Variations in melanin content with age in the human substantia nigra.

Evidence is presented here that the decrease in mean melanin content we have measured in old age is due, not to a general decline in melanin in all cells, but rather to a selective loss of those nerve cells containing greatest amounts of pigment. This cell loss is likely to be caused by the physical affects of pigment accumulation, per se, upon capacity for protein synthesis, as marked by losses of cytoplasmic RNA and nucleolar shrinkage in melanin rich cells.