Early myelination in the human fetal lumbosacral spinal cord: characterization by light and electron microscopy

Myelination in the human central nervous system is well documented after 20 weeks of gestation (WOG). However, earlier stages of this process have not been described in detail, although it is assumed that human myelinogenesis is similar to that observed in other animals. We used light and electron microscopy to study myelination in the human lumbosacral spinal cord during the second trimester of gestation. The kinetics of myelin-associated gene expression were analyzed by immunocytochemistry using antibodies to the myelin markers myelin basic protein (MBP) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase). These studies show that in 12-13 WOG specimens, occasional MBP-positive processes are found in developing white matter in areas distinct from the root entry zones. At this time, ultrastructural study revealed early investment of axons by glial processes and rare compacted myelin. CNPase staining was qualitatively and quantitatively less than that of MBP. The numbers of MBP- and CNPase-positive myelin sheaths increased with time, and by 24 WOG many were evident in all areas of the spinal cord except in the corticospinal tracts. Ultrastructural study of corresponding areas revealed many thin lamellae of compact myelin. This study provides initial normative data for early human myelination in the lumbosacral spinal cord and may serve as a baseline for future developmental and pathological studies.