Hemidesmosomes are found at the base of an epithelial cell and attach the cell to the basal lamina beneath the epithelium. •
Gap junctions—these consist of the connexin protein and allow for the direct exchange of both nutrients and intercellular signals between adjacent cells. Larger molecules can be exchanged through these hollow connections. They are found along the lateral surfaces of an epithelial cell, establishing the communication between cells.
While there are numerous specific tissue types, they can be organized into four basic categories of tissues. These include connective, epithelial, muscle, and nervous tissue. We will talk about each of these tissue types and how they function in the body.
TYPES OF TISSUES One of the four main types of tissue in the body is epithelial tissue or “epithelium.” This is what covers the entire surface of the body and the lining of most of the body’s internal cavities. Its functions include secretion, absorption, protection, and filtration. The skin on the outside of the body is epithelium; it protects the body from dust, dirt, bacteria, and microbes. The cells can be thin (as they are on the outermost layers), flat, elongated or cubic in their structural nature. These cells have many junctions, in keeping with their function. The most abundant tissue in the body is connective tissue—found everywhere in the body. This type of tissue is responsible for the support and protection of all of the organs and tissues of the body. Included as connective tissue are fatty tissue, loose connective tissue, dense fibrous tissue, bone, lymph tissue, cartilage, and blood. Muscle tissue comes in three types: skeletal, smooth, and cardiac muscle. Of these, only skeletal muscle is voluntary—used to control the skeletal parts of the body. Smooth muscle is found in the walls of the blood vessels and the internal organs of the body. It is completely involuntary. Cardiac muscle is found in the walls of the heart and acts as the “pumping tissue” of the heart. It is also involuntary. All types of muscle tissue are excitable electrically. Nerve tissue is found in the brain and in all parts of the body. Most nerve cells are long and stretch to connect to other nerve cells using chemo-electrical signaling between the cells. 28
