Connective tissue
Section of epididymis. Connective tissue (blue) is seen supporting the epithelium (purple).
Identifiers
MeSH	D003238
FMA	96404
Anatomical terminology [edit on Wikidata]

Connective tissue is biological tissue that is found in between other tissues in the body.^[1] Most types of connective tissue consists of three main components: elastic and collagen fibers, ground substance, and cells.^[1]

It is one of the four primary types of animal tissue along with epithelial tissue, muscle tissue, and nervous tissue.^[2] It develops mostly from the mesenchyme, derived from the mesoderm, the middle embryonic germ layer.^[1]

The three meninges, membranes that envelop the brain and spinal cord, are composed of connective tissue. Blood and lymph are classed as specialized fluid connective tissues that do not contain fiber. All are immersed in the body water. The cells of connective tissue include fibroblasts, adipocytes, macrophages, mast cells and leukocytes.

The term "connective tissue" (in German, Bindegewebe) was introduced in 1830 by Johannes Peter Müller. The tissue was already recognized as a distinct class in the 18th century.^[3][4]

Types

Connective tissue can be broadly classified into connective tissue proper (including loose connective tissue and dense connective tissue) and special connective tissue (including supportive connective tissue and fluid connective tissue).^[5][6]

Fibrous types

Fiber types found in the extracellular matrix are collagen fibers, elastic fibers, and reticular fibers.^[17]

Ground substance is a clear, colorless, and viscous fluid containing glycosaminoglycans and proteoglycans allowing fixation of collagen fibers in intercellular spaces. Examples of non-fibrous connective tissue include adipose tissue (fat) and blood. Adipose tissue gives "mechanical cushioning" to the body, among other functions.^[18][19] Although there is no dense collagen network in adipose tissue, groups of adipose cells are kept together by collagen fibers and collagen sheets in order to keep fat tissue under compression in place (for example, the sole of the foot). Both the ground substance and proteins create the matrix for connective tissue.

Type I collagen is present in many forms of connective tissue, and makes up about 25% of the total protein content of the mammalian body.^[20]

Function

Connective tissue has a wide variety of functions that depend on the types of cells and the different classes of fibers involved.

Loose and dense irregular connective tissue, formed mainly by fibroblasts and collagen fibers, have an important role in providing a medium for oxygen and nutrients to diffuse from capillaries to cells, and carbon dioxide and waste substances to diffuse from cells back into circulation. They also allow organs to resist stretching and tearing forces.^[21]

Dense regular connective tissue, which forms organized structures, is a major functional component of tendons, ligaments and aponeuroses, and is also found in highly specialized organs such as the cornea.^[21]

Mesenchyme is a type of connective tissue found in the developing organs of an embryo that is capable of differentiation into all types of mature connective tissue.^[22] Another type of relatively undifferentiated connective tissue is the mucous connective tissue known as Wharton's jelly, found inside the umbilical cord.^[21] This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.^[23]

Various types of specialized tissues and cells are classified under the spectrum of connective tissue, and are as diverse as brown and white adipose tissue, blood, cartilage and bone.^[21] Cells of the immune system—such as macrophages, mast cells, plasma cells, and eosinophils—are found scattered in loose connective tissue, providing the ground for starting inflammatory and immune responses upon the detection of antigens.^[21]

Clinical significance

There are many types of connective tissue disorders, such as:

• Connective tissue neoplasms including sarcomas such as hemangiopericytoma and malignant peripheral nerve sheath tumor in nervous tissue.
• Congenital diseases include Marfan syndrome and Ehlers-Danlos Syndrome.
• Myxomatous degeneration – a pathological weakening of connective tissue.
• Mixed connective tissue disease – a disease of the autoimmune system, also undifferentiated connective tissue disease.
• Systemic lupus erythematosus (SLE) – a major autoimmune disease of connective tissue
• Scurvy, caused by a deficiency of vitamin C which is necessary for the synthesis of collagen.
• Fibromuscular dysplasia is a disease of the blood vessels that leads to an abnormal growth in the arterial wall.

See also

• Endometrium
• Parametrium

Notes and references

External links

Wikimedia Commons has media related to Connective tissues.

• Overview, University of Kansas Archived 26 December 2010 at the Wayback Machine
• Connective tissue atlas, University of Iowa
• Heritable disorders of connective tissue US National Institute of Arthritis and Musculoskeletal and Skin Diseases
• Connective tissue photomicrographs