Pulpal axons in developing, mature, and aging feline permanent incisors. A study by electron microscopy

The ingrowth, maturation, and aging of pulpal axons was followed by electron microscopy in permanent mandibular incisors of 44 cats aged 35 days-11 years. The first axons entered the incisor pulps about 2 months after birth. Cells resembling Schwann cells, but lacking an axonal relation, were also observed at this time. Axonal ingrowth and maturation continued until 7 months, when the pulps contained 92-394 axons, 81-88% of which were unmyelinated. Between month 7 and 20 months the character of the incisor pulpal axons remained largely unchanged. In adult cats the myelinated pulpal axons had diameters of 1-6 micrometer. The relation between the number of myelin lamellae and the axon size correlated better with linear + logarithmic than linear functions. The g-ratios were higher than in most peripheral nerves, and tended to increase with axon diameter. In the old adult this tendency was more evident and the g-ratios were generally lower. From 3 years on the incisors displayed various age-related or pathological alterations. These were accompanied by axonal changes and axon loss. In contrast to earlier stages perineuriumlike sheaths were frequent in old teeth. The findings are discussed in relation to axon development in the inferior alveolar nerve and in primary incisor pulps.