The peculiar pattern of type IV collagen deposition in epiretinal membranes

Idiopathic epiretinal membranes are sheets of tissue that develop in the vitreoretinal interface. They are formed by cells and extracellular matrix and they are considered the expression of a fibrotic disorder of the eye. Confocal and immunoelectron microscopy of the extracellular matrix of excised membranes, revealed high contents of type IV collagen. It was distributed within epiretinal membranes in basement membrane-like structures associated with cells and in interstitial deposits. In both cases, type IV collagen was always associated with type I collagen. Col IV was also coupled with Col VI and laminin. At high magnification type IV collagen immunolabelling was associated with interstitial deposits and showed a reticular appearance due to the intersection of beaded microfilaments. The microfilaments are about 12 nm in diameter with interbead distance of 30-40 nm. Cells of the epiretinal membranes showed intracellular lysosome-like bodies heavily labelled for type IV collagen suggesting an active role in membrane remodelling. Hence, type IV collagen is not necessarily always associated with basement membranes; the molecular interactions that it may develop when not incorporated in basement membranes are still unknown. It is conceivable, however, that they might have implications in the progression of epiretinal membranes and other fibrotic disorders.