The postnatal development of the pont-and olivocerebellar neurons in the rat.

To study the postnatal development of the cerebellar afferent neurons in the inferior olivary nuclear complex as the climbing fiber and in the pontine nuclei as the mossy fiber, HRP (Sigma type VI) has been injected into cerebellar vermis and hemispheres of 1, 3, 5, 7, 14, 21 and 28 day after birth, and observed the HRP-labeled neurons in the inferior olivary nuclear complex and pontine nuclei with the light microscope.In the inferior olivary nuclear complex of the hemispheric injection the HRP-labeled neurons were mainly distributed throughout the principal nucleus and the rostral part of the medial accessory nucleus contralatelly, and in that of the injection into vermis the HRP-labeled neurons in the caudal part of the medial accessory nucleus bilaterally. Also, in the pontine nuclei of the hemispheric injection the HRP-labeled neurons were distributed thoughout almost the pontine nuclei contralaterally and partially the dorsal part of pontine nuclei ipsilaterally, and in them of the injection into vermis the HRP-labeled neurons the dorsal part of pontine nuclei bilaterally. These distributions in both nuclei are almost remained from 1 to 28 day after birth.Subsequently, we have studied the structure of the HRP-labeled neurons in the both nuclei at 1, 7 and 28 day after birth. In the pontine nuclei, at 1 day the HRP-labeled neurons were immature and the structure of their dendrites showed the profile of cytoplasmic processes, but at 7 and 28 day their differentiations were more advanced. While in the inferior nuclear complex, HRP-labeled neurons were already mature at 1 day after birth.From these findings it is concluded that the projection fibers derived from the inferior olivary nuclear complex and the pontine nuclei have already invaded into the cerebellum and the distribution of their cerebellar afferent neurons in both nuclei seems to be determined at 1 day after birth although the neurons are immature.