Structural Implications of Banding in Unstained Collagen

Abstract

THE three currently accepted three dimensional models for the structure of the collagen fibril predict alternating regions of high and low density along the length of the fibril, with a repeat length (D) of about 650 A (Fig. 1a)1–3. As in the previous two dimensional “modified quarterstagger” model4,5, the denser regions (A bands) are about 0.4 D in length, the less dense or “hole” regions (B bands) are about 0.6 D, and the tropocollagen molecular length is 4.4 D. The electron micrographs of negatively stained native and reconstituted collagen, on which the models are largely based, vividly display large banded features which have been interpreted as due to the higher permeability of the B bands to the dense metal stain6,7. Although electron micrographs of unstained collagen fibrils could be expected to reflect the intrinsic longitudinal density distribution, very few studies have included such micrographs, and these were not detailed8,9. Consequently, we have re-examined unstained collagen fibrils by a simple technique. The observed density patterns seem to conflict with the above models and with the prevailing interpretation of negative staining in collagen.