Axon number and size distribution in the developing feline inferior alveolar nerve.

The number and size distribution of axons in the developing feline inferior alveolar nerve (IAN) were examined by electron microscopy. Seven cat fetuses and thirty kittens and cats aged from 25 days post conception (dpc) to 11 years were used. The total number of IAN axons increased from 4,400 to 16-17,000 between 25 and 40 dpc, and then decreased to about 13,000 before birth. This level was maintained up to at least 11 years. Myelinated axons first appeared by 45 dpc and constituted 28% at birth. The young adult proportion of about 45% myelinated axons was established at 2 months. In the old adult (11 years) 55% of the IAN axons were myelinated. Size measurements showed that unmyelinated axons had diameters of 0.1-0.5 micrometer at 25 dpc. From 55 dpc and on the size range extended from 0.1 micrometer to 1 micrometer. The size range of myelinated axons was 1-4.5 micrometers at birth and 1-8 micrometers 2 months postnatally. A bimodal size distribution first appeared by 2 months, and the range was 1-13 micrometers from 6 months and on. The findings were compared with age-related changes in the primary and permanent dentitions.