Connective tissue is a form of fibrous tissue..

It is one of the four types of tissue in traditional classifications (the others being epithelial, muscle, and nervous tissue).

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.

Terminology

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.

Classification

Connective tissue can be classified into three categories: proper, embryonic, and specialized.

Connective tissue proper

Connective tissue proper includes the following five types: loose connective, dense connective, elastic, reticular, and adipose. They are called "proper" because they are the types usually meant when using the phrase "connective tissue".

• Areolar (or loose) connective tissue holds organs and epithelia in place, and has a variety of proteinaceous fibers, including collagen and elastin.
• Dense connective tissue forms ligaments and tendons. (Some classification systems include fibrous connective tissue instead. It is roughly equivalent to dense regular connective tissue.)
• Elastic tissue
• 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.)
• Adipose tissue contains adipocytes, used for cushioning, thermal insulation, lubrication (primarily in the pericardium) and energy storage.

Embryonic connective tissues

The two types of embryonic connective tissues are mesenchymal and mucous. They are of minimal importance in the adult.

• Mesenchymal connective tissue
• Mucous connective tissue

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.