Estimate of the total number of neurons and glial and endothelial cells in the rat spinal cord by means of the optical disector.

The total numbers of neurons and glial and endothelial cells in five rat spinal cords were estimated by stereological techniques. Each spinal cord was divided into 12 slabs of equal length. One transverse and one oblique slice was cut from each slab. The volumes of gray and white matter of each cord were then estimated by point-counting techniques on the transverse slices. By means of optical disectors and systematic sampling, the numerical densities of different cell types were estimated on 35 microns-thick plastic sections from the oblique slices. The total cell number was calculated by multiplying the numerical density by the total volume of gray and white matter. On average there were 15.1 and 21.1 million cells in white and gray matter, respectively. Of the cells in gray matter, 6.4 million were judged to be neurons, 4.3 million to be endothelial, and 10.3 million to be glial. Of the neurons, 1.7 million were located in the cervical region, 2.5 million in the thoracic, 1.6 million in the lumbar, and 0.6 million in the sacro-coccygeal region. The methods used are simple to perform, and the counting necessary to obtain a reliable estimate of cell number from one spinal cord can be carried out during the course of 1 day. The only major problem is reliable criteria for unambiguous cell classification.