Cell Types and Tissues

Isolated animal cells in tissue culture, no matter how highly differentiated, tend to revert quickly to one of three basic types known as epitheliocytes, mechanocytes, and amebocytes. Epitheliocytes are closely adherent cells derived from epithelial tissues and thought to be related in their origins to the two surface layers of the embryonic blastula. Mechanocytes, often called fibroblasts or fibrocytes, are derived from muscle, supporting, or connective tissue. Like the amebocytes, they arise from embryonic mesenchymal tissue cells that have migrated inward from the lower side of the blastula. Neurons, neuroglia, and lymphocytes are additional distinct cell types.
Tissues. Cells aggregate to form four major kinds of tissue. Epithelial tissues line the primary surfaces of the body: the skin, the digestive tract, urogenital tract, and glands. External skin is composed of flat platelike squamous epithelial cells whereas internal surfaces are often formed by colummar epithelial cells. Glands (sweat, oil, mammary, and internal secretory) as well as the sensory organs of the tongue, nose, and ear are all composed of epithelial cells. Epithelial cells are among the most highly polarized of cells. One side of each cell faces the outside, either air or water, while the other side is often directly against a basement membrane.
Supporting and connective tissues include the fatty adipose tissue as well as cartilage and bone. Both of the latter contain large amounts of intercellular material or ground substance consisting largely of complex polymers. Embryonic fibroblasts differentiate into white fibers, which produce collagen, and yellow fibers, which form elastin. The fibrils of both of these proteins are assembled in the intercellular space where they are embedded in the ground substance. Osteoblasts form bone by deposition of calcium phosphate in 3–7 µm thick layers within a ground substance that contains special proteins. A third tissue is muscle, which is classified into three types: striated (voluntary skeletal muscle), cardiac (involuntary striated muscle), and smooth (involuntary) muscle. There are two major groups of cells in nervous tissue, the fourth tissue type. Neurons are the actual conducting cells whose cell membranes
carry nerve impulses. Several kinds of glial cells lie between and around the neurons.
Blood cells. Blood and the linings of blood vessels may be regarded as a fifth tissue type. The human body contains 5 x 109 erythrocytes or red blood cells per ml, a total of 2.5 x 1013 cells in the five liters of blood present in the body. Erythrocytes are rapidly synthesized in the bone marrow. The nucleus is destroyed, leaving a cell almost completely filled with hemoglobin. With an average lifetime of 125 days, human red blood cells are destroyed by leukocytes in the spleen and liver. The white blood cells or leukocytes are nearly a thousandfold less numerous than red cells. About 7 x 106 cells are present per ml of blood. There are three types of leukocytes: lymphocytes (~26% of the total), monocytes (~7% of the total), and polymorphonuclear leukocytes or granulocytes (~70% of the total). Lymphocytes are about the same size as erythrocytes and are made in lymphatic tissue. Individual lymphocytes may survive for as long as ten years. They function in antibody formation and are responsible for maintenance of long-term immunity.
Monocytes, two times larger, are active in ingesting bacteria. These cells stay in the blood only a short time before they migrate into the tissues where they become macrophages, relatively fixed phagocytic cells. Macrophages not only phagocytize and kill invading bacteria, protozoa, and fungi but also destroy cancer cells. They also destroy damaged cells and cellular debris as part of the normal turnover of tissues. They play an essential role in the immune system by “processing” antigens and in releasing stimulatory proteins.
Granulocytes of diameter 9–12 µm are formed in the red bone marrow. Three types are distinguished by staining: neutrophils, eosinophils, and basophils. Neutrophils are the most numerous phagocytic cells of our blood and provide the first line of defense against bacterial infections. The functions of eosinophils and basophils are less well understood. The number of eosinophils rises during attacks of hay fever and asthma and under the influence of some parasites, while the basophil count is increased greatly in leukemia and also by inflammatory diseases. Granules containing histamine, heparin, and leukotrienes are present in the basophils. Blood platelets or thrombocytes are tiny (2–3 µm diameter) cell-like bodies essential for rapid coagulation of blood. They are formed by fragmentation of the cytoplasm of bone marrow megakaryocytes. One mature megakaryocyte may contribute 3000 platelets to the 1–3 x 108 per ml present in whole blood.
Cell culture. Laboratory growth of isolated animal cells has become very important in biochemistry. Sometimes it is necessary to have many cells with as nearly as possible identical genetic makeup. Such bacterial cells are obtained by plating out the bacteria and selecting a small colony that has grown from a single cell to propagate a “pure strain.” Similarly, single eukaryotic cells may be selected for tissue culture and give rise to a clone of cells which remains genetically identical until altered by mutations. The culture of embryonic fibroblasts is used to obtain enough cells to perform prenatal diagnosis of inherited metabolic diseases. Tissue culture is easiest with embryonic or cancer cells, but many other tissues can be propagated. However, the cells that grow best and which can be propogated indefinitely are not entirely normal; the well-known HeLa strain of human cancer cells which was widely grown for many years throughout the world contains 70–80 chromosomes per cell compared with the normal 46.