Structure and function of basement membrane.

Progress has been made in identifying and characterizing basement membrane macromolecules, including type IV collagen, laminin, a heparan sulfate proteoglycan and bullous pemphigoid antigen. Basement membrane contains a unique collagen, type IV collagen, which is formed of pro alpha 1(IV) (Mr = 185,000) and pro alpha 2(IV) (Mr = 170,000) chains. As opposed to the fibrillar pattern seen with other collagens, the type IV collagen molecules are thought to be arranged in a honey-comb or reticular pattern which provides the major structural element of the basement membrane. Consistent with this model, type IV collagen has been localized to the basement membrane lamina densa, a nonfibrillar structure. Laminin is a large (Mr = 1,000,000) noncollagenous glycoprotein with chains of 200,000 and 400,000 daltons. It has been localized to the basement membrane lamina lucida and functions to bind epithelial cells to the basement membrane. A heparan sulfate proteoglycan has also been identified in the basement membrane. Its biological function may be to restrict the penetration of anionic macromolecules through the basement membrane. In contrast to the above-mentioned components which are found in all tissue basement membranes, bullous pemphigoid antigen is only found in certain basement membranes, mostly those of stratified squamous epithelia. Bullous pemphigoid antigen is a protein, synthesized by keratinocytes in culture, with disulfide-linked chains (Mr = 220,000). By immunoelectron microscopy, it is localized in the lamina lucida of epidermal basement membrane and is closely associated with the basal cell surface. Its biological function is not known, but could involve epidermal basal cell-substrate interactions which occur when basal cells re-epithelialize wounds.