COUNTING OF NEURONS BY FLYING SPOT MICROSCOPE *

It is a common experience in biological research to be faced with the need for obtaining large quantities of statistically valid data without the expenditure of unreasonable amounts of time and effort. Quantitative studies of the microstructure of tissues and cells present part icularly complex problems because of the tedious nature of microscope observations. In neuropathology, as in other pathologies, situations occur where it is highly desirable to perform measurements such as cell population counts, cell dimensions, spectral absorptions, etc. For example, some forms of intellectual deterioration are believed to be linked to diffuse nerve cell loss in the entire brain or in certain focal regions. So far the most common method of making a determination of nerve cell loss has been to observe a few tissue section slides and offer a judgement based on years of experience. The judgement might simply be: “the cortex appears rather clear or depopulated”; or “the neuronal population density seems to have diminished.” Certainly, quantitative estimates of this type are not unique to pathology. The forestry expert, the reconnaissance pilot, the crowd-gaging policemen and many others all make quantitative estimates based on experience and training. Attempts have been made actually to count the numbers of cell elements in brain sect ions using visuomanual methods aided by special eyepiece graticules and filar micrometers. This technique has been reasonably successful for counting cells in very small, limited areas of tissue sections. The most recent of such attempts in neurology is the estimate of nerve cell population in the lateral geniculate body by Yakovlev’ and his associates. In the cortex or in the subcortical gray, however, the enormous number of cells make such visual counts completely impossible. Bok* has approached this problem by expressing quantitative relationships in terms of cortical profiles or counts along arbitrarily chosen cortical radii. This does not constitute an absolute count. Absolute counts are a necessity if one tries to evaluate the effect of a lesion in one area of the brain on another area. The best example of this is atrophy of the thalamus occurring after cortical lesions. Absolute counts may include the total neuronal population of the human brain, various correlations as between the right and left hemispheres, cortical versus thalamus populations, Golgi I1 type neurons versus large pyramidal neurons, and others. Because of the growing importance of accurate neuron population counts to * The work descrihed in this paper was supported in part by Grant B-3105 from the National Institute of Neurological Diseases and Blindness, Public Health Service, Bethesda, Md.