Despite its complexity, the organs of the human body are composed of only four basic tissue types: epithelial, connective, muscular, and nervous tissues. Each tissue is an assemblage of similarly specialized cells united in performing a specific function. The basic tissues, each containing extracellular matrix (ECM) as well as cells, associate with one another in the variable proportions and morphologies characteristic of each organ. The main features of the basic tissue types are summarized in Table 4–1.
TABLE 4–1Main characteristics of the four basic types of tissues. ||Download (.pdf) TABLE 4–1 Main characteristics of the four basic types of tissues.
|Tissue ||Cells ||Extracellular Matrix ||Main Functions |
|Epithelial ||Aggregated polyhedral cells ||Small amount ||Lining of surface or body cavities; glandular secretion |
|Connective ||Several types of fixed and wandering cells ||Abundant amount ||Support and protection of tissues/organs |
|Muscle ||Elongated contractile cells ||Moderate amount ||Strong contraction; body movements |
|Nervous ||Elongated cells with extremely fine processes ||Very small amount ||Transmission of nerve impulses |
Connective tissue is characterized by cells producing very abundant ECM; muscle tissue is composed of elongated cells specialized for contraction and movement; and nervous tissue is composed of cells with long, fine processes specialized to receive, generate, and transmit nerve impulses. Most organs can be divided into the parenchyma, which is composed of the cells responsible for the organ’s specialized functions, and the stroma, the cells of which have a supporting role in the organ. Except in the brain and spinal cord, the stroma is always connective tissue.
Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of organs and cover the body surface. Epithelia (Gr. epi, upon + thele, nipple) line all external and internal surfaces of the body and all substances that enter or leave an organ must cross this type of tissue.
The principal functions of epithelial tissues include the following:
Covering, lining, and protecting surfaces (eg, epidermis)
Absorption (eg, the intestinal lining)
Secretion (eg, parenchymal cells of glands)
Specific cells of certain epithelia may be contractile (myoepithelial cells) or specialized sensory cells, such as those of taste buds or the olfactory epithelium.
CHARACTERISTIC FEATURES OF EPITHELIAL CELLS
The shapes and dimensions of epithelial cells are quite variable, ranging from tall columnar to cuboidal to low squamous cells. The cells’ size and morphology are generally dictated by their function. Epithelial cell nuclei vary in shape and may be elliptic (oval), spherical, or flattened, with nuclear shape corresponding roughly to cell shape. Columnar cells generally have elongated nuclei, squamous cells have flattened nuclei, and cuboidal or pyramidal cells have more spherical nuclei (Figure 4–1).