Thickness and volume constants and ultrastructural organization of basement membrane (lens capsule).

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Abstract
The basement membrane of the crystalline lens of the rat had the following elastic constants: a Young''s Modulus of elasticity of 0.56 .+-. 0.38 .times. 106 N/m2 at low stress and 11.3 .+-. 1.9 .times. 106 N/m2 at rupture, an ultimate stress of 28.8 .+-. 4.5 .times. 105 N/m2 and a maximum elongation percentage of 41.3 .+-. 5.8. The ratio of initial thickness of the membrane to thickness at the point of rupture was 0.271 .+-. 0.02. The similar ratio for volume was 0.461 .+-. 0.031. EM observations of ultrasonicated fragments of the entire membrane showed long filaments in parallel arrays and sheets. The filaments had a periodicity of 3.7 nm and spacing of 3.5 nm. Collagenase-treated membrane had a poorly staining matrix associated with separate short straight non-periodic filaments 2.5 nm in diameter. Strands projected from the ends of the filaments with a diameter of 0.5-1.0 nm. A model of these filaments contained 3-5 parallel strands, 0.8 nm in diameter, wound in a superhelix. Thickness and volume changes were predicted satisfactorily in the model membrane when subjected to stress. The filaments apparently have a similar Young''s Modulus of elasticity and ultimate stress similar to collagen. If the basement membrane of the smallest retinal capillaries was subjected to a change in pressure of only 5 mmHg within the vessel lumen, the membrane may undergo some 30% reduction in thickness.