Computer-assisted morphometric study on the postnatal growth of axon size and myelin thickness in the ventral and dorsal roots of the rabbit

Abstract

The morphometric analysis of the myelinated fibers is of great value in developmental, experimental, and pathological studies. The present study was performed on the ventral and dorsal spinal roots (L7) of new-born, 10-, 15-, 20-, 30-, 60-, 120- and 240-day-old rabbits. Semi-thin cross-sections were investigated by OLYMPUS Video image analyser. The average axonal diameter was measured as an average length of Feret's diameters and the specific width of the myelin sheaths as a total width of the myelin sheath. In the ventral roots, the increase of the axon size is more intensive than the increase of the myelin thickness. In adult rabbits, the mean axonal diameter for the ventral root is approximately 15% greater than the mean axonal diameter for the dorsal root, whereas the mean myelin thickness values are practically equal for both roots. For the period newborn-adult (240-day-old) rabbits, a full correlation was found in respect to the increase of the dorsal root mean axonal diameter and mean myelin thickness (380% and 380%). In the ventral roots, the mean axonal diameter increased 420% whereas the mean myelin thickness increased 350%. It is intriguing whether the greater axonal size of the ventral roots might compensate their relatively thin myelin sheaths in comparison to the dorsal roots. Our findings of essential increase (16%) of the mean myelin thickness in the last (120-240 days) period in both, ventral and dorsal roots, points to a protracted period of myelinization in the rabbit.