High-Performance Polarization Microscopy Reveals Structural Remodeling in Rat Calcaneal Tendons Cultivated In Vitro

Collagenous tissues exhibit anisotropic optical properties such as birefringence and linear dichroism (LD) as a result of their structurally oriented supraorganization from the nanometer level to the collagen bundle scale. Changes in macromolecular order and in aggregational states can be evaluated in tendon collagen bundles using polarization microscopy. Because there are no reports on the status of the macromolecular organization in tendon explants, the objective of this work was to evaluate the birefringence and LD characteristics of collagen bundles in rat calcaneal tendons cultivated in vitro on substrates that differ in their mechanical stiffness (plastic vs. glass) while accompanying the expected occurrence of cell migration from these structures. Tendon explants from adult male Wistar rats were cultivated for 8 and 12 days on borosilicate glass coverslips (n = 3) and on nonpyrogenic polystyrene plastic dishes (n = 4) and were compared with tendons not cultivated in vitro (n = 3). Birefringence was investigated in unstained tendon sections using high-performance polarization microscopy and image analysis. LD was studied under polarized light in tendon sections stained with the dichroic dyes Ponceau SS and toluidine blue at pH 4.0 to evaluate the orientation of proteins and acid glycosaminoglycans (GAG) macromolecules, respectively. Structural remodeling characterized by the reduction in the macromolecular orientation, aggregation and alignment of collagen bundles, based on decreased average gray values concerned with birefringence intensity, LD and morphological changes, was detected especially in the tendon explants cultivated on the plastic substrate. These changes may have facilitated cell migration from the lateral regions of the explants to the substrates, an event that was observed earlier and more intensely upon tissue cultivation on the plastic substrate. The axial alignment of the migrating cells relative to the explant, which occurred with increased cultivation times, may be due to the mechanosensitive nature of the tenocytes. Collagen fibers possibly played a role as a signal source to cells, a hypothesis that requires further investigation, including studies on the dynamics of cell membrane receptors and cytoskeletal organization, and collagen shearing electrical properties.