Morphometrical synaptology of Clarke cells and of distal dendrites in the nucleus dorsalis: an electron microscopic study in the cat.

The fine structural synaptology of large Clarke cells in L3 has been investigated from a morphometrical point of view in both normal and adult cats which received horseradish peroxidase (HRP) injections in the cerebellum. This marking method made it possible to distinguish small or distal dendrites of large Clarke cells from those of interneurons and the marginal cells of Clarke's column. A total of 1036 boutons was observed on the perikarya of 21 large Clarke cells; 81.9% (848/1036) were small-sized boutons, the cross-sectional areas of which ranged between 0.3 and 2.9 sq. micrometer, while 18.1% (186/1036) were giant boutons ranging between 3.0 and 8.0 sq. micrometer. From 1075 boutons on 17 primary dendrites of Clarke cells, 72.4% (778/1075) were small-sized boutons and 27.6% (297/1075) were giant boutons. From 1679 boutons contacting 366 distal or small HRP-labeled dendrites, 89.9% (1507/1679) were small boutons and 10.1% were giant boutons. The giant boutons were more frequently located on the proximal dendrites than on the cell bodies or small distal dendrites of Clarke cells. The proportion of S- and F-type boutons was different in 3 parts of large Clarke cells. F-type boutons were more frequent on soma (55.0% 570/1036) and primary dendrites (59.4%, 635/1075). S-type boutons outnumbered the F-type on small or distal dendrites (62.6%, 1952/1679). The S/F ratio seemed to increase from the cell body toward the distal dendrites. The results suggest that Clarke cells receive predominantly small S-type boutons since the total receptive area of the dendrites is supposed to exceed that of the cell body.