Myelin basic protein immunohistochemistry: a study of the early stages of myelination in the brainstem of the rat.

An immunohistochemical study of MBP distribution in the brainstem of neonate till 16 d old rats based on the peroxidase-antiperoxidase method is described. Axons already invested with immunoreactive sheaths were found in neonate rats in the ventral funiculus of the cervical spinal cord and in the medial longitudinal fascicle of the medulla oblongata. Fibres commencing with myelination showed a closely spaced array of varicosities in longitudinal sections which diminished gradually. A caudo-rostral decrease in density of myelinated fibres in the brainstem was found in the medial and dorsal longitudinal fascicles. In contrast to other pathways, myelination in the fibres of the corticospinal tract in the brainstem occurred in a strictly synchronized pattern. The same temporal pattern of myelination was also observed in the cervical corticospinal tract, except that a few myelinated fibres had been visible much earlier within the area of the tract. At the exit of cranial nerves, the transitorial zone from central to peripheral myelin was outlined by a decrease in immunostaining.