A morphometric study of the soma, first-order dendrites and proximal axon of cat lumbar alpha-motoneurones intracellularly labelled with HRP.

Intracellularly HRP-labelled cat hindlimb alpha-motoneurones were reconstructed light microscopically from a series of 1 micron or 2 micron thick consecutive sections. The volume and surface area of the soma as well as the size of the very proximal part of the dendritic and axonal processes were estimated morphometrically. Similar measurements were also made on adjacent unlabelled neurons in the same series of sections. A close relation was found between the soma volume and surface area on one hand the combined cross-sectional area of the proximal dendrites and axon on the other. The combined axonal and dendritic bases occupied on the average 16% of the soma surface. The accuracy in using the diameters and cross-sectional area of the cell body as indirect estimates of soma volume and surface area was analyzed. Combined measurements in both the transversal and sagittal planes were then found to yield more satisfactory estimates then when the measurements were confined only to the transversal plane. Several different formulas using the soma axes for indirect calculations of the soma volume and surface area were compared with respect to the accuracy of the results.