Quantitative cytoarchitectonic distribution of neurons, glia, and DNA in rat cerebral cortex

A new technique combining microfluorometric DNA assays with differential cell counts was used to quantitate the intralaminar distribution of neuronal and non‐neuronal cells (chiefly glial) in rat somatosensory cortex (Charles River 250 gm males, C D® strain). The intracortical amounts of DNA per unit fresh volume were calculated from the DNA contents of serial frozen slices of known volume sampled serially from the pial surface to white matter in frozen cortical cylinders; respective amounts per unit solids were calculated from predetermined dry weights of the slices. Cell counts were performed on serial horizontal sections from formalin‐fixed cylinders stained by Nissl's method. Neuronal DNA and glial DNA were calculated based on the percentages of the respective cells counted. Total DNA averaged 5.43 μg/mg dry weight (1.19 μg mm 3 fresh volume). Values were highest in layers II and IV. Neuronal DNA paralleled total DNA in its intracortical distribution and showed distinct peaks in layers II and IV. Glial DNA showed an even distribution. Glia exceeded neurons only in layers I and VIc. The mean neuron/glia ratio was 2.5. This method gives a more precise estimate of the absolute numbers of neurons and glia than can be obtained by histological methods alone, since DNA assays eliminate the need to correct the histological counts for volume changes during fixation and staining.