How is the exact length of an internode determined.

(1) The length of the human ventral spinal roots in the newborn and in the adult were compared with the length of the internodes in these roots. Internodes of fibers of a given caliber were more than twice as long in the adult sacral roots than in the cervical roots. The factors of root elongation corresponded closely to the factors of internode elongation. (2) Internode length was in a statistically significant linear relation with fiber caliber in individual roots. The slope of the regression lines differed among roots, and were steeper caudally. If data from roots having different elongation factors were pooled there was no longer a linear relationship between fiber caliber and internode length. Internode length, therefore, correlated directly and quantitatively with root elongation; it was not directly linked to fiber caliber. (3) The total Schwann cell population per fiber was nearly stable from the newborn to the adult, but the sacral roots had 5 times as many Schwann cells as the cervical roots. Calculations based on the number of internodes and on the length of fetal roots at the onset of myelination showed that the mean "initial length" of the Schwann cell at the onset of myelination was 187 micrometer, the same for all roots. (4) The Schwann cell population per fiber is determined by 2 reciprocal growth phases: elongation of the fetal fiber up to the onset of myelination corresponds to a multiplication of Schwann cells each having a given initial length. Schwann cell populations stabilize after the onset of myelination; from then on the geometry of each internode is determined by its passive elongation. The timing of myelination is critical for determining the ultimate length of the Schwann cell. If myelination does not begin before a fiber system has attained approximately one eighth of its definitive size, the longest internodes of that system will average 1.5 mm and most will remain below 2 mm, regardless of body size.