Connective tissue is one of the four primary types ofanimal tissue, a group of cells that are similar in structure, along withepithelial tissue,muscle tissue, andnervous tissue.^[1] It develops mostly from themesenchyme, derived from themesoderm, the middleembryonicgerm layer.^[2] Connective tissue is found in between other tissues everywhere in the body, including thenervous system. The threemeninges, membranes that envelop thebrain andspinal cord, are composed of connective tissue. Most types of connective tissue consists of three main components:elastic andcollagen fibers,ground substance, andcells.^[2]Blood andlymph are classed as specialized fluid connective tissues that do not contain fiber.^[2][3] All are immersed in thebody water. The cells of connective tissue includefibroblasts,adipocytes,macrophages,mast cells andleukocytes.

Connective tissue
Section ofepididymis. Connective tissue (blue) is seen supporting theepithelium (purple).
Identifiers
MeSH	D003238
FMA	96404
Anatomical terminology [edit on Wikidata]

The term "connective tissue" (in German,Bindegewebe) was introduced in 1830 byJohannes Peter Müller. The tissue was already recognized as a distinct class in the 18th century.^[4][5]

Connective tissue can be broadly classified into connective tissue proper, and special connective tissue.^[6][7] Connective tissue proper includes loose connective tissue, and dense connective tissue. 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 includesreticular connective tissue, andadipose tissue.

Dense connective tissue also known asfibrous connective tissue^[8] is subdivided intodense regular anddense irregular connective tissue.^[9] Dense regular connective tissue, found in structures such astendons andligaments, is characterized bycollagen 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 consists ofcartilage,bone,blood andlymph.^[10] Other kinds of connective tissues include fibrous, elastic, andlymphoid connective tissues.^[11] Fibroareolar tissue is a mix of fibrous andareolar tissue.^[12] Fibromuscular tissue is made up of fibrous tissue andmuscular tissue. New vascularised connective tissue that forms in the process of wound healing is termedgranulation tissue.^[13] All of the special connective tissue types have been included as a subset offascia in thefascial system, with blood and lymph classed asliquid fascia.^[14][15]

Bone and cartilage can be further classified assupportiveconnective tissue. Blood and lymph can also be categorized asfluid connective tissue,^[2][16][17] andliquid fascia.^[14]

Membranes can be either of connective tissue orepithelial tissue. Connective tissue membranes include themeninges (the three membranes covering thebrain andspinal cord) andsynovial membranes that line joint cavities.^[18]Mucous membranes andserous membranes are epithelial with an underlying layer of loose connective tissue.^[18]

Fiber types found in theextracellular matrix arecollagen fibers,elastic fibers, andreticular fibers.^[19]Ground substance is a clear, colorless, and viscous fluid containingglycosaminoglycans andproteoglycans allowing fixation of Collagen fibers in intercellular spaces. Examples of non-fibrous connective tissue includeadipose tissue (fat) andblood. Adipose tissue gives "mechanical cushioning" to the body, among other functions.^[20][21] 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.^[22]

Connective tissue has a wide variety of functions that depend on the types of cells and the different classes of fibers involved.Loose anddense irregular connective tissue, formed mainly byfibroblasts andcollagen fibers, have an important role in providing a medium for oxygen and nutrients to diffuse fromcapillaries 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 oftendons,ligaments andaponeuroses, and is also found in highly specialized organs such as thecornea.^[23]Elastic fibers, made fromelastin andfibrillin, also provide resistance to stretch forces.^[23] They are found in the walls of large blood vessels and in certain ligaments, particularly in theligamenta flava.^[23]

Inhematopoietic andlymphatic tissues,reticular fibers made byreticular cells provide thestroma—or structural support—for theparenchyma (that is, the bulk of functional substance) of the organ.^[23]

Mesenchyme is a type of connective tissue found in thedeveloping organs of anembryo that is capable ofdifferentiation into all types of mature connective tissue.^[24] Another type of relatively undifferentiated connective tissue is themucous connective tissue known asWharton's jelly, found inside theumbilical cord.^[23] This tissue is no longer present after birth, leaving only scattered mesenchymal cells throughout the body.^[25]

Various types of specialized tissues and cells are classified under the spectrum of connective tissue, and are as diverse asbrown andwhite adipose tissue,blood,cartilage andbone.^[23] Cells of the immune system—such asmacrophages,mast cells,plasma cells, andeosinophils—are found scattered in loose connective tissue, providing the ground for startinginflammatory andimmune responses upon the detection ofantigens.^[23]

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

• Connective tissue neoplasms includingsarcomas such ashemangiopericytoma andmalignant peripheral nerve sheath tumor in nervous tissue.
• Congenital diseases includeMarfan syndrome andEhlers-Danlos Syndrome.
• Myxomatous degeneration – a pathological weakening of connective tissue.
• Mixed connective tissue disease – a disease of theautoimmune system, alsoundifferentiated connective tissue disease.
• Systemic lupus erythematosus (SLE) – a major autoimmune disease of connective tissue
• Scurvy, caused by a deficiency ofvitamin C which is necessary for the synthesis of collagen.
• Fibromuscular dysplasia is adisease of the blood vessels that leads to an abnormal growth in the arterial wall.

• Endometrium
• Parametrium

• Overview, University of KansasArchived 26 December 2010 at theWayback 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