An electron microscopic study of myelination of pyramidal fibres at the level of the pyramidal decussation in the human fetus.

The ultrastructure of pyramidal fibres at the level of the pyramidal decussation was studied in a 23 week-old human fetus (220 mm. C.-R.). It was found that the fibres are mainly unmyelinated. However, well-myelinated fibres were scattered above the pyramidal decussation and they were also present below the decussation in both the anterior and the lateral corticospinal tract. Within the tracts, well-differentiated glial cells were observed.