The collateral origin of a transient cerebrocerebellar pathway in kittens. A study using fluorescent double-labeling techniques

Abstract

A transient pathway from the sensorimotor neocortex to the cerebellum recently has been described in the kitten which appears late in the first postnatal week, develops maximally at days 9 and 10 and then gradually disappears. The transience of this pathway has been investigated in the present study using the retrograde transport of fluorescent dyes. In one set of experiments Fast Blue was injected into the caudal medulla of neonatal kittens while Nuclear Yellow was injected into the opposite cerebellum 3–10 days later. Neurons labeled with Fast Blue were found bilaterally throughout lamina V of the frontoparietal cortex. Neurons labeled with Nuclear Yellow also were found in lamina V. However, their distribution was predominantly ipsilateral to the cerebellar injection and generally was limited to the anterior and posterior sigmoid gyri, and the coronal gyrus. Most importantly, many neurons were labeled with both dyes, indicating that at least part of the transient cerebrocerebellar projection is derived from collateral branches of corticobulbar axons. A second set of experiments was done to determine if the transience of the cerebrocerebellar pathway is due to a retraction of collaterals of corticofuggal axons or due to the death of the cortical neurons themselves. In these experiments True Blue was injected into the cerebella of kittens 8–10 days after birth and allowed to survive for 40–75 days. No neurons were labeled with True Blue in the frontoparietal cortex, although numerous neurons were labeled in the precerebellar nuclei of the brainstem. In a control set of experiments, True Blue was injected into the caudal medulla in animals at 7 postnatal days. After 12–55 days, the frontoparietal cortex of these animals also was examined for neurons labeled with True Blue but, again, no cortical neurons were labeled. Thus, although the absence of cortical neurons labeled with True Blue after injections into the cerebellum would suggest that most, if not all, of the cortical neurons which project to the cerebellum in the neonate die during maturation, the negative results of the control experiments leave this question unanswered. These experiments also imply that True Blue may not be the appropriate marker for determining the ultimate fate of immature neurons in all developing systems.