Cell body and dendritic tree size of intracellularly labeled thalamocortical projection neurons in the ventrobasal complex of cat.

Fifteen thalamocortical projection (TCP) neurons from the adult cat ventrobasal complex (VB) were intracellularly labeled with horseradish peroxidase or neurobiotin and examined quantitatively. We find that cat TCP neurons share key morphological features and form one neuronal type. Previously reported variations in dendritic appearance cannot be supported by the present quantitative data. The number of dendrites varied between 4 and 13 (mean 9.1; +/- 4.0) and the total dendritic length of adult cat VB neurons varied between 9,421 and 19,646 microns (mean 13,120 microns; +/- 2,605). Linear regression analyses showed that soma diameter or cross-sectional area measurements provide a poor estimate for total dendritic length in TCP neurons. In contrast, the number of first order dendrites or the sum of first order dendrite diameters do provide a good estimate of overall TCP neuron size. This relationship is useful in predicting total dendritic length when it is not possible to reconstruct the entire dendritic tree. The mean dendritic path distance (distance from soma to the dendritic tip measured along the dendrite) was relatively constant for all neurons regardless of differences in total dendritic length or the number of branches that form the path distance.