Sequence of central nervous system myelination in human infancy. II. Patterns of myelination in autopsied infants.

The timing and synchronization of postnatal myelination in the human central nervous system (CNS) are complex. We found eight time-related patterns of CNS myelination during the first two postnatal years in autopsied infants. The intensity of myelination was graded in 162 infants with diverse diseases on an ordinal scale of degrees 0-4. The Ayer method for maximum likelihood estimates for censored data was utilized to generate curves of the temporal changes in the percent of infants with degrees 0 through 4 of myelin in 62 white matter sites. These sites fall into eight subgroups determined by the presence or absence of microscopic myelin (degree 1) at birth and the median age at which mature myelin (degree 3) is reached. There is variability in the timing of myelination within and across axonal systems, and early onset of myelination is not always followed by early myelin maturation. We reexamined general rules governing the timing of myelination proposed by previous investigators, and found that those rules are neither complete nor inviolate, and that there is a complex interplay among them. This study specifies distinct periods of maturation in which myelinating pathways are potentially vulnerable to insult during the first two postnatal years.