Connective tissue is a form of fibrous tissue.^[1]. 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.^[2]

Contents 1 Terminology 2 Classification 2.1 Connective tissue proper 2.2 Embryonic connective tissues 2.3 Specialized connective tissues 3 Fiber types 4 Disorders of connective tissue 5 Staining of connective tissue 6 References 7 External links

[edit] 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.

[edit] Classification

Connective tissue can be classified into three categories: proper, embryonic, and specialized.^[3]

[edit] Connective tissue proper

Connective tissue proper includes the following five types: loose connective, dense connective, elastic, reticular, and adipose.^[3] 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.

[edit] Embryonic connective tissues

The two types of embryonic connective tissues are mesenchymal and mucous.^[4] They are of minimal importance in the adult.

• Mesenchymal connective tissue
• Mucous connective tissue

[edit] Specialized connective tissues

The category "specialized connective tissue" consists of bone, cartilage, and blood.^[3] Although these items are connective tissue, they are often considered separately.

• 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.

The following two can be classified as "supportive connective tissue":^[5]

• Bone (osseous tissue) 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.

Dense connective tissue, also called dense fibrous tissue, has collagen fibers as its main matrix element. It is mainly composed of collagen type I. Crowded between the collagen fibers are rows of fibroblasts, fiber-forming cells, that manufacture the fibers. Dense connective tissue forms strong, rope-like structures such as tendons and ligaments. Tendons attach skeletal muscles to bones; ligaments connect bones to bones at joints. Ligaments are more stretchy and contain more elastic fibers than tendons. Dense connective tissue also make up the lower layers of the skin (dermis), where it is arranged in sheets.

[edit] Fiber types

Not to be confused with muscle fibers

Fiber types as follows:

• collagenous fibers
• elastic fibers
• reticular fibers

[edit] Disorders of connective tissue

Various connective tissue conditions have been identified; these can be both inherited and environmental.

• Marfan syndrome - a genetic disease causing abnormal fibrillin.
• Scurvy - caused by a dietary deficiency in vitamin C, leading to abnormal collagen.
• 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.
• Fibrodysplasia ossificans progressiva - disease of the connective tissue, caused by a defective gene which turns connective tissue into bone.
• Spontaneous pneumothorax - collapsed lung, believed to be related to subtle abnormalities in connective tissue.
• Sarcoma - a neoplastic process originating within connective tissue.

[edit] Staining of connective tissue

For microscopic viewing, the majority of the connective tissue staining techniques color tissue fibers in contrasting shades. Collagen may be differentially stained by any of the following techniques:

• Van Gieson's stain
• Masson's Trichrome stain
• Mallory's Aniline Blue stain
• Azocarmine stain
• Krajian's Aniline Blue stain

[edit] References

[edit] External links

• connective+tissue at eMedicine Dictionary
• Encyclopaedia Britannica, Connective Tissue
• Overview at kumc.edu
• UIUC Histology Subject 230
• Connective tissue atlas at uiowa.edu

v • d • e Biological tissue Animals Epithelium - Connective - Muscular - Nervous Plants Dermal - Vascular - Ground

v • d • e Histology: connective tissue Classification Proper/ Fibrous Loose Areolar · Reticular non-fibrous: Adipose (Brown, White) Dense Dense irregular connective tissue (Submucosa, Dermis) · Dense regular connective tissue (Ligament, Tendon, Aponeurosis) Embryonic Mucous · Mesenchymal Specialized Cartilage · Bone · Blood Composition Extracellular matrix (noncellular) Ground substance Tissue fluid Fibers Reticular fibers: Collagen Elastic fibers: Fibrillin (FBN1, FBN2, FBN3) Cells Resident Fibroblast · Reticular cell · Tendon cell Adipocyte Chondroblast · Osteoblast Wandering cell Mast cell · Macrophage see also soft tissue integument, SF, LCT navs: anat/physio, noncongen/congen/neoplasia, symptoms+signs/eponymous, proc muscle, DF+DRCT navs: anat/hist/physio, acquired myopathy/congenital myopathy/neoplasia, symptoms+signs/eponymous, proc