The number of neurons in dorsal root ganglia L4-L6 of the rat.

The number of neurons in the dorsal root ganglia L4-L6 of the rat was determined because published data are inconsistent and in general incompatible with the number of afferent axons in the sciatic nerve. Nucleoli were counted in serial sections; epoxy-resin sections 3 microns thick, or paraffin sections 5 microns thick, or unstained 12-microns paraffin sections of osmicated tissue were used. Correction factors for split and multiple nucleoli were obtained by counting nucleolar profiles in consecutive sections of identified cells. Dividing the number of nucleolar profiles into the number of cells gave the factor by which the counts of nucleolar profiles had to be multiplied to obtain the number of neurons. The ganglia L4, L5, and L6 contained about 12,000, 15,000 and 14,000 neurons, respectively, when resin sections were used. The standard deviation for the average of 41,000 neurons in the three ganglia was 8% of the mean value. The results compare well with the number of dorsal root fibers, and with the fact that the sciatic nerve at midthigh, to which less than half of the neurons connect, contains 19,000 afferent axons. The data obtained from the paraffin series were 23% smaller, but still considerably higher and less variable than all previously reported data. The main problem with stained paraffin sections was that most small neurons had multiple nucleoli attached to the membrane of the nuclei, which only measured 10 microns in diameter. The nucleoli often projected into the dark cytoplasm and were difficult to identify.