Volume densities, surface densities, length densities and numerical densities of several structures in the neocerebellar lobule VIa and the archicerebellar lobule X of six-month old male Han: WIST-rats were estimated by point- and intersection-counting. The volume densities of dendritic spines (ca. 6.5), parallel fiber varicosities (ca. 25) and processes of Bergmann glial cells (ca. 21) were similar in the upper third of the molecular layer of lobule VIa and X respectively. The surface density of the spine membrane was 31 mm2/mm3in lobule X and 32 mm2/mm3in lobule VIa (p=0.4375; paired Pitman permutation test). The length density of dendritic spines varied from 793 meters/mm3in lobule VIa to 675 meters/mm3in lobule X (p=0.0938). The mean caliper diameter of parallel fiber-Purkinje cell synapses was estimated by Mayhew's (1979) method and calculated by Cruz-Orive's (1983) computer program. Both tests yielded nearly identical numerical densities of parallel fiber synapses in lobule VIa (6.558x108/mm3) and in lobule X (4.892x108/mm3;p=0.0313). The area of synaptic apposition relative to the postsynaptic dendritic spine surface was higher in lobule VIa (13.3) than in lobule X (10.4;p=0.0313). The data provide electron microscopic evidence of regional differences in spine morphology, which together with different spiny branchlet diameter and numerical density of parallel fiber synapses may be of importance in Purkinje cell physiology.
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