Myelin sheath thickness and internodal length of nerve fibres in the developing feline inferior alveolar nerve.

The relation between the number of myelin lamellae (nl) and axon size (d) was examined in the developing and adult feline inferior alveolar nerve (IAN). The internodal lengths (L) and total diameters (D) were measured on teased IAN specimens from kittens and cats. The results show that relations nl/d and L/D principally similar to those in young adult cats had been established 2 months after birth. This coincides with the maturation of the primary dentition. During the first 3 postnatal weeks signs of a developmental demyelination were common. Comparisons between the internodal elongation of early myelinating axons and the longitudinal growth of the IAN in the mandibular canal indicated that some 50% of all prospective large internodes must be removed. Between 2 months and the young adult stage, when the permanent dentition is established, the relations nl/d and L/D were essentially unaltered, but the ranges extended towards larger sizes. In the young adult the average g-value was 0.67. In the old adult cat the relation nl/d was less uniform than in young adults, and the average g-value had decreased to 0.59. In addition, successive short (100-150 micrometer) internodes were found, indicating a senescent de- and remyelination in the IAN. These alterations may be related to the age-dependent deterioration of the mandibular dentition.