Hypothermic decrease in microtubule density and birefringence in unimyelinated axons

The density of microtubules, T.D. (number of tubules/unit area) is highest in small crab nerve axons. After cooling of the nerve at 0 degrees C, the tubule density tends to zero (T.D. 2% of the normothermic value). Once the cooled nerves are returned in normothermia, the reconstitution of the microtubules is evidenced (T.D. = 90% of the initial normothermic value). Therefore cooling "depolymerizes" the tubules, whereas rewarming leads to their "repolymerization." These results definitely improve the interpretation of the birefringence decrease by cooling: as tubules and filaments contribute to the positive fraction of the total positive birefringence, their depolymerization by cooling explains to a large extent the decrease in total birefringence.