The Animal Body : Introduction to Structure and Function Tissue is a group of cells that have similar structure and that function together as a unit. ✔ A nonliving material, called the intercellular matrix, fills the spaces between the cells. ✔This may be abundant in some tissues and minimal in others. ✔ The intercellular matrix may contain special substances such as salts and fibers that are unique to a specific tissue and gives that tissue distinctive characteristics. ✔ There are four main tissue types in the body: epithelial, connective, muscle, and nervous. ✔Each is designed for specific functions.EPITHELIAL TISSUES Epithelial tissues are widespread throughout the body. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. They perform a variety of functions that include protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception. The cells in epithelial tissue are tightly packed together with very little intercellular matrix. Because the tissues form coverings and linings, the cells have one free surface that is not in contact with other cells. Opposite the free surface, the cells are attached to underlying connective tissue by a non-cellular basement membrane. This membrane is a mixture of carbohydrates and proteins secreted by the epithelial and connective tissue cells.Epithelial cells may be squamous, cuboidal, or columnar in shape and may be arranged in single or multiple layers. Epithelia are classified based on cell shape Squamous epithelial cells are platelike Cuboidal epithelial cells appear cube shaped Columnar epithelial cells are rectangular, with the nucleus near the basal end of the cell Epithelia may be simple (one layer of cells) or stratified (made of two or more layers) Simple epithelia allow more rapid passage of materials through the layer Stratified epithelia afford protection Pseudostratified epithelium appears to be stratified, but all cells are in contact with the basement membraneSimple cuboidal epithelium is found in glandular tissue and in the kidney tubules. Simple columnar epithelium lines the stomach and intestines. Pseudostratified columnar epithelium lines portions of the respiratory tract and some of the tubes of the male reproductive tract. Transitional epithelium can be distended or stretched. Glandular epithelium is specialized to produce and secrete substances. CONNECTIVE TISSUE Connective tissues bind structures together, form a framework and support for organs and the body as a whole, store fat, transport substances, protect against disease, and help repair tissue damage. They occur throughout the body. Connective tissues are characterized by an abundance of intercellular matrix with relatively few cells. Connective tissue cells are able to reproduce but not as rapidly as epithelial cells. Most connective tissues have a good blood supply but some do not.There are several types of connective tissue Loose connective tissue is the most widespread connective tissue Forms a thin filling between body parts Attaches skin to muscles Dense connective tissue consists mainly of fibers In irregular dense connective tissue, collagen fibers run in all directions Tendons and ligaments are composed of regular dense connective tissue Elastic tissue is found in structures that must expand Elastic connective tissue is composed of parallel elastic fibers This tissue type is found in lung tissue and the walls of large arteriesReticular connective tissue provides support Reticular fibers form a framework in many organs, such as the liver, spleen, and lymph nodes Adipose tissue stores energy Adipose tissue cushions and protects organs Numerous cell types are found in connective tissue. Three of the most common are the fibroblast, macrophage, and mast cell.MUSCLE TISSUE Muscle tissue is composed of cells that have the special ability to shorten or contract in order to produce movement of the body parts The tissue is highly cellular and is well supplied with blood vessels. The cells are long and slender so they are sometimes called muscle fibers, and these are usually arranged in bundles or layers that are surrounded by connective tissue. Actin and myosin are contractile proteins in muscle tissue. Muscle tissue can be categorized into skeletal muscle tissue, smooth muscle tissue, and cardiac muscle tissue.Skeletal muscle fibers are cylindrical, multinucleated, striated, and under voluntary control. Smooth muscle cells are spindle shaped, have a single, centrally located nucleus, and lack striations. They are called involuntary muscles. Cardiac muscle has branching fibers, one nucleus per cell, striations, and intercalated disks. Its contraction is not under voluntary controlNervous Tissue Nervous tissue is found in the brain, spinal cord, and nerves. It is responsible for coordinating and controlling many body activities. It stimulates muscle contraction, creates an awareness of the environment, and plays a major role in emotions, memory, and reasoning. To do all these things, cells in nervous tissue need to be able to communicate with each other by way of electrical nerve impulses. The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. These cells have three principal parts: the dendrites, the cell body, and one axon. The main part of the cell, the part that carries on the general functions, is the cell body. Dendrites are extensions, or processes, of the cytoplasm that carry impulses to the cell body. An extension or process called an axon carries impulses away from the cell body. Nervous tissue also includes cells that do not transmit impulses, but instead support the activities of the neurons. These are the glial cells (neuroglial cells), together termed the neuroglia. Supporting, or glia, cells bind neurons together and insulate the neurons. Some are phagocytic and protect against bacterial invasion, while others provide nutrients by binding blood vessels to the neurons.