Distribution and morphological characteristics of axons in the periodontal ligament of cat canine teeth and the changes observed after reinnervation.

The distribution and morphological characteristics of myelinated and non-myelinated axons innervating the lower canine periodontal ligament (PDL) in adult cats have been analysed. After perfusion fixation and decalcification, the teeth were slit transversely, divided into segments, and embedded in plastic. Ultrathin sections of each segment were examined in the electron microscope and used to reconstruct the whole PDL at 1, 4, 7, and 9 mm from the tooth apex. One millimeter from the tooth apex there were a mean of 920 myelinated axons and 1,415 non-myelinated axons. The numbers of axons declined toward the tooth crown. Bundles of myelinated and small non-myelinated axons lay adjacent to the blood vessels midway between the bone and cementum. Isolated myelinated axons appeared to have split away from these main nerve bundles and entered the avascular zone of the ligament, where they lost their myelin sheaths to become large non-myelinated axons rich in mitochondria. These non-myelinated axons sometimes appeared to be linked to collagen fibres and were thought to be the mechanoreceptor terminals. Twelve weeks after sectioning and inferior alveolar nerve, the total number of axons innervating the periodontal ligament was 50% of that found in the contralateral controls. The large non-myelinated axons had smaller mean diameters and contained fewer mitochondria, a change which may be consistent with a reduction in mechanoreceptor excitability.