Regeneration of three populations of olfactory axons as a function of temperature

Following injury near the mucosa, 3 populations of regenerating axons have been shown to invade successively the olfactory nerve. At 21 degrees C, each of the first 2 phases (I and II) represents 3-5% of the original axonal population. Phase III corresponds to the massive arrival of new axons and contains 50-70% of an intact nerve axonal population. The influence of temperature on the elongation rates of these 3 phases of growing fibers has been followed from 10 to 35 degrees C. No regeneration occurs at 10 degrees C. Between 14 and 31 degrees C the elongation velocity of the most rapidly growing fibers (phase I) increases linearly from 1.6 to 10.3 mm/day. These values are identical to the rates of slow intra-axonal flow measured in all 3 phases of regenerating axons. In the same range of temperature, the elongation velocity of the other two populations, increases exponentially from 0.5 to 9 mm/day for phase II and from 0.3 to 6.3 mm/day for phase III. Above 33 degrees C, rates of phases II and III fibers do not reach the values predicted from the exponential functions. The 3 phases fused as a single axonal population whose elongation velocity was shown at 34 and 35 degrees C to increase linearly according to the equation defined for phase I. Since a similar rate of slow flow was measured in all 3 populations, it can be concluded that slow flow defines the upper limit of the elongation velocity of a regenerating fiber. However, other factors acting on the regenerating neuron (mainly protein synthesis), might prevent it from reaching the maximal possible elongation velocity.