Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, muscle, and nervous tissue.) It is largely a category of exclusion rather than one with a precise definition, but all or most tissues in this category are similarly:
Characterized largely by the traits of non-living tissue.
Blood, cartilage, and bone are usually considered connective tissue, but because they differ so substantially from the other tissues in this class, the phrase "connective tissue proper" is commonly used to exclude those three. There is also variation in the classification of embryonic connective tissues; on this page they will be treated as a third and separate category.
Collagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% of the total protein content.
Bone makes up virtually the entire skeleton in adult vertebrates.
Cartilage makes up virtually the entire skeleton in chondrichthyes. In most other vertebrates, it is found primarily in joints, where it provides cushioning. The extracellular matrix of cartilage is composed primarily of collagen.
Adipose tissue contains adipocytes, used for cushioning, thermal insulation, lubrication (primarily in the pericardium) and energy storage. [fat]
Reticular connective tissue is a network of reticular fibres (fine collagen, type III) that form a soft skeleton to support the lymphoid organs (lymph nodes, bone marrow, and spleen.)
Ehlers-Danlos syndrome - deficient type III collagen- a genetic disease causing progressive deterioration of collagens, with different EDS types affecting different sites in the body, such as joints, heart valves, organ walls, arterial walls, etc.
Loeys-Dietz syndrome - a genetic disease related to Marfan syndrome, with an emphasis on vascular deterioration.
Pseudoxanthoma elasticum - an autosomal recessive hereditary disease, caused by calcification and fragmentation of elastic fibres, affecting the skin, the eyes and the cardiovascular system.
Systemic lupus erythematosus - a chronic, multisystem, inflammatory disorder of probable autoimmune etiology, occurring predominantly in young women.
Osteogenesis imperfecta (brittle bone disease) - caused by insufficient production of good quality collagen to produce healthy, strong bones.