Topographic organization of the corticonuclear and corticovestibular projections from the pyramis and copula pyramidis in the albino rat. An autoradiographic orthograde tracing study

The topographic organization of the corticonuclear and corticovestibular fibers from the pyramis and copula pyramidis in the albino rat was investigated by the autoradiographic orthograde tracing technique. Cortical efferent fibers from the medial part of the pyramis project to the caudoventral part of the caudomedial and middle subdivisions of the medial cerebellar nucleus, whereas those arising from the lateral part of the pyramis project to the caudomedial part of the posterior interpositus nucleus and also to the dorsal part of the lateral vestibular nucleus via the juxtarestiform body. Fibers from the medial part of the copula pyramidis project to the medial part of the anterior interpositus nucleus. On the other hand, fibers from the medial portion of the lateral part of the copula pyramidis project to the posterior interpositus nucleus, while those from the lateral portion, including the lateral limit of the copula pyramidis, project to the rostroventral part of the lateral cerebellar nucleus. These results indicate that the cortical efferent projections from the pyramis and the copula pyramidis are clearly oriented mediolaterally in the albino rat.     

The dentatorubral projection in the rat: an autoradiographic study

The dentatorubral projection has been mapped in rats using autoradiography. Any part of lateral cerebellar nucleus (NL) projects throughout the contralateral parvocellular red nucleus (NRp) rostrocaudally; the projection is topographically organized: (1) a caudorostral shift in the NL corresponds to a dorsoventral displacement through the NRp; matching of this arrangement with the origin of rubrospinal projections is discussed; (2) only ventral parts of the NL, including the parvocellar subnucleus, project to the lateral edge of the NRp.     

Efferent projections from the flocculus in the albino rat as revealed by an autoradiographic orthograde tracing method

The terminal sites of floccular efferent fibers were investigated in the albino rat by an autoradiographic orthograde method. The corticonuclear fibers terminated in the caudoventral part of the lateral cerebellar nucleus and in the caudoventral region of the lateral part of the posterior interpositus nucleus. A few fibers from the rostral flocculus terminated in the granular cell layer of the basolateral part of the nodulus and uvula as mossy fiber type terminals. The projection to the nodulus and uvula was confirmed, by an additional retrograde HRP study, to originate from scattered spindle-shaped cells in the floccular stalk. The corticovestibular fibers terminated massively in the subnucleus y. The fibers passing through the subnucleus y divided into two bundles; one bundle coursed rostrally to terminate in the lateral and ventral parts of the superior vestibular nucleus, while the other bundle passed through the lateral and then ventral parts of the lateral vestibular nucleus, supplying a few terminals to these regions, to terminate sparsely in the rostral to intermediate part of the medial vestibular nucleus and the rostroventral part of the spinal vestibular nucleus. Some fibers passing through the lateral vestibular nucleus coursed rostrally to terminate in the medial part of the superior vestibular nucleus. Sparse terminals derived from the rostral flocculus were found in the prepositus hypoglossal nucleus. No definitive differential efferent projections were demonstrated in the rat flocculus.     

Organizational features of the cat and monkey cerebellar nucleocortical projection.

The organization of the cerebellar nucleocortical projection in the cat and the monkey has been studied using orthograde and retrograde neuroanatomical tracing techniques. Injections of tritiated leucine in the cat cerebellar nuclei orthogradely labeled nucleocortical fibers throughout their course to the cerebellar cortex. Their branch points in the corpus medullare, in the folial white matter, and in the granular layer were evident from the dense, continuous distribution of silver grains overlying these labeled axons. The results from the cat showed that the cerebellar nucleocortical projection is organized principally into three rostrocaudally oriented longitudinal cortical zones. Fastigial nucleocortical fibers were directed principally to the medial 1.5-2.5 mm of the ipsilateral vermis, with a lighter projection to the lateral vermis ipsilaterally and to the medial area of the vermis contralaterally. The interposed nuclei projected mainly to the paravermis-medial hemispheric zone of the cerebellar cortex. Nucleocortical fibers from the posterior interposed nucleus projected principally to the paramedian lobule, to the medial hemispheric area of Crus I and the lobus simplex, and to the flocculus and paraflocculus. Nucleocortical projections from the anterior interposed nucleus coursed to the anterior lobe paravermis and to the ventral folia of the paramedian lobule. A lighter projection from the interposed nuceli was found to the lateral edge of the vermis and into intermediate areas of the hemisphere. Dentatocortical fibers were directed into the lateral folia of Crus I and Crus II of the lateral hemispheric zone, with a ligher projection to intermediate areas of the hemisphere of the posterior lobe and along the lateral edge of the anterior lobe hemisphere. Along the periphery of each cortical zone, the nucleocortical projection from adjacent deep nuclei overlapped slightly. The retrograde transport of horseradish peroxidase (HRP) from injection sites in the lateral hemisphere, in the medial hemisphere--paravermis, and in the vermis labeled neurons localized mainly within the dentate, interposed, and fastigial nuclei, respectively. Retrograde labeling experiments carried out in monkeys indicated that the organization of the nucleocortical projection in this species is different than that of the cat. In the primate, the nucleocortical projection to the lateral hemisphere, to the medial hemisphere--paravermis, and to the vermis appeared to arise principally from the dentate nucleus. There was a secondary input to the paravermis and vermis arising from the interposed and fastigial nuclei, respectively. This evidence suggests that the cerebellar nucleocortical system undergoes a significant phylogenetic change in its organization between the cat and primate. These organization differences are discussed in light of possible functional implications.     

Topographical localization in the olivocerebellar projection in the rat: An autoradiographic study

The extent of the olivocerebellar projection was examined in the rat using autoradiographic techniques. In animals in which injections of [3H]leucine encompassed the whole olive unilaterally (4 cases), the vast majority of olive cells was densely labelled and climbing fibres were heavily labelled throughout the contralateral hemicerebellum, except for some small gaps which were not consistently located between cases. The multiple injections required to cover the oliver inevitably labelled cells in the reticular formation surrounding the olive, and it is possible that these neurones might also provide climbing fibres to the cerebellum. To control for this possibility, the inferior olive was pharmacologically destroyed (4 cases) prior to [3H]leucine injections similar in size and placement to those given to normal animals. Examination of the cerebellar cortex of these pretreated animals revealed no molecular layer labelling despite identification of labelled reticular neurones. It was thus demonstrated that all regions of the cerebellar cortex receive afferents from the inferior olive which terminate as climbing fibres. The distribution of these terminations over the entire cortex permits the conclusion that the inferior olive is the major source of climbing fibres in the rat. The same conclusions are reached using [3H]methionine as the tracer (4 cases).     

Neurogenesis in the trigeminal ganglion of the albino rat: A quantitative autoradiographic study

The time of neuron origin in the trigeminal ganglion was examined in autoradiograms of 60-day-old rats that were exposed to a single pulse of ³H-thymidine on day 11, 12, 13, 14, or 15 of gestation. Heavily labeled neurons, representing cells in or near their last mitotic division at the time of the pulse label, were present in animals injected between embryonic days 11 and 13 with a peak on day 12. Within this time period, larger neurons were generated prior to smaller neurons with a peak for larger cells on day 12 and for smaller cells on day 13. Thus, the majority of trigeminal ganglion neurons are generated over a three-day period just after the midpoint of gestation. Neuron number, size, type, and cytoarchitectural organization were also examined in the ganglion. The mean neuron count per ganglion was 52,372. The size distribution of these cells ranged continuously from 7–61 μm (mean diameter) with no evidence for clearly defined subpopulations. The staining intensity and distribution patterns of the Nissl substance varied greatly from cell to cell precluding the classification of cells as light or dark. Little correspondence between these Nissl features and cell size was found. Among the clusters and rows of neurons in the ganglion, we did not see consistent cytoarchitectonic patterns which might reflect specific sensory receptive fields.     

Retinopretectal projections in albino and pigmented rabbits: an autoradiographic study

The ipsilateral and contralateral retinal projection was studied in pigmented rabbits and in 3 strains of albino rabbits by anterograde transport of [3H]proline and [3H]fucose combined with autoradiographic techniques. Special attention was paid to the terminals in the pretectal area of both the pigmented and albino strains. On the contralateral side terminal labeling was found in both pigmented and albino rabbits in the nucleus of the optic tract (NOT), the anterior pretectal nucleus (PA), the posterior pretectal nucleus (PP) and the pretectal olivary nucleus (PO). Ipsilaterally labeling was found only in the pigmented strain in small patches in the PP. Ipsilateral projection was not found in the albinos in the pretectal area. The results are in agreement with the findings of Scalia in pigmented rabbits. The absence of ipsilateral labeling in the pretectal region in albinos is in contrast with earlier findings of Giolli and Takahashi et al., in pigmented rabbits but is in agreement with the observations of Takahashi and Oyster. Since no radioactively labeled fibers were found to project to the NOT in either pigmented or albino rabbits, these results do not support the hypothesis of Collewijn that the inverted optokinetic nystagmus in albinos is due to misrouting of the ipsilateral retinal fibers to the NOT.     

Topographic organization of suprachiasmatic nucleus projection neurons

The mammalian circadian pacemaker, the hypothalamic suprachiasmatic nucleus (SCN), has two subdivisions. The core is located above the optic chiasm, receives primary and secondary visual afferents, and contains neurons producing vasoactive intestinal polypeptide and gastrin-releasing peptide. The shell largely surrounds the core, receives input from nonvisual sources and contains neurons producing arginine vasopressin and calretinin. In this study, we tested the hypothesis that SCN efferent projections are topographically organized with respect to the subdivision of origin. Injections of retrograde tracers were placed in major sites of efferent termination, described from prior studies that used anterograde tracers (Watts and Swanson, [1987] J. Comp. Neurol. 258:230-252; Watts et al. [1987] J. Comp. Neurol. 258:204-229). After retrograde tracer injections in the medial preoptic area, dorsomedial and paraventricular hypothalamic nuclei, bed nucleus of stria terminalis, paraventricular thalamic nucleus, zona incerta, and medial subparaventricular zone, retrogradely labeled SCN cells are clustered in the shell with few labeled neurons in the core. After injections centered in the lateral subparaventricular zone, peri-suprachiasmatic region, lateral septum, or ventral tuberal area, the majority of neuronal label is in the core with moderate to sparse neuronal label in the shell. Both subdivisions are labeled after injections in the paratenial thalamic nucleus. The same pattern of retrograde labeling is found with four tracers, cholera toxin-beta subunit, Fluoro-Gold, the Bartha strain of pseudorabies virus, and biotinylated dextran amine. These data extend our understanding of the significance of the division of the SCN into shell and core by demonstrating that the subdivisions differ in the pattern of projections. Together with prior observations that the subdivisions differ with respect to afferents, local connections, and neuroactive substances, the present study provides an anatomic basis for discrete control of circadian function by the SCN core and shell. In this novel view, the nature of the signal conveyed to areas receiving core or shell projections varies as a function of the subdivision from which innervation is derived.     

Cerebello-olivary projections in the rat: an autoradiographic study

The cerebello-olivary projection was studied in the albino rat using conventional autoradiographic techniques. The results indicated that the cerebello-olivary projection in the rat is topographically organized in a pattern similar to other mammalian species. The anterior interpositus projects to the dorsal accessory olive, the posterior interpositus to the medial accessory olive, and the dentate to both lamellae of the principal nucleus. A point of controversy may arise, however, when one considers the fastigio-olivary projection.     

Topographic organization of the corticonuclear fibers from the tuber vermis and paramedian lobule in the albino rat

The topographic organization of the corticonuclear fibers from the tuber vermis and paramedian lobule in the albino rat was investigated by autoradiographic anterograde tracing method. The medial portion of the tuber vermis projects to the dorsal part of the caudomedial subdivision of the medial cerebellar nucleus (MNcm), whereas the lateral portion of the tuber vermis projects to the dorsal part of the MNcm and the caudal part of the middle subdivision of the medial nucleus. The intermediate cortex of the paramedian lobule can be subdivided mediolaterally into three portions which project to the dorsolateral protuberance of the medial cerebellar nucleus, the rostrodorsal part of the posterior interpositus nucleus, and the caudodorsal part of the lateral anterior interpositus nucleus, respectively. The lateral cortex of the paramedian lobule can also be subdivided mediolaterally into two portions: the medial portion projects to the dorsolateral hump, and the lateral one to the lateral cerebellar nucleus. These results indicate that the cortical efferent fibers from the tuber vermis and paramedian lobule are clearly organized in the mediolateral direction in the albino rat.     

Histogenesis of the deep cerebellar nuclei in the mouse: an autoradiographic study.

In order to tag cells when they arise, pregnant mice were injected usually once or in some cases multiple times at a known time of gestation with tritiated thymidine. The offspring were killed and their brains prepared for autoradiography. Distribution of labeled cells was plotted using a drawing apparatus. Neurons of the deep cerebellar nuclei arise on gestation days 10-17. (Later periods were not studied.) Most neurons arise on gestation day 11. Many medium and small sized neurons arise after gestation day 11 with a limited number of small neurons observed to arise through the 17th day. Neurons for all parts of the complex arise at the same time, thus no gradients could be established.     

Anatomical organization of retinotectal afferents in the cat: an autoradiographic study.

The distribution of retinotectal afferents has been studied by autoradiography in 4 adult cats. The findings suggest that crossed and uncrossed retinal fibers terminate in a striking cluster-and-sheet pattern that varies systematically with respect to the retinotopic map of the colliculus. Following unilateral eye injection, labelling was most pronounced in the contralateral colliculus but a suprising volume of label appeared on the ipsilateral side in all cases in the form of dense clumps of silver grains separated by sparsely labelled zones. The contralateral projection appeared densest in the most superficial of the 3 laminae of the stratum griseum superficiale; appreciable labelling was present also in the middle lamina at all survival times used (23-72 h). Near the area centralis representation labelling in both contralateral tiers weakened markedly and local gaps appeared densest in the more dorsal band. Elsewhere, labelling in this dorsal band was generally dense, though sharply interrupted at the optic disc representation and in a curious, elongated lateral zone at mid-collicular levels. In the caudal half of the binocular zone rarefications or 'holes', about 200 mum wide, appeared in the more ventral tier between more densely labelled zones of roughly similar width. On the ipsilateral side, labelling was sparse or absent at the rostral and caudal collicular poles, and was also weak in the region of the area centralis representation save for occasional very superficial grain-clusters. Farther caudally, however, prominent approx. 200 mum wide 'puffs' of label marked the middle lamina of the superficial gray layer. The puffs were most regular in shape in the caudal half of the ipsilateral zone and these were spaced at roughly 200 mum intervals. Puffs lateral to the horizontal meridian representation tended to lie more dorsal than those medial to this line and some of the most lateral puffs at mid-collicular levels invaded the upper lamina of the superficial gray layer. The optic disc representation was marked by a column of label extending through the upper and middle laminae. Similar experiments in cat fetuses suggest that these staggered--and possible even complementary--patterns of crossed and uncrossed retinotectal projection are innate: ipsilateral 'puffs' of labelling and contralateral 'holes' appear in the superior colliculus at least one week before term, as does the ipsilateral filling-in and contralateral gap at the optic disc representation. These observations suggest that in the cat, a vertical as well as horizontal organization may characterize the superficial layers of the superior colliculus. The additional finding of a similar, interrupted puff-like pattern of labelling in the stratum griseum medium following injections in the region of the substantia nigra makes it likely that a somewhat comparable cluster-and-sheet organization may exist also in the deep collicular layers.     

Development of the cerebellar cortical efferent projection: an in-vitro anterograde tracing study in rat brain slices.

This study reports the results of an anterograde HRP analysis of the development of cerebellar cortical efferents studied in-vitro with cerebellum-brainstem slices from embryonic (E-18) through newborn rats. From E-18, Purkinje cells in the medial region of the cerebellum are seen to project to the medial cerebellar and vestibular nuclei, their appropriate targets in the adult animal. Thus, the cortical efferent system is forming during late embryogenesis and may establish synaptic contacts with target cells as early as E-20.     

An autoradiographic study of the organization of intrahippocampal association pathways in the rat

The longer associational connections of the hippocampal formation have been studied autoradiographically in a series of adult rats after small injections of ³H-amino acids into each of its various cytoarchitectonic fields. The major findings can be summarized as follows. The dentate gyrus projects in a topographically ordered manner upon the pyramidal cells of the regio inferior by way of the supra- and infrapyramidal bundles of mossy fibers. Certain cells in the hilar region of the dentate gyrus (which operationally may be defined as constituting field CA₄ of Ammon's horn) give rise to a hippocampodentate projection to the inner one-quarter of the molecular layer of the dentate gyrus. Either the same or closely related cells give rise to fibers which join the Schaffer collateral system from field CA₃ to the stratum radiatum and stratum oriens of the regio superior. The regio inferior is also characterized by a longitudinally directed associational bundle which runs throughout the septo-temporal extent of the hippocampus and is centered in the region of subfield CA_3a. The regio superior has no reciprocal projection to the regio inferior but sends a substantial projection back to the subiculum and to the entorhinal area. There is also a projection to the subiculum from the regio inferior, and the subiculum itself probably contributes significantly to the projection to the entorhinal and perirhinal cortices. There is a striking parallelism between certain of these associational connections and the commissural projections to the hippocampus and dentate gyrus. Each cytoarchitectonic field that contributes a commissural projection also gives rise to an ipsilateral associational pathway which in its intrahippocampal course and its mode of termination exactly matches that of the commissural projection, although in general, the associational connections are more extensive in their distribution along the septo-temporal extent of the hippocampus than the corresponding commissural connections. The reverse is not true; there are a number of associational projections which are not paralleled by commissural projection. All of the associational projections are topographically arranged, but those which extend across the transverse axis of the hippocampus usually show considerable divergence so that afferents from different levels overlap fairly considerably within their respective projection fields.     

Parasagittal organization of the rat cerebellar cortex: direct comparison of Purkinje cell compartments and the organization of the spinocerebellar projection.

Retrograde and anterograde transport of tracers, electrophysiological recording, somatotopic mapping, and histochemical and immunological techniques have all revealed a parasagittal parcellation of the cerebellar cortex, including its efferent and many of its afferent connections. In order to establish whether the different compartments share a common organizational plan, a systematic comparative analysis of the patterns of parasagittal zonation in the cerebellar cortex of the rat has been undertaken, by using the parasagittal compartmentation of zebrin I+ and zebrin I- Purkinje cells as revealed by monoclonal antibody Q113 as a reference frame. The distribution of mossy fiber terminals originating from the lower thoracic-higher lumbar spinal cord was compared to the distribution of zebrin I bands. Three-dimensional reconstructions from alternate frontal sections processed either for the anterograde transport of tracer or for zebrin I immunoreactivity reveal that the limits of the spinocerebellar terminal fields in the granular layer correlate well with the boundaries of some, but not all, zebrin I compartments in the molecular layer above. This leads to a subdivision of the zebrin I compartments into spinal receiving and spinal nonreceiving portions. In lobules II and VIII, the spinocerebellar terminal fields assume different positions relative to the zebrin I compartments in the ventral compared to the dorsal faces. Thus, each longitudinal compartment may be further divided transversely into subzones, each receiving a specific combination of mossy fiber afferents. The further subdivision of zebrin I compartments by mossy fiber terminal fields increases the resolution of the topography to such a point that anatomical compartment widths become compatible with the width of the microzones and the patches identified by electrophysiological methods.     

Topographic organization of the interpositorubral connections in the rat. A WGA-HRP study

The projections from the anterior (NIA) and posterior (NIP) interposed nuclei to the magnocellular red nucleus (RNm) have been investigated in the rat, using the retrograde transport of horseradish peroxidase-wheatgerm agglutinin conjugate. Projections from the NIA extend throughout the RNm, whereas those from the NIP only reach its medial aspect. In addition, a topographical organization of the NIA-RNm pathway was found, such that the medial NIA projects ventrally, the lateral NIA projects dorsally.