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:

• Involved in structure and support.
• Derived from mesoderm, usually.
• 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.

Classification

The old classification system for connective tissue is proper versus specilized. There has been a new classifications system proposed, however, and it is as follows:

Loose connective tissue

• Areolar
• Adipose
• Reticular

Dense connective tissue

• Regular
• Irregular
• Elastic

Cartilage

• Hyaline
• Fibrocartilage
• Elastic

Other

• Bone
• Blood
• Lymphatics

Connective tissue proper

• Areolar (or loose) connective tissue holds organs and epithelia in place, and has a variety of proteinaceous fibres, including collagen and elastin.
• Fibrous connective tissue forms ligaments and tendons. Its densely packed collagen fibers have great tensile strength.

Specialized connective tissues

• Blood functions in transport. Its extracellular matrix is blood plasma, which transports dissolved nutrients, hormones, and carbon dioxide in the form of bicarbonate. The main cellular component is red blood cells.
• 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.
• 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.)