Connective tissue

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Connective tissue is biological tissue that is found in between other tissues in the body.<ref name="Biga" /> Most types of connective tissue consists of three main components: elastic and collagen fibers, ground substance, and cells.<ref name="Biga" />

It is one of the four primary types of animal tissue along with epithelial tissue, muscle tissue, and nervous tissue.<ref name="oregon">Template:Cite web</ref> It develops mostly from the mesenchyme, derived from the mesoderm, the middle embryonic germ layer.<ref name="Biga">Template:Citation</ref>

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, Template:Lang) was introduced in 1830 by Johannes Peter Müller. The tissue was already recognized as a distinct class in the 18th century.<ref>Mathews, M. B. (1975). Connective Tissue, Macromolecular Structure Evolution. Springer-Verlag, Berlin and New York. link.</ref><ref>Template:Cite journal</ref>

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).<ref>Template:Cite web</ref><ref name="PorthMatfin2010">Template:Cite book</ref>

Connective tissue proper

Loose and dense connective tissue are distinguished by the ratio of ground substance to fibrous tissue. Loose connective tissue has much more ground substance and a relative lack of fibrous tissue, while the reverse is true of dense connective tissue.

Loose connective tissue includes reticular connective tissue, and adipose tissue.

Dense connective tissue is subdivided into dense regular and dense irregular connective tissue.<ref>Template:Cite web</ref> Dense regular connective tissue, found in structures such as tendons and ligaments, is characterized by collagen fibers arranged in an orderly parallel fashion, giving it tensile strength in one direction. Dense irregular connective tissue provides strength in multiple directions by its dense bundles of fibers arranged in all directions.Template:Cn

Special connective tissue

Special connective tissue consists of supportive connective tissue (comprising bone and cartilage) and fluid connective tissue (comprising blood and lymph).<ref name="Biga" /><ref name="libre">Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref> Special connective tissue are a form of fascia, with blood and lymph being known as liquid fascia.<ref name="Biga" /><ref name="Bordoni">Template:Cite journal</ref><ref name="Schleip">Template:Cite journal</ref>

Other kinds of special connective tissues include fibrous, elastic, and lymphoid connective tissues.<ref>Template:Cite news</ref> Fibroareolar tissue is a mix of fibrous and areolar tissue.<ref name="Merriam">Template:Cite web</ref> Fibromuscular tissue is made up of fibrous tissue and muscular tissue.

New vascularised connective tissue that forms in the process of wound healing is termed granulation tissue.<ref>Template:Cite web</ref>

Membranes

Membranes can be either of connective tissue or epithelial tissue. Connective tissue membranes include the meninges (the three membranes covering the brain and spinal cord) and synovial membranes that line joint cavities.<ref name="SEER">Template:Cite web</ref> Mucous membranes and serous membranes are epithelial with an underlying layer of loose connective tissue.<ref name="SEER" />

Fibrous types

Fiber types found in the extracellular matrix are collagen fibers, elastic fibers, and reticular fibers.<ref name="Ushiki">Template:Cite journal</ref>

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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>

Types of fibers
Tissue	Purpose	Components	Location
Collagen fibers	Bind bones and other tissues to each other	Alpha polypeptide chains	Tendon, ligament, skin, cornea, cartilage, bone, blood vessels, gut, intervertebral disc; loose and dense-irregular connective tissue<ref name="Ross" />
Elastic fibers	Allow organs to recoil; provide resistance to stretch forces<ref name="Ross" />	Elastic microfibril, elastin, fibrillin	Extracellular matrix, walls of large blood vessels, certain ligaments (e.g. ligamenta flava)<ref name="Ross" />
Reticular fibers	Form a scaffolding for other cells; provides the stroma for the parenchyma of an organ<ref name="Ross" />	Type III collagen	Liver, bone marrow, and lymphatic organs; hematopoietic and lymphatic tissue<ref name="Ross" />

Function

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Hypermobility as a result of an inherited connective tissue defect

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.<ref name="Ross" />

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.<ref name="Ross">Template:Cite book</ref>

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.<ref name="wheater">Template:Cite book</ref> Another type of relatively undifferentiated connective tissue is the mucous connective tissue known as Wharton's jelly, found inside the umbilical cord.<ref name="Ross" /> This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.<ref name="Openstax Anatomy & Physiology attribution">Template:CC-notice Template:Cite book</ref>

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.<ref name="Ross" /> 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.<ref name="Ross" />