Table 1.

Defining features of collagen types.

Collagen Type	Subfamily	Molecular Species	Supramolecular Structure and Structural Features	Distributions	Function
I	Fibril Forming	• [α1(I)]2, α2(I) • [α1(I)]3	300 nm molecule, 67 nm banded fibril [35]	Ubiquitous, predominant in skin, tendons, ligaments, bones [35]	Key structural component of tissues [35]
II	Fibril Forming	• [α1(II)]3		Cartilage	Confers tensile strength and elasticity to cartilage [36], endochondral bone formation [37]
III	Fibril Forming	• [α1(III)]3	Stabilizing C-terminal cystine knot [38]	Dermis, aorta [28], uterus, blood vessels, bowel, wound healing, blood clotting cascade [39]	Tensile strength and integrity [39]
IV	basement membrane and associated collagens	• [α1(IV)]2, α2(IV) • α3(IV), α4(IV), α5(IV) • [α5(IV)]2, α6(IV)	COL domain with 21–26 interruptions [40]	Basement membranes	Cell adhesion, migration, differentiation, growth [41]
V	Fibril Forming	• [α1(V)]2, α2(V) • [α1(V)]3 • [α1(V)]2, α4(V) • α3(XI), α1(V), α3(XI)	Thrombospondin domain [6]	Interstitial tissue [42], dermal-epidermal junction [43]	Regulates collagen fibrillogenesis [44]
VI	beaded filamentforming collagen	• α1(VI),α2(VI),α3(VI)	C-terminal propeptide endotrophin hormone [45,46] Much lower frequency of GPO repeat [28]	Ubiquitous [28], basement membrane-interstitial matrix interface [46]	Maintains the integrity of skeletal muscle [46]
VII	basement membrane and associated collagens	• [α1(VII)]3	Exceptionally long triplehelix domain, Kunitz domain [6]	Epidermal–dermal Junction [6]	Anchoring collagen, binds fibril forming collagens [47]
VIII	hexagonal network collagens	• [α1(VIII)]2, α2(VIII) • α1(VIII), [α2(VIII)]2 • [α1(VIII)]3 • [α2(VIII)]3	C1q domain [6]	Descemet’s membrane [48], vascular smooth muscle [49]	Mechanical stability of vascular wall, bridge between ECM components [50]
IX	FACIT	• α1(IX), α2(IX), α3(IX)	Thrombospondin domain, three COL and three NC domains [6]	Articular cartilage [51]	Stabilizes fibrillar collagen network in the cartilage matrix, anchors matrilin 3 and proteoglycans [52]
X	hexagonal network collagens	• [α1(X)]3	C1q domain [6]	Hypertrophic cartilage [53]	Regulates endochondral ossification of articular cartilage [54,55]
XI	Fibril Forming	• α1(XI), α2(XI), α3(XI) • α1(XI), α1(V), α3(XI)	Thrombospondin domain [56]	Minor component of hyaline cartilage collagen fibrils [57], broadly distributed in testis, trachea, tendons, trabecular bone, skeletal muscle, placenta, lung, and the neoepithelium of the brain [56]	Nucleates and controls cartilage collagen fibril formation [57]
XII	FACIT	• [α1(XII)]3	Largest FACIT collagen, Triple armed NC3 domain [58], two variants [59] NC3 domain carries glycosaminoglycan chains [59]	Mesenchymal tissue during development [60], periodontal ligament [61], dermis around hair follicles [62], cornea of the eye [63] in adults	Temporarily stabilizes collagen fibrils during development [59,60,64]
XIII	transmembrane collagens	• [α1(XIII)]3		Connective tissue, blood vessel and junctions [65]	Bone formation [66], regulates formation of neuromuscular synapse [67]
XIV	FACIT	• [α1(XIV)]3		Skin, tendon, cornea, articular cartilage [68]	Fibrillogenesis regulation, Maintaining mechanical integrity [68], embryonic development [60]
XV	basement membrane and associated collagens	• [α1(V)]3	Bonded to chondroitin sulfate via disulfide-bonds [69]  Flexible due to knot/figure-of-eight/pretzel configuration [70]  Thrombospondin domain [6]	Cardiac and skeletal muscles, basement membrane zones [71]	Maintains integrity of ECM [71]
XVI	FACIT	• [α1(VI)]3	Flexible due to kinks in structure, 10 COL and 11 NC domains [72–74]	Territorial cartilage matrix [74,75], integrated into fibrillin-1-rich microfibrils containing in skin [74]	Hypothesized to stabilize ECM by organizing and connecting fibrillar networks, cell adhesion and invasion [72,76]
XVII	transmembrane collagens	• [α1(XVII)]3	15 COL and 16 NC domains [6]	Basement membrane zone, specifically hemidesmosomes [77,78], central nervous system neurons [79]	Epidermal cell adhesion [80]  Proliferation of epidermis [81]
XVIII	basement membrane and associated collagens	• [α1(XVIII)]3	Thrombospondin domain, 10 triple helical COL domains 11 NC domains [6]	Basement membrane zones [82]	Eye development [83], maintaining basement membrane integrity [84]
XIX	FACIT	• [α1(XIX)]3	Thrombospondin domain [6]	Vascular, neuronal, mesenchymal, epithelial basement membrane zones [85], hippocampal neurons [86]	Affects the phenotype for smooth muscle motor dysfunction and hypertension sphincter [87]
XX	FACIT	• [α1(XX)]3	von Willebrand factor A, fibronectin type III repeat, thrombospondin domain [6,88]	Possibly bile ducts, breast, cerebellum, smooth muscle cells [89]
XXI	FACIT	• [α1(XXI)]3	two collagenous domains interrupted by three noncollagenous domains [6,90]	heart, stomach, kidney, skeletal muscle, placenta [90]	May play a role in blood vessel assembly [91]
XXII	FACIT	• [α1(XXII)]3	Does not directly polymerize with fibrillar collagens, but rather associates with components of microfibrils such as fibrillins [92]	Tissue junctions: myotendinous junctions, articular cartilage-synovial fluid junction, border between the anagen hair follicle and the dermis in the skin [92]	Possibly mechanical stability of myotendinous junctions [92]
XXIII	transmembrane collagens	• [α1(XXIII)]3	N-terminal cytoplasmic domain, a transmembrane domain, and extracellular triple helical domains [93]	Lung, cornea, skin, tendon, amnion [94]	Cancer, cell-cell and cell-matrix adhesion mediation [95]  Expression elevated in prostate cancer recurrence and distant metastases [96]
XXIV	FACIT	• [α1(XXIV)]3	Thrombospondin-Nterminal like motif and charged segments with tyrosine residues on amino-terminal domain [97]	Bone and cornea [97]	Marker of osteoblast differentiation and bone formation [98]
XXV	transmembrane collagens	• [α1(XXV)]3	COL domain interrupted four times (two four-residue imperfections and two large NC sequences) [99]	Neurons predominantly of brain, also of heart, testis, eye [100–102]	Also known as collagen-like amyloidogenic component, isolated from Alzheimerdiseases brains, component of amyloid plaques [99]
XXVI	Not assigned	• [α1(XXVI)]3	Two collagenous regions and no obvious sequence homology [103]	Testes, ovary [103]	Testis and ovary development [103]
XXVII	Fibril Forming	• [α1(XXVII)]3	Nonstriated fibrils, 10 to 80 nm fibril width [104,105]	Adult cartilage [106]	Cartilage calcification, possibly cartilage transition to bone [107]
XXVIII	Not assigned	• [α1(XXVIII)]3	528 amino-acid collagenous domain flanked by two von Willebrand factor A [6,108]  Structurally resembles collagen VI	Basement membranes of peripheral nervous system [109,110]