By use of a highly sensitive method for measuring slight variations in birefringence it is shown here that a strong reversible correlation exists between rat tail tendon birefringence and temperature. This phenomenon is totally different from the loss of birefringence that results from a denaturation process. Below the threshold temperature leading to denaturation, an increase in temperature is systematically accompanied by a reversible increase in birefringence (0.25%℃~(-1)). This phenomenon is observed at very fast heating rates (250,000℃ s~(-1)), such as those induced by pulsed infrared lasers, and confirmed by experiments conducted with slow homogeneous heating of the sample medium (0.1℃s~(-1)). The good correlation between birefringence and temperature observed during the fast heating suggests that there are only small modifications of the tissue structure at the fibril level.
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