LEARNING OBJECTIVES: Identify the types of tissues in the human body and their functions.
Tissues are groups of specialized cells similar in structure and function. They are classified into fourmain groups: epithelial, connective, muscular, and nervous.
The lining tissue of the body is called epithelium. It forms the outer covering of the body known as the free surface of the skin. It also forms the lining of the digestive, respiratory, and urinary tracts; blood and lymph vessels; serous cavities (cavities which have no communication with the outside of the body, and whose lining membrane secretes a serous fluid), such as the peritoneum or pericardium; and tubules (small tubes which convey fluids) of certain secretory glands, such as the liver and kidneys. Epithelial tissues are classified according to their shape, arrangement, and the function of their cells. For example, epithelial tissues that are composed of single layers of cells are called “simple,” while cells with many layers are said to be “stratified.” In the following paragraphs we will discuss the three categories of epithelial tissue: columnar, squamous, and cuboidal.
Columnar Epithelial Tissue
Epithelial cells of this type are elongated, longer than they are wide. Columnar tissue is composed of a single layer of cells whose nuclei are located at about the same level as the nuclei in their neighboring cells (fig. 1-3). These cells can be located in the linings of the uterus, in various organs of the digestive system, and in the passages of the respiratory system. In the digestive system, the chief function of columnar tissue is the secretion of digestive fluids and the absorption of nutrients from digested foods. In certain areas (such as the nostrils, bronchial tubes, and trachea), this tissue has a crown of microscopic hairlike processes known as cilia. These cilia provide motion to move secretions and other matter along the surfaces from which they extend. They also act as a barrier, preventing foreign matter from entering these cavities.
Figure 1-3.—Columnar epithelial tissue.
Squamous Epitheial Tissue
Squamous epithelial tissue is composed of thin platelike or scalelike cells forming a mosaic pattern (fig. 1-4). This tissue is found in the tympanic membrane (eardrum) as a single layer of cells, or in the free skin surface in multiple layers. Squamous tissue is the main protective tissue of the body.
Figure 1-4.—Squamous epithelial tissue.
Cuboidal Epithelial Tissue
The cells of cuboidal tissue are cubical in shape (fig. 1-5) and are found in the more highly specialized organs ofthe body, such as the ovary and the kidney. In the kidneys, cuboidal tissue functions in the secretion and absorption of fluids.
Figure 1-5.—Cuboidal epithelial tissue.
This is the supporting tissue of the various structures of the body. It has many variations and is the most widespread tissue of the body. Connective tissue is highly vascular, surrounds other cells, encases internal organs, sheathes muscles, wraps bones, encloses joints, and provides the supporting framework of the body. Structures of connective tissue differ widely, ranging from delicate tissue-paper membranes to rigid bones. Connective tissue is composed of cells and extracellular materials (materials found outside the cells). Extracellular materials include fibers and the ground substance. The ground substance contains proteins, water, salts, and other diffusible substances. These extracellular materials give connective tissue varying amounts of elasticity and strength, depending on the type of tissue and location. In the following paragraphs we will discuss the three predominant types of connective tissue: areolar, adipose, and osseous.
Areolar Connective Tissue
Areolar tissue consists of a meshwork of thin fibers that interlace in all directions, giving the tissue both elasticity and tensile strength (fig. 1-6). This type of connective tissue is extensively distributed throughout the body, and its chief function is to bind parts of the body together. Areolar tissue allows a considerable amount of movement to take place because of its elasticity. It is found between muscles and as an outside covering for blood vessels and nerves. The areolar tissue layer connects the blood vessels and nerves to the surrounding structures.
Figure 1-6.—Areolar connective tissue.
Adipose Connective Tissue
Adipose tissue is “fatty tissue.” The adipose cell at first appears star-shaped. When the cell begins to store fat in its cytoplasm, it enlarges, losing its star shape as the nucleus is pushed to one side (fig. 1-7). When this process occurs to many cells, the other cell types are crowded out and adipose tissue is formed. Adipose tissue is found beneath skin, between muscles, and around joints and various organs of the body. Adipose tissue acts as a reservoir for energy-producing foods; helps to reduce body heat loss (because of its poor heat conductivity); and serves as support for various organs and fragile structures, such as the kidneys, blood vessels, and nerves.
Figure 1-7.—Adipose connective tissue.
Osseous Connective Tissue
This type of tissue, known as “bone tissue,” is a dense fibrous connective tissue that forms tendons, ligaments, cartilage, andbones (fig. 1-8). These tissues form the supporting framework of the body.
Figure 1-8.—Osseous (bone) connective tissue.
Muscular tissue provides for all body movement. Contracting muscles cause body parts to move. The three types of muscle tissue are skeletal, smooth, and cardiac.
Skeletal Muscle Tissue
Skeletal (voluntary) muscle fiber is striated, or striped, and is under the control of the individual's will (fig. 1-9). For this reason, it is often called “voluntary” muscle tissue. Skeletal muscle tissues are usually attached to bones. When muscle fibers are stimulated by an action of a nerve fiber, the fibers contract and relax. This interaction between muscle and nervous fibers produces movement.
Figure 1-9.—Skeletal muscle tissue.
Smooth Muscle Tissue
These muscle fibers are smooth, or nonstriated, and are not under the control of the individual's will (fig. 1-10). For this reason, this type of muscle tissue is called “involuntary.” Smooth muscle tissue is found in the walls of hollow organs, such as the stomach, intestines, blood vessels, and urinary bladder. Smooth muscle tissues are responsible for the movement of food through the digestive system, constricting blood vessels, and emptying the bladder.
Figure 1-10.—Smooth muscle tissue.
Cardiac Muscle Tissue
The cardiac muscle cells are striated and are joined end to end, resulting in a complex network of interlocking cells (fig. 1-11). Cardiac muscles are involuntary muscles and are located only in the heart. These tissues are responsible for pumping blood through the heart chambers and into certain blood vessels.
Figure 1-11.—Cardiac muscle tissue.
Nerve tissue is the most complex tissue in the body. It is the substance of the brain, spinal cord, and nerves. Nerve tissue requires more oxygen and nutrients than any other body tissue. The basic cell of the nerve tissue is the neuron (fig. 1-12). This highly specialized cell receives stimuli from, and conducts impulses to, all parts of the body.