are specialized to carry out a common function. This specialization occurs because different cell types read out different parts of the DNA blueprint and therefore make different proteins. In animals there are four major tissue types: epithelium, connective tissue, nervous tissue, and muscle. Some examples of the cells that make up these tissues are shown in Figure 1.5.
Epithelia are sheets of cells that cover the surface of the body and line its internal cavities, such as the lungs and intestine. The cells may be columnar, meaning taller than they are broad (Figure 1.5b), or squamous, meaning flat (e.g. the capillary epithelial cell in Figure 1.4). They are often polarized, meaning that one surface of the cell is distinct in its organization, composition, and appearance from the other. In the intestine, the single layer of columnar cells lining the inside, or lumen, has an absorptive function that is increased by the folding of the surface into villi (Figure 1.6). The luminal surfaces of these polarized cells have microvilli that increase the surface area even further. The basal (bottom) surface sits on a thin planar sheet of specialized extracellular matrix called the basement membrane or basal lamina. Many of the epithelial cells of the airways, for instance those lining the trachea and bronchioles, have cilia on their surfaces (Figure 1.7). These are hairlike appendages that actively beat back and forth, moving a layer of mucus away from the lungs (Chapter 13). Particles and bacteria are trapped in the mucus layer, preventing them from reaching the delicate membranes in the lung that carry out air exchange. In the case of the skin, the epithelium is said to be stratified because it is composed of several layers.
Connective tissues provide essential support for the other tissues of the body. They include bone, cartilage, and adipose (fat) tissue. Unlike other tissues, connective tissue contains relatively few cells within a large volume of extracellular matrix that consists of different types of fiber embedded in amorphous ground substance (Figure 1.6). The most abundant of the fibers is collagen, a protein with the tensile properties of steel that accounts for about a third of the protein of the human body. Other fibers have elastic properties that permit the supported tissues to be displaced and then to return to their original position. The amorphous ground substance contains large quantities of water, facilitating the diffusion of metabolites, oxygen, and carbon dioxide to and from the cells in other tissues and organs. Of the many cell types found in connective tissue, two of the most important are fibroblasts, which make and secrete the ground substance and fibers, and macrophages, which remove foreign, dead, and defective material. A number of inherited diseases are associated with defects in connective tissue. Osteogenesis imperfecta or Brittle Bone Disease, for example, is characterized by short height, loose joints, and bones that break easily. These characteristics result from a defect in the organization of the collagen fibers (see Medical Relevance 3.2 on page 47).
Source: Image by Giorgio Gabella, Department of Cell and Developmental Biology, University College London. Reproduced by permission.
Source: Image by Giorgio Gabella, Department of Cell and Developmental Biology, University College London. Reproduced by permission.
Nervous tissue is a highly modified epithelium that is composed of several cell types. Principal among these are the nerve cells, also called neurons (Figure 1.5c), along with a variety of supporting cells that help maintain them. Neurons extend processes called axons, which can be over a meter in length. Neurons constantly monitor what is occurring inside and outside the body. They integrate and summarize this information and mount appropriate responses to it (Chapters 9–11). Another type of cell, glia, has other roles in nervous tissue, including forming the electrical insulation around axons.
Muscle tissue can be of two types, smooth or striated. Smooth muscle cells are long and slender and are usually found in the walls of tubular organs such as the intestine and many blood vessels. In general, smooth muscle cells contract slowly and can maintain the contracted state for a long time. There are two classes of striated muscle: cardiac and skeletal. Cardiac muscle cells (Figure 1.4) make up the walls of the heart chambers. These are branched cells that are connected electrically by gap junctions (page 26), and their automatic rhythmical contraction powers the beating of the heart. Each skeletal muscle is a bundle of hundreds to thousands of fibers,