(A) Transmission electron microscopic image of tumor tissue showing dense type I collagen fibrils (reprinted with permission from the Journal of Biological Chemistry, ref. 126). Rectangle indicates a single fibril. Below, a sketch represents type I collagen molecules (wavy lines) packed in a fibril in parallel and staggered with respect to one another by approximately 67 nm. (B) Cross-linking sites of adjacent collagen molecules. Initial cross-links are indicated by dashed lines. Two telopeptidyl lysine aldehydes (O=CH) can cross-link within the same molecule (intramolecular cross-link). Telopeptidyl lysine- and hydroxylysine-aldehyde (O=CH) can condense with ε-amino groups (NH₂) of juxtaposed helical lysine or hydroxylysine residues to form intermolecular cross-links. Helical cross-linking hydroxylysine residues near the N-terminus are one of the major glycosylation sites (galactose or galactose-glucose indicated in tan) in type I collagen, contributing to glycosylated cross-linking. (C) Major cross-linking pathways characterized in fibrotic and cancer tissues (for comprehensive cross-linking pathways, see ref. 65). Lysyl oxidases (LOXs) initiate cross-linking by converting lysine or hydroxylysine residues in the telopeptides to aldehyde (t-Lys^ald and t-Hyl^ald, respectively). Then the aldehyde condenses with another t-Lys^ald in the same molecule or juxtaposed helical Lys (h-Lys) or Hyl (h-Hyl) on a neighboring molecule. These divalent cross-links can then mature into tri- and tetravalent cross-links. All cross-links are intermolecular cross-links except an aldol condensation product (ACP). In fibrosis and cancer, the pathway is driven toward LH2-mediated Hyl^ald-derived pathway, as indicated by blue boxes (115). d, deoxy; deH, dehydro; DHLNL, dihydroxylysinonorleucine; h, helical; HHMD, histidinohydroxymerodesmosine; HLNL, hydroxylysinonorleucine; LH2, lysyl hydroxylase-2; Pyr, pyridinoline; t, telopeptidyl.