Structure and composition of the inner limiting membrane of the retina. SEM on frozen resin-cracked and enzyme-digested retinas of Macaca mulatta

After a historical introduction describing previous observations and views on the structure and composition of the internal limiting membrane of the retina (MLI), it is concluded that no definite unifying concept exists concerning the MLI structure. The authors applied frozen resin cracking with hexamethyldisilazane (HMDS) desiccation on normal and enzyme-digested specimens of monkey retinas. Distinct differences were observed between equatorial, macular, and optic disc regions. The MLI in the equatorial and disc regions was about 70 nm thick with an outer dense, basement membranelike part and an inner, loose fibrillar meshwork fusing with the vitreous fibers. The foveal region was 400 nm thick; the dense outer layer was thicker than normal basement membrane, whereas the inner loose meshwork had only a few fibrils. The enzyme-digestion experiments showed that the fibrillar meshwork consisted mainly of collagen fibers surrounded by predominantly hyaluronic acid.