Reduced inferior olivary neuron number in early Down syndrome.

Counts of total neuron number per section and of neurons per microscopic field of inferior olivary principal nuclei were made on sections from 10 patients with Down syndrome (DS) aged 0.36 to 28 months and seven control (C) patients aged 1 to 29 months. After stereologic and appropriate shrinkage corrections of the count data, the ratios of values for DS/C were calculated. For mean principal olivary nucleus neuron number, DS/C = 0.64; for mean number of neurons per field, DS/C = 0.84; for mean volume of olivary neuronal band per section, DS/C = 0.79; and for mean volume of neuronal band per neuron, DS/C = 1.27. The data are in accord with other data suggesting that (1) numbers of cells in various cell populations, including various areas of the cerebrum, in DS approximate two-thirds normal (DS/C approximately 0.67); (2) for the volumes of such cell populations, DS/C = 0.82 normal; and (3) for volumes of individual cells, DS/C = 1.22 normal. The data of the present study suggest that the inferior olivary nuclei in DS are affected in the same way and to a similar degree as other brain areas, with the age distribution and histologic features of the specimens studied suggesting that the reduced olivary principal nucleus number in early Down syndrome results from reduced initial neuron production rather than postnatal neuron loss.