The sequence of myelination in the brainstem of the rat monitored by myelin basic protein immunohistochemistry

The temporal order which is observed by nervous pathways in their myelination was investigated in the brainstem of neonatal to 16-day-old albino rats. The immunohistochemical detection of myelin basic protein in newly forming myelin sheaths provided a highly sensitive and specific criterion to judge the time of onset and the progress of myelination in its initial stages. Immunohistochemistry was performed on paraffin-embedded tissue according to the unlabeled peroxidase-antiperoxidase method. A small number of immunoreactive fibers was already present in neonate rats in the ventral funiculus of the cervical spinal cord and in the medial longitudinal fascicle of the medulla oblongata. By the 4th postnatal day myelination had commenced in many sensory tracts and in the trigeminal root, in the tectospinal tract, in the inferior cerebellar peduncle and in the roots of most cranial motor nerves. The auditory pathway up to the inferior colliculus, the gracile, spinocerebellar and rubrospinal tracts and the superior cerebellar peduncle had acquired some myelin on the 7th day. Phylogenetic late fiber systems also appeared to myelinate comparatively late: the transverse pontine fibers and the medial cerebellar peduncle started on the 10th day, and in the corticospinal tract a conspicuously synchronized myelination appeared on the 13th day. All pathways of the brainstem possessed myelinated fibers on the 16th day and some tracts had approached their relative maturity.