Some characteristics of the darkly stained area of the medial preoptic area of rats.

Male rats have larger darkly stained area (DSA) within the medial preoptic area (MPOA) than female rats and this sex difference appears at the age of day 5 and continues to persist up to senescence. This sex dimorphism seems to be independent of gonadal hormones in the adult. In male rats the DSA has higher neuron density and larger neurons than the surrounding non-DSA. The total neuron number in MPOA is not different between young male rats and young female rats. In old age, neuron number of MPOA decreased in female rats but not in male rats.