 Fibrillar collagens include types I, II, III, V, and XI collagen molecules. These types are characterized by uninterrupted glycine–proline–hydroxyproline repeats and aggregate to form 68-nm-banded fibrils (as diagramed in Fig. 6.7a).
 Fibril-associated collagens with interrupted triple helixes (FACITs) have interruptions in their triple helixes that provide flexibility to the molecule. They are located on the surface of different fibrils and are represented by types IX, XII, XIV, XVI, XIX, XX, XXI, and XXII  collagens. For instance, type IX collagen molecule binds and interacts with type II collagen in the cartilage at the intersections of the fibrils. It serves to stabilize this tissue by binding type II collagen fibrils with proteoglycans of the ECM.
 Hexagonal network–forming collagens are represented by collagen types VIII and X.
 Transmembrane collagens are represented by types: XIII (found in the focal adhesions), XVII (found within the hemidesmosomes), XXIII (found in metastatic cancer cells), and XXV (a brain-specific collagen).
 Multiplexins (collagens with multiple triple-helix domains and interruptions) comprise collagen types XV and XVIII, which reside in the basement membrane zones.
 Basement membrane–forming collagens include type IV collagen, which is responsible for the collagen suprastructure in the basement membrane of epithelial cells (page 139), type VI collagen, which forms beaded filaments, and type VII collagen, which forms anchoring fibrils that attach the basement membrane to the ECM.
Table 6.2 lists the collagens that have been characterized to date (I to XXV), including their structural variations and some of the roles presently ascribed to them. Recently identified collagen types (XXVI to XXVIII) have not been fully characterized and are not included in the table.
Biosynthesis and Degradation of Collagen Fibers
Collagen fiber formation involves events that occur both within and outside the fibroblast.
The production of fbrillar collagen (I, II, III, V, and XI) involves a series of events within the fibroblast that leads to production of procollagen, the precursor of the collagen molecule. These events take place in membrane-bounded organelles within the cell. Production of the actual fibril occurs outside the cell and involves enzymatic activity at the plasma membrane to produce the collagen molecule, followed by assembly of the molecules into fibrils in the ECM under guidance by the cell (Fig. 6.8).