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.[4][5]
Types
Connective tissue can be broadly classified into connective tissue proper, and special connective tissue.[6][7]
Connective tissue proper
Connective tissue proper consists of loose connective tissue (including reticular connective tissue and adipose tissue) and dense connective tissue (subdivided into dense regular and dense irregular connective tissues.)[8] 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. 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.[citation needed]
Special connective tissue
Special connective tissue consists of cartilage, bone, blood and lymph.[9] Other kinds of connective tissues include fibrous, elastic, and lymphoid connective tissues.[10] Fibroareolar tissue is a mix of fibrous and areolar tissue.[11] 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.[12] All of the special connective tissue types have been included as a subset of fascia in the fascial system, with blood and lymph classed as liquid fascia.[13][14]
Bone and cartilage can be further classified as supportiveconnective tissue. Blood and lymph can also be categorized as fluid connective tissue,[2][15][16] and liquid fascia.[13]
Fiber types found in the extracellular matrix are collagen fibers, elastic fibers, and reticular fibers.[18]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.[19][20] 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 (fibers) 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.[21]
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.[22]: 161 Elastic fibers, made from elastin and fibrillin, also provide resistance to stretch forces.[22]: 171 They are found in the walls of large blood vessels and in certain ligaments, particularly in the ligamenta flava.[22]: 173
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.[23] Another type of relatively undifferentiated connective tissue is the mucous connective tissue known as Wharton's jelly, found inside the umbilical cord.[22]: 160 This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.[24]
^Mathews, M. B. (1975). Connective Tissue, Macromolecular Structure Evolution. Springer-Verlag, Berlin and New York. link.
^Aterman, K. (1981). "Connective tissue: An eclectic historical review with particular reference to the liver". The Histochemical Journal. 13 (3): 341–396. doi:10.1007/BF01005055. PMID7019165. S2CID22765625.
^Young B, Woodford P, O'Dowd G (2013). Wheater's Functional Histology: A Text and Colour Atlas (6th ed.). Elsevier. p. 65. ISBN978-0702047473.
^ This article incorporates text available under the CC BY 4.0 license. Betts, J Gordon; Desaix, Peter; Johnson, Eddie; Johnson, Jody E; Korol, Oksana; Kruse, Dean; Poe, Brandon; Wise, James; Womble, Mark D; Young, Kelly A (26 June 2023). Anatomy & Physiology. Houston: OpenStax CNX. 4.3 Connective Tissue supports and protects. ISBN978-1-947172-04-3.