A tissue is a group of connected cells that have a similar function within an organism. The simplest living, multicellular organisms, sponges, are made of many specialized types of cells that work together for a common goal. Such cell types include digestive cells, tubular pore cells, and epidermal cells. Though the different cell types create a large organized, multicellular structure—the visible sponge—they are not organized into true tissues. If a sponge is broken up by passing it through a sieve, the sponge will reform on the other side.
More complex organisms, such as jellyfish, coral, and sea anemones, have a tissue level of organization. For example, jellyfish have tissues that have separate protective, digestive, and sensory functions. There are four basic types of tissues in the bodies of all animals including the human body. These make up all the organs, structures, and other contents of the body. The four basic types of tissues are epithelial, muscle, nervous, and connective.
Epithelial tissue is made up of a layer or layers of tightly packed cells that line the surfaces of the body. The largest example of epithelial tissue (also the largest organ in the human body) is the skin. Mammalian skin consists of stratified epithelium, which has several layers of cells. The outermost layers of cells, called squamous cells, are flat plate-like cells, while the deeper layers are roughly cube shaped and called cuboidal cells. Epithelial tissue has multiple functions, but it serves primarily to protect, absorb, and secrete. As you probably already know, our skin organ covers our entire body and protects underlying tissues from bacteria, chemicals, and other injury. Epithelial cells also line the small intestine where they absorb nutrients, and similar cells in the glands secrete enzymes and hormones.
Muscle tissue encompasses not only the muscles, such as those in our legs or fingers, that we actively control, but also the tissue that forms most of our internal organs. There are three types of muscle tissue: skeletal, cardiac, and smooth. Skeletal muscle tissue forms what we think of as our muscles; it is attached to our bones by our tendons and can be relaxed or contracted voluntarily. Similar in structure to skeletal muscle, cardiac muscle is found exclusively in the walls of the heart. The major difference, however, is that cardiac muscle is involuntary and cannot be actively controlled. Similarly, smooth muscle, which forms the muscle layers in internal organs such as the digestive tract and bladder, is an involuntary tissue. Smooth muscle tissue controls slow involuntary movements such as stomach wall contractions and the contractions of arteries to regulate blood flow.
Nervous tissue is made up of the nerve cells (neurons) that form the nervous system, including the brain and spinal cord. These cells are especially responsive to stimuli, allowing nervous tissue to transmit stimuli from the brain to the body extremely rapidly.
Connective tissue connects, supports, or separates other tissues and organs. Connective tissue proper, a form of connective tissue, can be either loose or dense. Adipose tissue, or fat, is loose connective tissue, while tendons and ligaments, composed of collagen, are examples of dense connective tissue. Other forms of connective tissue include blood (fluid connective tissue) and cartilage and bone (both forms of supporting connective tissue).
Human Anatomy 