Development of myelination in optic tract of the cat.

The postnatal development of myelin in the optic nerve and tract of normal and dark reared cats has been studied quantitatively with light and electron microscopy. In the newborn cat few myelinated fibers (3% of the population) are seen in the optic tract. Until the end of the second postnatal week, the total number of myelinated axons in the tract remains low (23%). At this time, however, there is an explosive increase in the rate of myelination and by the end of the fourth postnatal week 80% of the optic tract axons have acquired a myelin sheath. Thereafter, the number of myelinated axons increases gradually, reaching adult levels (100%) at 12 weeks. During the initial period of myelination, the average axon diameter is 0.6 mu for unmyelinated fibers and a.2 mu for myelinated fibers. Both of these means remain substantially unchanged until myelination is completed, suggesting that initial myelination of an axon is not a continuous process but rather proceeds in a step-wise manner. Dark rearing appears to have no effect on the initiation of myelination.