• of mollusks (crabs, ...) and arthropods is made up of chitin (a b(1->4) N-acetyl-b-D-glucosamine omopolysaccharide)
• of Tunicates is made up of cellulose (a b(1-4) D-glucose homopolysaccharide, breakable by cellulase produced by some Bacteria (symbiont in ruminants gut), some Protozoa (symbiont in termites gut) and some Fungi)

• proteins
• collagen : a fibrous insoluble protein found in the connective tissue, including skin, bone, ligaments, and cartilage that keeps your skin taut. Collagen represents about 30% of the total body protein
• type I
• a₁
• a₂
• type II
• a₁
• type III
• a₁
• type IV : the type IV collagen a-chains have similar domain structures and share between 50-70% homology at the amino-acid level. The a-chains can be separated into 3 domains : an amino-terminal 7S domain, a middle triple-helical domain, and a carboxy-terminal globular non-collagenous (NC)-1 domain. The triple-helical part is the longest domain, at about 1,400 amino acids (aa) in length, with about 22 interruptions within the classical Gly-X-Y sequence motif that is characteristic of collagens. The NC1 domain of each a-chain is about 230 aa in length. As with all collagens, the NC1 domain is considered important for the assembly of the type IV collagen trimeric structure. The assembly of a particular trimer begins when the 3 NC1 domains initiate an as yet unknown molecular interaction between 3 a-chains. Promoter trimerization proceeds like a zipper form the carboxy-terminal end, resulting in a  fully assembled protomer. The assembled protomer is flexible and can bend at many triple-helical interruption points in the molecule. The next step in the assembly is the type IV collagen dimer formation. 2 type IV collagen protomers associate via their carboxy-terminal NC1 trimers to forman NC1 hexamer. Next, 4 protomers interact at the glycosylated amino-terminal 7S region to form tetramers. These interactions form the nucleus for a type IV collagen scaffold. The scaffold evolves into a type IV collagen suprastructure, with the help of end to end associations and also lateral associations between type IV collagen protomers.
• a₁
• a₂
• a₃ (Goodpasture's syndrome autoantigen) (binding protein) => tumstatin
• a₄
• a₅
• a₆
• type V
• a₁
• a₂
• a₃
• type VI
• a₁
• a₂
• a₃
• type VII
• a₁
• anchoring fibril : a type VII collagen fibril having a central cross-banded region and a fanlike group of filaments at each end that attach the dermis to the basement membrane by interweaving with collagen fibers in the dermis and lamina densa; such fibers also occur in the gingiva, where they attach the epithelium to the lamina propria.
• type VIII
• a₁
• a₂
• type IX
• a₁
• a₂
• a₃
• type X
• a₁
• type XI
• a₁
• a₂
• type XII
• a₁
• a₁-like
• type XIII
• a₁
• type XIV
• a₁ (undulin)
• type XV
• a₁
• type XVI
• a₁
• type XVII
• a₁
• type XVIII
• a₁ => endostatin
• type XIX
• a₁
• type XXI
• a₁
• type XXII
• a₁
• type XXIV
• a₁
• type XXVI
• a₁
The a_vb₃ ligand gelatin is derived by MT1-MMP induced cleavage of collagen.
• precollagenous fibers : a name given reticular fibers on the supposition that they are immature collagenous fibers.
• collagen or collagenous fibers : the soft, flexible, white fibers which are the most characteristic constituent of all types of connective tissue, consisting of the protein collagen, and composed of bundles of fibrils that are in turn made up of smaller units (unit fibrils or microfibrils) which show a characteristic crossbanding with a major periodicity of approximately 65 nm. In describing the hierarchy of arrangement of collagen structure, the terms fiber and fibril are sometimes loosely interchanged
• collagen fibrils : delicate fibrils of collagen in connective tissue, composed of molecules of tropocollagen in linear arrays. In Type I collagen, the most common type, the tropocollagen molecules are associated in periodic, staggered arrays that give the appearance of cross-banding, with a period of approximately 65 nm in the unit fibril (or microfibril); these unit fibrils are aggregated in bundles to form larger fibrils, with longitudinal striations, which may themselves be aggregated into fibers. Some other types of collagen also associate into fibrils (e.g., Types II, III, VI) but may not aggregate to show cross-banding or to form fibers. The terms fiber and fibril are sometimes interchanged loosely in descriptions of the hierarchy of collagen aggregation
• reticular fibers / argentaffin fibers / argentophilic fibers / lattice fibers / Gitterfasern : immature connective tissue fibers, staining with silver, forming the reticular framework of lymphoid and myeloid tissue and occurring also in the interstitial tissue of glandular organs, the papillary layer of the skin, and elsewhere
• Sharpey's fibers / bone fibers : collagenous fibers that pass from the periosteum and are embedded in the outer circumferential and interstitial lamellae of bone
Collagens represent the most abundant group of proteins in vertebrates where they play important roles in the development, morphogenesis, and growth of many tissues^{ref1, ref2}. In addition to their mechanical properties, collagens also act as substrates for cell attachment and migration and mediate signaling events by binding to cells surface receptors such as
• integrins
• discoidin domain receptors (DDRs)
• glycoprotein VI^ref.
To maintain correct tissue homeostasis, the synthesis, deposition, and degradation of collagens, which is mainly carried out by fibroblasts, must be coordinately regulated. Imbalances in collagen metabolism are manifested in pathological conditions such as inflammation, fibrosis, and tumor metastasis^{ref1, ref2}. To date, several different mechanisms have been proposed for the remodeling of existing collagen networks.
• matrix metalloproteinases (MMPs) in collagen dissolution^ref. MMPs have been linked to the extracellular breakdown of collagens during inflammation^ref and cancer progression^ref
• an alternative intracellular degradation pathway takes place at other sites of rapid collagen turnover such as periodontium^ref and healing wounds^ref. This intracellular catabolytic pathway occurs
• predominantly via phagocytosis of collagen fibrils mediated by integrins, primarily by integrin a₂b₁ with a potential regulatory contribution by integrin a₃b₁. The ingested collagen is subsequently transported to lysosomes and degraded by proteases of the cathepsin family^{ref1, ref2}.
• a separate tissue specific system for the uptake of denatured collagen has been described in liver endothelial cells where a collagen scavenger receptor, which only recognizes monomeric collagen a-chains but not their native triple helical form, mediates the elimination of circulating collagen waste products from the blood stream^{ref1, ref2}. In contrast to the integrin-mediated phagocytic route, the collagen scavenger receptor internalizes collagen via an endocytic route with subsequent delivery of the collagenous cargo from the endosomes to lysosomal compartments^ref. Endo180 binds collagens type I, II, IV, and V^ref
• elastin : a yellow scleroprotein, the essential constituent of yellow elastic connective tissue: it is brittle when dry, but when moist is flexible and elastic
• elastin binds elastin receptor 1
• elastin-like 1
• laminin : an adhesive glycoprotein component of the basement membrane; it binds to heparan sulfate, type IV collagen, and specific cell-surface receptors and is involved in the attachment of epithelial cells to underlying connective tissue.
• fibronectin : any of several related adhesive glycoproteins. One form circulates in plasma, acting as an opsonin; another is a cell-surface protein that mediates cellular adhesive interactions. Fibronectins are important in connective tissue, where they cross-link to collagen, and are also involved in aggregation of platelets.
• osteopontin (OPN) / secreted phosphoprotein 1 (SPP1) /  bone sialoprotein I / early T-lymphocyte activation 1 : a secreted adhesive glycophosphoprotein expressed by several cell types. It is normally produced in bone, teeth, kidney and epithelial lining tissues and is found in plasma and breast milk. It is involved in a number of physiologic and pathologic events including angiogenesis, apoptosis, inflammation, wound healing and tumor metastasis^ref
• proteoglycans : any of a group of polysaccharide-protein conjugates occurring primarily in the matrix of connective tissue and cartilage, composed mainly of polysaccharide chains, particularly glycosaminoglycans, as well as minor protein components. Individual subunits consist of a polypeptide backbone to which many glycosaminoglycan chains are covalently linked; large aggregates resembling bottle brushes are formed when many such subunits are attached via small proteins to long hyaluronic acid chains.
• chondroitin sulfate proteoglycans (CSPG)
• aggrecan/versican proteoglycan family
• chondroitin sulfate proteoglycan 1 / aggrecan 1
• chondroitin sulfate proteoglycan 2 / versican
• chondroitin sulfate proteoglycan 3 / neurocan
• chondroitin sulfate proteoglycan 4 (melanoma-associated)
• chondroitin sulfate proteoglycan 5 (neuroglycan C)
• chondroitin sulfate proteoglycan 6 (bamacan)
• chondroitin sulfate proteoglycan BEHAB/brevican
• interphotoreceptor matrix proteoglycan 1
• interphotoreceptor matrix proteoglycan 2
• proteoglycan 1, secretory granule (serglycin)
• dermatan sulfate proteoglycan (DSPG)
• class I small LRR proteoglycan (SLRP) : 12 LRRs with (STT)₄ arrangement
• DSPG I / biglycan binds to TLR2 and TLR4^ref
• DSPG II / decorin
• class III small LRR proteoglycan (SLRP) : 7 LRRs with (ST)T(ST)₂ arrangement
• DSPG III / epiphycan/PG-Lb
• heparan sulfate proteoglycans (HSPG)
• agrin binds to MuSK
• glypican-related integral membrane proteoglycan(GRIPs) family  contain a core protein anchored to the cytoplasmic membrane via a glycosyl phosphatidylinositol linkage
• glypican 1
• glypican 2 (cerebroglycan)
• glypican 3
• glypican 4
• glypican 5
• glypican 6
• syndecan 1
• heparan sulfate proteoglycan 1 (syndecan 2, cell surface-associated, fibroglycan)
• syndecan 3 / N-syndecan
• heparan sulfate proteoglycan 2 (perlecan)
• proteoglycan 2, bone marrow (NK cell activator, eosinophil granule MBP)
• proteoglycan 3 / MPB homolog (MBPH)
• syndecan 4 (amphiglycan, ryudocan)
• keratan sulfate proteoglycans (KSPG)
• leucine proline-enriched proteoglycan (leprecan) 1
• class II small LRR proteoglycan (SLRP) : 12 LRRs with (STT)₄ arrangement
• fibromodulin
• keratocan
• lumican
• osteoadherin
• proline arginine-rich end leucine-rich repeat protein (PRELP) [not a proteoglcycan ?]
• class III small LRR proteoglycan (SLRP) : 7 LRRs with (ST)T(ST)₂ arrangement
• osteoglycin (osteoinductive factor, mimecan)
• both chondroitin- and keratan sulfate proteoglycans
• proteoglycan 4, (megakaryocyte stimulating factor)
• both chondroitin- and heparan sulfate proteoglycans
• sparc/osteonectin, cwcv and kazal-like domains proteoglycan (testican)
• sparc/osteonectin, cwcv and kazal-like domains proteoglycan (testican) 2
• sparc/osteonectin, cwcv and kazal-like domains proteoglycan (testican) 3
All proteoglycans contains O-linked glycosaminoglycans (GAGs) linked via a trisaccharide (-b1,3-Gal-b1,3-Gal-b1,4-Xyl-b1-O-Ser). Some forms of keratan sulfates are linked to the protein core through an N-asparaginyl bond. The protein cores of proteoglycans are rich in S and T residues, which allows multiple GAG attachments.
• heparan sulfates (basement membranes, components of cell surfaces) bind to TLR4
• heparin (intracellular granules of mast cells)

Synthesis :
• polymer of D-GlcUA and GlcNAc
• GlcNAc => GlcN
• GlcN => GlcNSO₄^2-
• GlcNAc => L-iduronic acid
• O-sulfatation of GlcUA and iduronic acid in C-2
• O-sulfatation of GlcNAc in C-3 and C-6
• chondroitin sulfates (CS)
• chondroitin sulfate A (CSA) / chondroitin 4-sulfate (cartilage, bone, heart valves; the most abundant GAG)
• chondroitin sulfate B (CSB) / dermatan sulfate(skin, blood vessels, heart valves)
• chondroitin sulfate C (CSC) / chondrotin 6-sulfate (major GAG in fish)
• keratan sulfate(cornea, bone, cartilage aggregated with chondroitin sulfates)
• hyaluronan / hyaluronic acid (HA)(synovial fluid, vitreous humor, ECM of loose connective tissue) is a linear GAG composed of multiple copies of the disaccharide units of glucuronic acid (GlcA) and N-acetylglucosamine (GlcNAc; [b-1,4-GlcA-b-1,3-GlcNAc-]_n). With > 10,000 disaccharide repeats (n = > 10,000), a HA molecule can be easily > 4,000 kDa (with each disaccharide unit being about 400 Da) in its molecular mass, showing an extended molecular size of > 10 mm. About 50% of the HA in the body is found in the skin, where HA serves as a major structural component of the dermal ECM and the intercellular space in epidermis. Despite its chemical simplicity, HA exhibits diverse biological functions, presumably reflecting its unique ability to bind to many different proteins via a highly conserved domain known as the link module.
• CD44
• link protein
• aggrecan 1
• versican
• hyaluronectin
• neurocan
• lymphatic vessel endothelial HA receptor-1 (LYVE-1) is a recently identified HA receptor that is expressed selectively on the surface of lymph vessel endothelium; this receptor also contains a link module
• receptor for HA-mediated motility is distinct from other HA-binding proteins in its binding to HA via a non-link module sequence.
Unlike conventional GAGs that are synthetised in the Golgi network, HA is synthetiseed at the inner face of the plasma membrane, and the growing polymer is extruded through the membrane to the outer surface as it is being synthesized. 3 HA synthases (termed HAS1, HAS2, HAS3) polymerize the disaccharide units by alternate addition of GlcA and GlcNAc to the growing chain of HA polymers using UDP-GlcA and UDP-GlcNAc as substrates. These enzymes differ from each other in catalytic activities as well as in the sizes of the final products. HAS1 and HAS2 polymerize long stretches of GlcA-GlcNAc disaccharide chains, whereas HAS3 polymerizes relatively short stretches (< 300 kDa). HA polymers are degraded into monosaccharides by 3 enzymatic reactions. Hyaluronidases (Hyal) degrade HA polysaccharides to oligosaccharides, which are further digested into GlcA and GlcNAc by b-D-glucuronidase and b-N-acetyl-D-hexosaminidase (a and b subunits), respectively. 7 distinct Hyal have been identified: Hyal-1, Hyal-2, Hyal-3, Hyal-4, Hyal-5, meningioma expressed Ag 5, and the sperm-specific sperm adhesion molecule 1 / PH-20 hyaluronidase, zona pellucida binding. These enzymes appear to exhibit different catalytic profiles; e.g. Hyal-1 cleaves HA polymers into small oligosaccharides, whereas Hyal-2 cleaves them into intermediate size fragments of about 20 kDa. HA had long been recognized to serve primarily as a filling material of extracellular spaces. This classic view was rapidly revised in 1990, when several groups reported that CD44 acts as a cell surface receptor of HA. Identification of 2 additional surface HA receptors (receptor for HA-mediated motility and LYVE-1) has further supported this concept. More recently, degradation products of Ha ranging from 1.5 kDa (hexasaccharides) to 500 kDa have been shown to trigger the secretion of various cytokines and chemokines by macrophages. Thus, HA may potentially regulate diverse cellular functions.
• basement membrane : a term first used by ultrastructural histologists when they observed a sheet of amorphous, dense and proteinaceous extracellular material interposed basolateral to various cellular elements in many tissues :
• epithelial cells
• endothelial cells
• Debove's membrane : the delicate layer between the epithelium and the tunica propria of the bronchial, tracheal, and intestinal mucous membranes
• muscle cells
• Schwann cells
• fat cells
 ... and the underlying connective tissue. It comprises 2 layers :
• basal lamina : the layer of the basement membrane lying next to the basal surface of the adjoining cell layer, composed of an electron-dense lamina densa and an electron-lucent lamina lucida; sometimes used to denote the lamina densa alone.
• lamina densa : an electron-dense layer of the basal lamina, consisting mainly of type IV collagen fibrils and heparan sulfate proteoglycans (HSPG) (which is unique to basement membranes), that closely follows the plasma membrane of the basal aspect of the adjacent cell layer, from which it is separated by the lamina lucida. In the renal glomeruli and the pulmonary alveoli, it is bounded by the lamina rara externa and the lamina rara interna.
• lamina lucida : an electron-lucent layer of the basal lamina, composed of laminin, fibronectin, and proteoglycans and lying between the lamina densa and the adjoining cell layer; in the pulmonary alveoli and renal glomeruli, there is a lamina lucida, termed the lamina rara interna and lamina rara externa, on either side of the lamina densa.
• reticular lamina / lamina reticularis : a layer of the basement membrane, adjacent to the connective tissue, seen in some epithelia; it is of variable thickness and is composed of condensed connective tissue with a reticulum of collagen fibers

• crystal : a homogeneous angular solid formed from a chemical element, compound, or isomorphous mixture, having a definite form in which the ultimate units from which it is built up are systematically arranged
• calcium pyrophosphate dihydrate (CPPD) crystals : microscopic crystals of calcium pyrophosphate dihydrate occurring in the synovial fluid in calcium pyrophosphate deposition disease
• hedgehog crystals : spiny form of uric acid concretions
• calcite crystal (CaCO₃) : found nonpathologically in otoconia and pineal gland. The complex texture structure of the microcrystals may lead to crystallographic symmetry breaking and possible piezoelectricity
• hydroxyapatite or hydroxylapatite crystal : an inorganic compound, (Ca₃(PO₄)₂)₃·Ca(OH)₂, found in the matrix of bone and the teeth, which gives rigidity to these structures. Microscopic crystals of hydroxyapatite occur nonpathologically in pineal gland and pathologically in joints or bursae in a variety of connective tissue disorders.

Web resources :

• The extracellular matrix (ECM)
• Extracellular matrix at COPE
• The Virtual Library - Cell adhesion and extracellular matrix (ECM)

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