A revised morphological model for the crimp structure of tendon is presented. The 300–500 μ diameter tendons of the mature rat tail are comprised of from one to more than ten substructures, called fascicles, of 80–320μ diameter. Fascicles each possess a “crimp structure” demonstrable in the polarizing microscope and neighboring fascicles within a tendon usually exhibit crimp registry.

The fascicle itself is shown to be a cylindrical array of planar-zig-zag crimped 500–5000Å diameter collagen fibrils. The approximate cylindrical symmetry of the fascicle is demonstrated by SEM and polarizing optical microscopy. A method of replacing native water with other liquids of refractive index near to that of collagen is utilized to reduce or eliminate light diffusion and thereby greatly improve OM observations. Small bunches of collagen fibrils removed from the tendon are shown to exhibit the simple planar zig-zag morphology described in previous literature. The planar crimping of collagen fibrils and their assemblage into cylindrically symmetric fascicles is verified by small angle X-ray diffraction.