Number and size spectra of non-myelinated axons of human premolars.

The aim of this study was to determine the number and size of apical non-myelinated (C) axons of healthy human premolars. The material was derived from a large collection of specimens prepared for a previous quantitative investigation on the myelinated (A) axons of human premolars. A total of 16 teeth (six maxillary first and five each of mandibular first and second premolars), removed from adolescents for orthodontic reasons, were used. Root discs of about 0.6 mm thickness were prepared at about 2 mm cervical to the root apex and processed for light and electron microscopy. The number of non-myelinated axons was determined by taking a total census of such fibres that could be identified and reconstructed by standardized composite electron micrographs from each root disc. The measurement of axons was done on a statistically representative sample of axons (n = 1810) using an electronic image processing unit. The 16 teeth had an average of 2000 +/- 1023 non-myelinated axons at the juxta-apical level (range 534-3912). The average diameter of the non-myelinated axons was found to be 0.5 +/- 0.4 microns (range 0.05-2.4 microns).