The elastic constants and ultrastructural organization of a basement membrane (lens capsule)

Abstract

The elastic constants and ultrastructure of the basement membrane of the crystalline lens of the adult cat have been investigated. Negatively stained specimens examined by electron microscopy revealed fragments of parallel filaments showing little tendency to cross over or link with each other. High resolution micrographs also showed that the filament spacing was about 4.3 nm while the filaments had a regular periodicity of 4.1 nm along their length. Optical defraction analysis of the filaments suggested a possible helical array, the angle of tilt of the helices being about 50°. The elastic properties of the basement membrane were compared with those of a lightly vulcanized rubber membrane of the same thickness. At low stress values the Young modulus of elasticity of the basement membrane (0.82 x 10 6 Nm -2 ) and rubber membrane (1.32 x10 6 N m -2 ) were similar, but at moderate extension the basement membrane had a Young modulus of elasticity almost ten times greater than rubber which in contrast showed a slight decrease in elasticity. Also basement membrane had a low percentage of elongation (25%) compared with rubber (750 %) but the ultimate stress required to rupture basement membrane was similar to that of rubber. These data suggest that the extension of coiled superhelices of the filaments rather than the extension of non-extensile randomly linked filaments, would be an appropriate model of basement membrane. This satisfactorily predicts the stress-strain curve, and ultimate stress of the intact membrane, while on the molecular level it predicts the angle of tilt of the superhelix (53°) and indicates that the elasticity modulus of the filaments of which the membrane is composed is similar to the elasticity modulus of collagen filaments. Furthermore the change of entropy (1.0 → 1.6 J K -1 per mole of collagenous protein polypeptide residue) has been calculated from the external work necessary to rupture the membrane. In terms of the model this presumes that all the superhelices of which the membrane is composed have passed from a helical to a non-helical or extended state before rupture. A similar estimate for the growth entropy of polypeptide chains has previously been made from thermodynamic measurements on binary polypeptide solutions which induce either a random or helical configuration.