External skin is one form of the epithelial tissue type.
External skin is one form of the epithelial tissue type.

The body contains four tissue types, each encompassing a variety of subtypes. The four big groups are epithelial tissue, connective tissue, muscle tissue and nervous tissue. While the embryological origin of each type of tissue is an important factor for the four main types, the subtypes are often differentiated by their appearance under the microscope or by their function.

Epithelial Tissue Divisions

The cells in the epithelial group are involved in lining organs, covering the surfaces of the body or gland formation.The primary subcategories of epithelial tissue are based on appearance. Epithelial tissue that is arranged in a single layer is called "simple epithelium," and tissue that has more than one layer is termed "stratified epithelium." If the cells look stratified but in reality are simple but just have misleading shapes, then this type is called "pseudo-stratified epithelium." Flat-looking cells are called "squamous," cube-shaped cells are termed "cuboidal" and taller cells are called "columnar." There are two other epithelial subtypes, "glandular," which refers to cells that release secretions, and "transitional," which describes epithelium found in specific places like the bladder.

Connective Tissue Subtypes

The greatest variety of cells can be found in the connective tissue group. Connective tissue cells are involved in physical or functional support of the other three tissue groups. It can be roughly divided into two groups -- the connective tissue proper and the specialized connective tissue. Connective tissue proper is made up of three further groups -- loose connective tissue, which tends to be found underneath epithelium covering the outside and inside surfaces of the body. Dense regular tissue is another type, forming all the ligaments and tendons. Dense irregular connective tissue is the third type, containing lots of fibers and found in places where resistance to stretch is important, such as hollow organs.

Muscle Tissue Subcategories

All muscle cells have a movement and contraction function. There are three subtypes of muscle. Skeletal muscle is the type that is under voluntary control, and is attached to bones to allow the body to move. Cardiac muscle is found only in the heart. Both skeletal and cardiac muscle can also be grouped under the term "striated muscle." Smooth muscle is found in places like the digestive tract and the blood vessels, and is under unconscious control.

Nervous Tissue Divisions

All the nervous tissue cells play roles in informational sensation or transfer around the body. The nervous tissue group is split up into neurons, which are the cells that transmit information, and support cells. There are various types of support cells, all of which may be referred to as "glia." The glia in the brain and spinal cord include the oligodendrocytes, astrocytes, microglia and ependymal cells. Oligodendrocytes provide myelin to insulate neurons and astrocytes have several functions such as protecting the blood-brain barrier. Microglia are part of the immune defense system and ependymal cells make cerebrospinal fluid for a healthy brain. For nervous tissue outside of the spinal cord and brain, the main support cells are called Schwann cells and satellite cells. Schwann cells wrap neurons in myelin for insulation, and satellite cells are similar to astrocytes.