Organization of collagen fibers in rat tail tendon at the optical microscope level.

The collagen fibers of tendon have a wavy configuration which is important for the mechanical function of the tissue. An investigation into the organization of collagen fibers in rat tail tendon at the level of the light microscope has led us to propose a new model for the basic tendon unit. This unit, which is termed the fascicle, is usually triangular in cross-section and 150 to 300 microns in diameter. The fibrous entities which comprise the fascicle take the planar waveform configuration seen in the polarized transmission microscope and in longitudinal histology sections. The plane of the waveform is parallel to the long side of the fascicle and adjacent planes are arranged with the waveform in registry. Other structural features observed in the microscope can be produced by defects in this ordered arrangement. Thus slip parallel to the plane of the waveform can produce the crimp reversal observed in through-focus photomicrographs. The ridges and valleys which characterize the surface of the fascicle result from out-of-plane crimping. The surface topology is probably important in maintaining registry between neighboring fascicles.