Absolute number of parvicellular and magnocellular neurons in the red nucleus of the rat midbrain: a stereological study

Abstract

The absolute number of parvicellular and magnocellular neurons in the red nucleus was estimated using design-based stereological counting methods and systematic random sampling techniques. Six young adult male rats, and a complete set of serial 40-μm glycolmethacrylate sections for each rat, were used to quantify neuronal numbers. After a random start, a systematic subset (i.e. every third) of the serial sections was used to estimate the total volume of the red nucleus using Cavalieri's method. The same set of sampled sections was used to estimate the number of neurons in a known subvolume (i.e. the numerical density Nv ) by the optical disector method. Multiplication of the total volume by Nv yielded the absolute number of neurons. It was found that the right red nucleus consisted, on average, of 8400 parvicellular neurons (with a coefficient of variation of 0.16) and 7000 magnocellular neurons (0.12). These total neuronal numbers provide important data for the transfer of information through these nuclei and for species comparisons.

Keywords: Cavalieri's method; magnocellular neurons; optical disector method; parvicellular neurons; red nucleus; species comparisons; transfer of information.

Figure 1

Light micrographs of coronal sections showing the boundaries (see yellow contour) of the right red nucleus. Every third section from the beginning to the end of the red nucleus of one rat has been photographed and illustrated (i.e. rat 6, sections 190–217). SNC, substantia nigra compacta; SNR, substantia nigra reticulata. Scale bar: 200 μm.

Figure 2

Light micrographs of parvicellular and magnocellular neurons of the red nucleus captured at different disector heights. Taken from rat 7, section 239. (a) A parvicellular neuron (arrow) in focus. However, this neuron was not counted as a disector neuron as its nucleus fell on an exclusion line of the unbiased sampling frame. (b) Two magnocellular neurons (arrows) in focus at a different focal depth to (a). These neurons were counted as disector neurons because their nucleus fell either on the inclusion lines, or within the middle, of the unbiased sampling frame. Scale bar: 30 μm.