Macromolecular constituents of basement membranes: a review of current knowledge on their structure and function.

Macromolecules which appear to be integral constituents of basement membranes include type IV collagen, the glycoprotein laminin, and heparan sulphate proteoglycan. Another glycoprotein, fibronectin, may occupy an intermediate position between some lining cells and their basement membranes but is not, however, restricted to this location. An additional form of collagen, genetic type V which differs significantly from type IV collagen in structure, appears to be associated with some basement membranes, possibly linking them to underlying connective tissue. The main structural features of each of these macromolecules, as presently understood, are reviewed here as a background to the experimental papers in this "minisymposium."