title: The organization of hypothalamocerebellar cortical fibers in the squirrel monkey (Saimiri sciureus).
abstract: The organization and distribution of hypothalamocerebellar cortical fibers in squirrel monkey were investigated by using horseradish peroxidase (HRP, WGA-HRP) and 3H-leucine as anterograde tracers. Following hypothalamic injections, anterogradely labeled fibers coursed bilaterally through the periventricular gray (ipsilateral preponderance) and into the cerebellar white matter. Sparse numbers of labeled fibers appeared to descend into the reticular formation and enter the cerebellum via the brachium pontis. The pattern of cerebellar cortical labeling does not conform to that of mossy or climbing fibers. Labeled axons enter and branch within the granular layer, proceed around Purkinje cell somata, and enter the molecular layer. Within the latter some labeled fibers branch outwardly in a fanlike manner whereas others ascend before branching. Many fibers within the molecular layer ultimately assume an orientation that is similar to that of parallel fibers. The distribution patterns of hypothalamocerebellar cortical axons resemble those reported for monoaminergic fibers in the cerebellar cortex. Afferent fibers to the cerebellar cortex (including hypothalamocerebellar) that do not terminate as mossy or climbing fibers may collectively constitute a third general category of cerebellar afferent axons. On the basis of their distribution within all cortical layers these fibers are designated as multilayered fibers. The morphology of multilayered fibers stands in contrast to the presumptive mossy fiber labeling seen in lobules IX and X following large injections. Such labeling may represent a subpopulation of hypothalamocerebellar fibers or result from enzyme deposition in areas bordering the hypothalamus that project to cerebellar structures.
mesh_terms: D000818:Animals; D002525:Cerebellar Cortex / Q000033:anatomy & histology*; D005260:Female; D007031:Hypothalamus / Q000033:anatomy & histology*; D008297:Male; D009434:Neural Pathways / Q000033:anatomy & histology; D012453:Saimiri