Macromolecular organization and functional architecture of basement membranes.

Basement membranes (BMs) are extracellular laminar matrices produced by endothelial and epithelial cells. They are composed by three major intrinsic macromolecules: type IV collagen, laminin and heparan sulfate proteoglycan (HSP), and by two major extrinsic ones: fibronectin and type V collagen. The intrinsic components are assembled in a three-dimensional network (type IV collagen) to which cells stick (by laminin) and through which (HSP) they interact with the stromal compartment. The BM is a barrier to be crossed by any cell that leaves the stroma to enter into the circulation or vice versa. Metastatic tumor cells secrete a protease which specifically degrades the BM collagen and some evidence suggests that the same enzyme is used also by normal monocytes.