This is a big — and interesting — subject. I checked in with some of my science colleagues to give you the CliffsNotes version of an answer that could stretch for pages and pages.
Tissues are groups of similar cells that perform a common function. There are four categories of tissues in the human body: epithelial, connective, nervous, and muscle.
Epithelial tissue protects your body from moisture loss, bacteria, and internal injury. There are two kinds of epithelial tissues:
- Covering and lining epithelium covers or lines almost all of your internal and external body surfaces; for example, the outermost layer of your skin and other organs, and the internal surface lining of your lymph vessels and digestive tract.
- Glandular epithelium secretes hormones or other products such as stomach acid, sweat, saliva, and milk.
Connective tissue generally provides structure and support to the body. There are two types of connective tissue:
- Loose connective tissue holds structures together. For example, loose connective tissue holds the outer layer of skin to the underlying muscle tissue. This tissue is also found in your fat layers, lymph nodes, and red bone marrow.
- Fibrous connective tissue also holds body parts together, but its structure is a bit more rigid than loose connective tissue. Fibrous connective tissue is found in ligaments, tendons, cartilage, and bone.
Nervous tissue forms the nervous system, which is responsible for coordinating the activities and movements of your body through its network of nerves. Parts of the nervous system include the brain, spinal cord, and nerves that branch off of those two key parts.
Nervous tissue consists of two kinds of nerve cells:
- Neurons are the basic structural unit of the nervous system. Each cell consists of the cell body, dendrites, and axon.
- Neuroglia, or glial cells, provide support functions for the neurons, such as insulation or anchoring neurons to blood vessels.
Muscle tissue differs from other tissue types in that it contracts. Muscle tissue comes in three types: cardiac, smooth, and skeletal. Those muscle tissues are made up of muscle fibers. The muscle fibers contain many myofibrils, which are the parts of the fiber that actually contract. There are three kinds of muscle tissues:
- Skeletal muscle is attached to bones and causes movements of the body.
- Cardiac muscle is found in the heart.
- Smooth muscle lines the walls of blood vessels and certain organs such as the digestive and urogenital tracts.