Development of myelin in human optic nerve and tract. A light and electron microscopic study.

The early development of myelin in human optic nerve and tract was studied in plastic-embedded material from post-mortem examinations of 18 infants and children. Specimens were examined by light and electron microscopy, and a good correlation was found between the data obtained by the two techniques. The characteristic lamellae of myelin were first seen around a few fibers of tract and intracranial optic nerve at 32 weeks of gestation. By term, these myelin sheaths had become thicker, and a majority of the nerve fibers had become myelinated. In the optic nerve near the globe, myelin was first seen at term and virtually all fibers were myelinated by 7 months of age. Significant increases in sheath thickness were seen in the first two years, and modest increases were found thereafter. These results are in agreement with earlier observations that, in the optic nerve, myelination proceeds from the brain toward the eye. The present data suggest that a significant amount of myelination in the human optic nerve occurs after a full-term (40-week) gestation, during a period of rapid postnatal visual development.