Groups of cells collectively organize into a structure known as a tissue. The properties tissue is determined by the different cell types that form it and there are generally four types of tissue. Each tissue type has its own subgroups, but the subgroups have similar properties and functions.
1. Muscle Tissue – Muscle tissue is assembled by myocytes, or muscle cells, and allows the skeletal system to move using contractile forces. There are three different groups of muscle tissues which differ in their physical properties and their location in the body. Striation is a physical characteristic used to recognize different muscle tissues and typically means the sarcomeres are repeating. The striated muscles visually look distinct because of repeating segments of thick and thin protein filaments. Skeletal muscle is striated muscle and controlled voluntarily. Smooth muscle is located along the walls of the digestive tract, blood vessels, uterus, and urinary bladder. It is nonstriated and involuntary. Cardiac muscle is located within the heart, it is striated, and involuntary.
2. Nervous Tissue – Nervous tissue are composed of neurons. This tissue has the specialized role of responding to external and internal stimuli by transmitting impulses from one area of the body to another. The tissue can induce a response in distant muscles to contract, glands to secrete, or regulate other body processes. The nervous tissue composes the nervous system. Nervous tissue forms the central nervous system brain and spinal cord as well as the peripheral sensory receptors.
3. Epithelial Tissue – This tissue type has many subgroups, but for the most part have similar characteristics. Epithelial tissue covers the the external surfaces of the body as different layers of skin. It lines the internal tubes and cavities of the body. Characteristically it has compactly aggregated cells, no blood vessels, and has limited intercellular space.Epithelial cells are also polarized where the top and bottom of the tissue is quite distinct. Additionally the tissue is derived from all three germ layers.
4 Connective Tissue – The last of the four tissue types is like packaging material of the body. Connective tissue supports the other tissues of the body. It is developed from the mesoderm and contains ground substance, fibers, and cells. The ground substance is an amorphous material found between the cells and fibers and acts route of passage of nutrients and waste to and from the cells. The fibers add support and strength to the connective tissue. The cells of connective tissue are typically nonmotile, but some have the ability to wander. Fibroblasts are found most abundantly in connective tissue especially in ligaments and tendons. Additionally free and fixed macrophages uptake foreign material in connective tissue acting as an arm of the innate immune system. Adipocytes or fat cells are large and round cells with a large lipid droplet taking up the majority of their volume.