Connective tissue

Section of epididymis. Connective tissue (blue) is seen supporting the epithelium (purple)

Connective tissue (CT) is one of the four types of biological tissue that support, connect, or separate different types of tissues and organs in the body. The other three types are epithelial, muscle, and nervous tissue. Connective tissue is found in between other tissues everywhere in the body, including the nervous system. In the central nervous system, the three outer membranes (the meninges) that envelop the brain and spinal cord are composed of connective tissue.

All connective tissue apart from blood and lymph consists of three main components: fibers (elastic and collagenous fibers), ground substance and cells. (Not all authorities include blood or lymph as connective tissue.) Blood and lymph lack the fiber component. All are immersed in the body water.

The cells of connective tissue include fibroblasts, adipocytes, macrophages, mast cells and leucocytes.

Structure

Connective tissue can be broadly subdivided into connective tissue proper, and special connective tissue. Connective tissue proper consists of loose connective tissue and dense connective tissue (which is further subdivided into dense regular and dense irregular connective tissues.) Special connective tissue consists of reticular connective tissue, adipose tissue, cartilage, bone, and blood. Other kinds of connective tissues include fibrous, elastic, and lymphoid connective tissues.

Fibroblasts are the cells responsible for the production of some CT.

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.

Characteristics

Characteristics of CT:

• Cells are spread through an extracellular fluid.
• Ground substance - A clear, colorless, and viscous fluid containing glycosaminoglycans and proteoglycans to fix the body water and the collagen fibers in the intercellular spaces. Ground substance slows the spread of pathogens.
• Fibers. Not all types of CT are fibrous. Examples of non-fibrous CT include adipose tissue and blood. Adipose tissue gives "mechanical cushioning" to our body, among other functions. 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). The matrix of blood is plasma.
• Both the ground substance and proteins (fibers) create the matrix for CT.

Types of fibers:

Tissue	Purpose	Components	Location
Collagenous fibers	Bind bones and other tissues to each other	Alpha polypeptide chains	tendon, ligament, skin, cornea, cartilage, bone, blood vessels, gut, and intervertebral disc.
Elastic fibers	Allow organs like arteries and lungs to recoil	elastic microfibril & elastin	extracellular matrix
Reticular fibers	Form a scaffolding for other cells	Type-III collagen	liver, bone marrow, and lymphatic organs

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. 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. Elastic fibers, made from elastin and fibrillin, also provide resistance to stretch forces. They are found in the walls of large blood vessels and in certain ligaments, particularly in the ligamenta flava.

In hematopoietic and lymphatic tissues, reticular fibers made by reticular cells provide the stroma—or structural support—for the parenchyma—or functional part—of the organ.

Mesenchyme is a type of connective tissue found in developing organs of embryos that is capable of differentiation into all types of mature connective tissue. Another type of relatively undifferentiated connective tissue is mucous connective tissue, found inside the umbilical cord.

Various type 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. 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.

Clinical significance

Main articles: Connective tissue disease and Connective tissue neoplasm

It is estimated that 1 in 10 people have a connective tissue disorder. Diseases of connective tissue include:

• 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.
• Scurvy, caused by a deficiency of vitamin C which is necessary for the synthesis of collagen.

Staining of connective tissue

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

• Van Gieson's stain
• Masson's trichrome stain
• Mallory's trichrome stain
• Aniline blue stain
• Eosin

See also

• Connective tissue in skeletal muscle
• Fascia

External links

• Encyclopaedia Britannica, Connective Tissue
• Overview at kumc.edu
• UIUC Histology Subject 230
• Connective tissue atlas at uiowa.edu
• Questions and Answers about Heritable Disorders of Connective Tissue - US National Institute of Arthritis and Musculoskeletal and Skin Diseases