Which type of collagen forms basement membranes?

• A. Type III
• B. Type I
• C. Type II
• D. Type IV

Answer: (D)

Basement membranes rely on a network-forming collagen that creates a stable, sheet-like matrix under epithelia and endothelia. This is provided mainly by type IV collagen, which unlike the fibrillar collagens forms a two-dimensional, non-woven network. The type IV network is stabilized by interactions with laminin and other components like nidogen and proteoglycans, giving the basement membrane its characteristic scaffold, filtration properties in some tissues, and a surface for cell attachment and signaling.

The other collagens—types I, II, and III—are fibrillar. They assemble into long fibers that provide tensile strength in skin, bone, cartilage, and other connective tissues, not the sheet-like basement membrane. That’s why they don’t form the basement membrane’s characteristic network structure.