Connective tissue is one of the four traditionally classified types of biological tissue. There are many different kinds, and they mainly serve as structure and support, often connecting two other types of tissue to each other. This tissue usually derives from the mesoderm, the middle of three layers in an animal embryo. Its characteristics are largely derived from the extracellular matrix, non-living material that surrounds and supports the living cells.
The older classification of this tissue had two subtypes: proper, which covered areolar and fibrous tissue, and specialized, which included bone, blood, cartilage, adipose (fat) tissue, and reticular tissue. The newer classification has four categories: loose connective tissue, dense connective tissue, cartilage, and other.
Loose connective tissue includes areolar, adipose, and reticular tissue. Areolar tissue is a mesh-like tissue with a fluid matrix that supports the epithelium, tissue that makes up the skin and other membranes. Adipose tissue is fat, which provides cushioning and insulation, lubrication in some areas, and energy storage. Reticular tissue is similar to areolar tissue, but contains only reticular fibers made of type-III collagen in its matrix. Reticular tissue supports a number of bodily structures, notably the organs of the lymphatic system.
Dense connective tissue is divided into regular and irregular types. Both have a matrix composed mainly of collagen fibers, although dense regular tissue has a matrix of parallel collagen fibers. It is very strong and connects other tissue types to each other; tendons connect muscle to bone and ligaments connect bone to bone. Dense irregular tissue has irregularly arranged collagen fibers and comprises the lower layers of the dermis, or skin.
Cartilage makes up nearly the entire skeleton of some animals, while in others, including humans, it serves mainly to cushion the joints. There are three types of cartilage: elastic, hyaline, and fibrocartilage. Elastic cartilage in rich in elastin and is found in the outer ear, the Eustachian tubes, and the epiglottis. Hyaline cartilage is characterized by a large amount of collagen and is the hardest type of cartilage. It is found on the ends on bones, in the nose and larynx, and between the ribs and sternum. Fibrocartilage contains even more collagen than hyaline cartilage, particularly type-I collagen. It is tough, heavy, white, and found in areas of high stress, such as the intervertebral discs.
The "other" category includes bone, which makes up the majority of the skeleton in adult vertebrates; blood, which transports nutrients and hormones throughout the body; and lymphatic tissues, which help transport nutrients between the blood and other cells and produce immune cells.