The cerebellorubral projection in the rat: retrograde anatomical study

The cerebellorubral projections have been studied in the rat using the retrograde transport of horseradish peroxidase-wheat germ agglutinin conjugate. The lateral cerebellar nucleus projects to the parvocellular red nucleus (RN), the anterior (NIA) and posterior (NIP) interposed nuclei project to the magnocellular RN. Whereas the projections from the NIP are limited to the medial aspect of the RN, those from the NIA extend throughout the magnocellular RN. NIA-RN projections are topographically arranged: the medial NIA projects ventrally, the lateral NIA projects dorsally. Functionally, this differential distribution seems to fit the hindlimb-forelimb areas of origin of the rubrospinal tract.

Compensatory climbing fiber innervation after unilateral pedunculotomy in the newborn rat: origin and topographic organization

In neonatal rats the unilateral transection of the cerebellar peduncles causes a fast and complete degeneration of the contralateral inferior olive. Axons from the remaining olive recross the cerebellar midline and partially innervate the deprived hemicortex. Analysis of the topographic organization of this compensatory projection studied with the axonal tracing method provided the following results: Retrograde tracing experiments revealed that the bulk of compensatory afferents originates from neurons in the ipsilateral medial accessory olive, especially from its medial region, whereas afferents from the principal olive and the dorsal accessory olive contribute to a much lesser degree. In case of incomplete neonatal pedunculotomy, neurons with a similar location in the ipsilateral intact olive still contribute to the innervation of the partially deprived hemicortex, along with the atrophic contralateral olive. Moreover, these experiments revealed important information about the organization of the compensation. Although its specificity was not totally maintained, the mediolateral distribution of sprouted afferents in the cerebellum matched the caudorostral disposition of parent neurons in the olive, as in the case in normal olivocerebellar projection. Anterograde studies showed that compensatory fibers recrossing the cerebellar midline spread throughout the whole extent of the deprived cortex and terminate solely in the molecular layer as typical climbing fibers. The latter were not homogeneously distributed, their density being markedly reduced according to a mediolateral gradient. Compensatory projection followed a sagittal striped pattern, as does the normal climbing fiber projection. Moreover, if the cortex is divided broadly into vermal, intermediate, and hemispheral regions, an apparent reciprocity seems to exist concerning the relative involvement of the various cortical subdivision in both hemicerebella. Our present results indicate that the immature olivocerebellar system is capable of anatomical plasticity, although to a limited extent. More important, they suggest that a certain degree of specificity is maintained during the process of sprouting, resulting in a topographical arrangement of the transcommissural climbing fiber projection. This indicates, in turn, that cues which guide the growth of olivocerebellar fibers during normal development could also direct the compensatory innervation.

Topographic organization of the cerebellothalamic projections in the rat. An autoradiographic study

The topographical organization of the subnuclear projections towards the thalamus was studied with autographic methods in adult Wistar rats. The four cerebellar deep nuclei give rise to projections to the ventral region of the rostral thalamus. Most of the fibers end contralaterally, according to a topographical pattern; however, some fibers from each of the cerebellar nuclei recross the midline at the thalamic level and terminate ipsilaterally, within regions symmetric to those receiving the densest contralateral projection. These ipsilateral cerebellothalamic components arise in decreasing order from the caudal nucleus lateralis, the ventrocaudal nucleus medialis and the nucleus interpositus, respectively. The projections of the nucleus lateralis directed to the contralateral thalamus are topographically organized. (1) Within the nucleus ventralis lateralis, the rostral and caudal parts of the cerebellar nucleus lateralis project respectively to rostral and caudal regions; lateral and medial zones of the nucleus lateralis project, respectively, to medial and central aspects of the nucleus ventralis lateralis. (2) The nucleus ventralis medialis and particularly its caudal portion appears to receive the bulk of its afferents from the ventromedial portion of the nucleus lateralis including the "subnucleus lateralis parvocellularis". (3) The nucleus centralis lateralis receives fibers from most parts of the nucleus lateralis including the "dorsolateral hump". (4) The nucleus interpositus anterior projects to the dorsomedial aspect of the rostral nucleus ventralis lateralis. In the latter nucleus, the ventrolateral aspect of the central region receives projections in cases in which the nucleus interpositus posterior is largely involved. A particular emphasis is put on the different projections from the various subnuclear regions of the lateral nucleus. A comparison is attempted with the situation in the primates, particularly with regard to the question of the parvocellular subdivision of the lateral nucleus.

The distribution and origin of the ipsilateral descending limb of the brachium conjunctivum. An autoradiographic and horseradish peroxidase study in the rat

The distribution, organization and origin of the ipsilateral descending limb of the Brachium Conjunctivum (B.C.), have been studied in the rat by using anterograde and retrograde tracing techniques. After injections of tritiated leucine/proline into the lateral cerebellar nucleus, covering both its medial part, corresponding to the dorsolateral hump (DLH) of Goodman et al. (1963) and its lateral part, (designated here as the lateral dentate, LD), and the neighboring interposed nucleus (NI), emerging fibres are numerous and leave laterally from the B.C. On the contrary, injections restricted to LD reveal very few such fibers. Within the lateral parvocellular reticular formation (LPRF) terminal labelling is heavy, and moderate to sparse within the adjacent trigeminal complex. Rostro-caudally, silver grain accumulation within the LPRF extends from the level of the motor trigeminal nucleus (VM) to the pyramidal decussation, exhibiting a cephalocaudal decrease of grain density. Within the trigeminal complex, labelling occurs in the caudal VM, the dorsal portion of the principal sensory nucleus, and within and around the trigeminal spinalis oralis. In addition, the area surrounding the VM (in part corresponding to the supratrigeminal region of Lorente de Nó 1922, 1933) is moderately labelled. After injections of HRP into various levels of the ipsilateral descending B.C.'s projection field, retrogradely labelled cells are numerous within the DLH. A slightly lesser amount of labelled cells are found in the lateral half of the NI, primarily concerning the nucleus interpositus posterior. Within the LD, only a few labelled cells are observed: these are mainly restricted to the dorsal portion at rostral levels of the nucleus. The results obtained by both the anterograde and retrograde studies suggest an absence of a topographic organization within this descending B.C. component. The possible functional meaning of these results is discussed.

An autoradiographic study of the cerebellopontine projections in the rat. I. Projections from the medial cerebellar nucleus

The medial cerebellar nucleus of the rat is shown by the autoradiographic technique to project to both the contralateral nucleus reticularis tegmenti pontis and the pontine nuclei proper. The former projection is more concentrated in the medial--parvocellular --region. In the pontine gray, the bulk of the projection concerns the dorsal aspect of the medial nucleus. Rostral parts of the medial cerebellar nucleus project to caudal pontine levels whereas caudal parts seem to project throughout the rostrocaudal extent of the basilar pons.

Ultrastructural evidence for compensatory sprouting of climbing and mossy afferents to the cerebellar hemisphere after ipsilateral pedunculotomy in the newborn rat

Unilateral section of the inferior and middle cerebellar peduncles was performed in rats at postnatal days 1 or 2. The ultrastructure of the cerebellar hemispheric cortex ipsilateral to the lesion was examined 3 months later. The absence of contralateral inferior olive and of ipsilateral middle peduncle, together with a marked regression of the contralateral pontine gray, were indicative of successful pedunculotomy. In spite of a relative atrophy of the hemisphere, its cytological structure was qualitatively normal. Mossy and climbing fibers were present and their terminal varicosities disclosed normal features. The density of climbing fiber terminals was reduced compared to control cerebellum, whereas the density of mossy terminals seemed unchanged. subsequent to the reduction of climbing afferents two subclasses, or types, of Purkinje cells were present: A "normal" type characterized by its climbing fiber innervation and a "hyperspiny" type devoid of climbing fiber. In some of the adult rats pedunculotomized at birth, section of the contralateral peduncles was performed 24 hours before fixation. Terminal degeneration of climbing and mossy fibers was observed in the neonatally deprived hemisphere, providing the proof that these fibers result from a compensatory transcommissural sprouting of afferents destined to the contralateral hemicerebellum. These results demonstrate that the cerebellar cortex neonatally deprive of its main afferents can be innervated by climbing and mossy fibers through a process of transcommissural sprouting. Although the newly formed synapses maintain their target specificity, a functional reorganization must occur because of the altered distribution of both systems of afferents.

Comparative study of the posterior red nucleus in baboons and gibbons

The posterior red nucleus (PRN) was studied in two species of primates by the technique of retrograde degeneration of rubrospinal cells following transection of the spinal cord at different levels. The form of the PRN was reconstructed for both a quadruped monkey (baboon) and an anthropoid with erect posture (gibbon). The PRN contains polymorphic cells characterized by their very chromophilic and granular Nissl substance. These neurons vary in diameter from 25 micrometer to 70 micrometer. Some of them give rise to the rubrospinal tract. Baboon: The approximately 1,300 rubrospinal cells in this species are divided into two equal groups, one related to the contralateral forelimb, with axons ending between the second cervical and third thoracic segment, and the other related to the contralateral hindlimb, projecting caudally beyond T3. Following a high cervical lesion, nondegenerated cells of similar description remain throughout the nucleus. A significantly large group of these cells occurs medially and may be the source of fibers ending in the brain stem or cerebellum. Gibbon: In this species, the number of rubrospinal cells controlling the hindlimb is less than half that found in the baboon. This reduction in the gibbon is much greater for medium-sized cells, but is also significant for the giant cells. These results obtained from primates are compared with those reported for the cat. A possible function for the PRN in the control of limb movements is discussed from the viewpoint of phylogeny.

Embryonic development of the nucleus isthmo-opticus in the chick: a Golgi and electron microscopic study

The cytology and the synaptology of the nucleus isthmo-opticus have been studied in the chick embryo at three stages during (incubation days 14 and 15) and after (day 18) the neurone death period of the nucleus. The maturation process is not synchronous throughout the nucleus:the dorsal most region lags behind the rest of the nucleus. On the other hand, cell degeneration is equally distributed over the nucleus at days 14 and 15. At day 14, the nucleus shows little cellular alignment. The neuropil is stocked with immature neurites. Synaptogenesis essentially concerns axodendritic contacts, and the presynaptic partners contain only round vesicles. A few axons undergo myelination; among them, some are degenerating. At day 15, cellular alignment is much more pronounced. Synaptogenesis is very active; it concerns axosomatic as well as axodendritic contacts. A majority of the presynaptic bags contain only round vesicles, although some have a pleomorphic population of vesicles. At day 18, the histological structure of the nucleus is similar to that found in the adult. Although the neuropil presents mature profiles (development of dendritic spines, presence of synaptic glomeruli), it still contains numerous immature features (active synaptogenesis, persistent growth cones). Possible functional implications of this maturation are discussed. In particular, the fact that the nuclear neuropil is not fully developed by the end of the incubation period suggests that the accomplishment of the feedback role of the nucleus is determined by function.

Diversity of mossy fibres in the cerebellar cortex in relation to different afferent systems: an experimental electron microscopic study in the cat

The evolution of the terminal degeneration has been compared in two systems of mossy fibres: the spinocerebellar and the pontocerebellar projections. The two systems exhibit both dense and clear types of terminal degeneration. However, there are important differences between the evolutive processes of terminal degeneration in the two systems: (i) the time course of the degenerating process is much faster for spinocerebellar than for pontocerebellar rosettes, and (ii) the glial phagocytic process accompanying the dense type of degeneration is different for the two systems. Spinocerebellar rosettes are generally removed from their glomerular central position by reactive glia, leaving fragments of the presynaptic membrane attached to their postsynaptic partner. This feature is exceptional for pontocerebellar rosettes which, in the course of their glial engulfment, leave free the postsynaptic differentiation of their former target granule cell dendrites. These differences of terminal degenerative processes have been reconciled with optical microscope observations by Brodal and Drablos1 of morphological differences between the rosettes of two different fibre systems.