Connective tissue

Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue.[1] It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer.[2] Connective tissue is found in between other tissues everywhere in the body, including the nervous system. The three meninges, membranes that envelop the brain and spinal cord are composed of connective tissue. Most types of connective tissue consists of three main components: elastic and collagen fibers, ground substance, and cells.[2] Blood, and lymph are classed as specialized fluid connective tissues that do not contain fiber.[2][3] All are immersed in the body water. The cells of connective tissue include fibroblasts, adipocytes, macrophages, mast cells and leucocytes.

Connective tissue
Section of epididymis. Connective tissue (blue) is seen supporting the epithelium (purple)
Identifiers
MeSHD003238
FMA96404
Anatomical terminology

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 and dense connective tissue (which is further 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.

Special connective tissue

Special connective tissue consists of reticular connective tissue, adipose tissue, cartilage, bone, and blood.[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.[13][14]

Bone and cartilage can further be grouped into supportive connective tissue. Blood and lymph can further be classed as fluid connective tissues.[2][15][16]

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.[17] Mucous membranes and serous membranes are epithelial with an underlying layer of loose connective tissue.[17]

Fibrous types

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]

Types of fibers
TissuePurposeComponentsLocation
Collagen fibersBind bones and other tissues to each otherAlpha polypeptide chainstendon, ligament, skin, cornea, cartilage, bone, blood vessels, gut, and intervertebral disc.
Elastic fibersAllow organs like arteries and lungs to recoilElastic microfibril and elastinextracellular matrix
Reticular fibersForm a scaffolding for other cellsType III collagenliver, 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.[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 

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.[22]: 171 

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 

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.[22]: 158  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.[22]: 161 

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.
Hypermobility as a result of an inherited connective tissue defect.

See also

Notes and references
  1. Biga, Lindsay M.; Dawson, Sierra; Harwell, Amy (26 September 2019). "4.1 Types of Tissues". Retrieved 30 July 2022.
  2. Biga, Lindsay M.; Dawson, Sierra; Harwell, Amy; Hopkins, Robin; Kaufmann, Joel; LeMaster, Mike; Matern, Philip; Morrison-Graham, Katie; Quick, Devon (2019), "4.3 Connective Tissue Supports and Protects", Anatomy & Physiology, OpenStax/Oregon State University, retrieved 16 April 2021
  3. "5.3.4: Fluid Tissues". Biology LibreTexts. 21 May 2021. Retrieved 2 August 2022.
  4. Mathews, M. B. (1975). Connective Tissue, Macromolecular Structure Evolution. Springer-Verlag, Berlin and New York. link.
  5. 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. PMID 7019165. S2CID 22765625.
  6. Shostak, Stanley. "Connective Tissues". Retrieved 9 December 2012.
  7. Carol Mattson Porth; Glenn Matfin (1 October 2010). Essentials of Pathophysiology: Concepts of Altered Health States. Lippincott Williams & Wilkins. pp. 24–. ISBN 978-1582557243. Retrieved 11 May 2011.
  8. Potter, Hugh. "The Connective Tissues". Archived from the original on 31 October 2012. Retrieved 9 December 2012.
  9. Caceci, Thomas. "Connective Tisues". Archived from the original on 6 January 2013. Retrieved 9 December 2012.
  10. King, David. "Histology Intro". Retrieved 9 December 2012.
  11. "Medical Definition of FIBROAREOLAR". www.merriam-webster.com. Retrieved 11 October 2018.
  12. "Granulation Tissue Definition". Memidex. Archived from the original on 16 November 2018. Retrieved 7 May 2016.
  13. Bordoni, Bruno; Mahabadi, Navid; Varacallo, Matthew (2022). "Anatomy, Fascia". StatPearls. StatPearls Publishing. PMID 29630284. Retrieved 3 August 2022.
  14. Schleip, R; Hedley, G; Yucesoy, CA (October 2019). "Fascial nomenclature: Update on related consensus process". Clinical Anatomy. 32 (7): 929–933. doi:10.1002/ca.23423. PMC 6852276. PMID 31183880.
  15. "Supporting Connective Tissue | Human Anatomy and Physiology Lab (BSB 141)". courses.lumenlearning.com. Retrieved 16 April 2021.
  16. Karki, Gaurab (23 February 2018). "Fluid or liquid connective tissue: blood and lymph". Online Biology Notes. Retrieved 16 April 2021.
  17. "Membranes | SEER Training". training.seer.cancer.gov. Retrieved 3 August 2022.
  18. Ushiki, T (June 2002). "Collagen fibers, reticular fibers and elastic fibers. A comprehensive understanding from a morphological viewpoint". Archives of Histology and Cytology. 65 (2): 109–26. doi:10.1679/aohc.65.109. PMID 12164335.
  19. Xu, H.; et al. (2008). "Monitoring Tissue Engineering Using Magnetic Resonance Imaging". Journal of Bioscience and Bioengineering. 106 (6): 515–527. doi:10.1263/jbb.106.515. PMID 19134545. S2CID 3294995.
  20. Laclaustra, M.; et al. (2007). "Metabolic syndrome pathophysiology: The role of adiposetissue". Nutrition, Metabolism and Cardiovascular Diseases. 17 (2): 125–139. doi:10.1016/j.numecd.2006.10.005. PMC 4426988. PMID 17270403.
  21. Di Lullo; G. A. (2002). "Mapping the Ligand-binding Sites and Disease-associated Mutations on the Most Abundant Protein in the Human, Type I Collagen". Journal of Biological Chemistry. 277 (6): 4223–31. doi:10.1074/jbc.M110709200. PMID 11704682.
  22. Ross M, Pawlina W (2011). Histology: A Text and Atlas (6th ed.). Lippincott Williams & Wilkins. pp. 158–97. ISBN 978-0781772006.
  23. Young B, Woodford P, O'Dowd G (2013). Wheater's Functional Histology: A Text and Colour Atlas (6th ed.). Elsevier. p. 65. ISBN 978-0702047473.
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