Morphology of the olivocerebellar projection of the chick: An axonal reconstruction study

Abstract

The projection pattern of the olivocerebellar (OC) axons, which terminate mainly as climbing fibers (CFs) in the cerebellar cortex, tightly reflects the compartmental and developmental organization of the cerebellum as revealed by mapping and reconstruction studies in the rat. The avian cerebellum is well lobulated and longitudinally compartmentalized like the mammalian cerebellum. However, the projection pattern of the OC axons has not been studied in detail for most areas of the avian cerebellum. In the present study, we reconstructed labeled chick OC axons resulting from a small focal injection of biotinylated dextran amine into the inferior olive to investigate their morphological characteristics, and to determine their relationship to the general morphology of the chick cerebellum. Labeled CFs were distributed basically in a single longitudinally elongated narrow band-shaped area in lobules I-VIII, but in multiple, transversely widened, band-shaped areas in lobules IX-X. Three of the four reconstructed OC axons terminated in a single longitudinally band-shaped area in lobules IXa-c, whereas the other one terminated in multiple mediolaterally separated areas in lobule IXc, which is part of the flocculus. Single OC axons branched into 14 CFs on average. Two CFs occasionally merged to form a single terminal arbor. Axons also had thin, non-CF collaterals that projected either to a cerebellar nucleus or to the cortex. The results indicate that the morphological characteristics of OC axons, including branching and termination, are basically conserved between the chick and the rat. J. Comp. Neurol. 521:3321-3339, 2013.

Compartmentalization of the chick cerebellar cortex based on the link between the striped expression pattern of aldolase C and the topographic olivocerebellar projection

“…Projections were classified according to the position of the origin within the DL and the position of the labeled climbing fibers in the cerebellar cortex, which were topographically related to each other (A-E). Two additional cases of similar anterograde labeling (F), in which aldolase C was not immunostained are cited from our previous study (Sasamura et al, 2013) and shown for comparison to others. For anterograde labeling experiments, the injection site is shown with a colored spot circumscribed by a continuous black line in the inferior olive and the labeled climbing fibers are shown with dots in the same color in the cerebellar cortex.…”

“…This was presumably partly because the olivocerebellar projections have transverse branching in lobules IXc and X (green dots in Fig. C; Pakan et al, ; Sasamura et al, ).…”

“…Our previous study has shown that a single olivocerebellar axon branches into 14 climbing fibers on average and terminates in a single band‐shaped area or in multiple mediolaterally separate areas in the cerebellar cortex contralateral to its origin (Sasamura et al, ). This suggests a topographic pattern of the olivocerebellar projection.…”

“…We used chicks 7 days after hatching (P7) because of their availability and convenience (Sasamura et al, 2013). Since the chick cerebellum is morphologically well developed at P7 (Sasamura et al, 2013), we assumed that the compartmental and topographic patterns represent their mature and permanent forms.…”

“…The olivocerebellar projection is highly preserved between mammals and birds in terms of the morphological properties of its climbing fiber arbor (Ramón y Cajal, ; Freedman et al, ), single axonal morphology (Sasamura et al, ), common molecular marker expression (FoxP2, Fujita and Sugihara, ), and topographic projection originating from different subnuclei of the inferior olive to different longitudinally band‐shaped areas in the cerebellar cortex (Arends and Voogd, ; Pakan and Wylie, ). Based on the topography of the olivocerebellar and corticonuclear projections, the avian cerebellar cortex has been subdivided into five or six longitudinal zones in pigeon (A, B, C, E, and F‐zones, Arends and Voogd, ).…”