Stereological sex difference during development of the magnocelluar subdivision of the medial preoptic nucleus (MPN mag).
In Syrian hamsters, reproductive behaviors are initiated in the presence of appropriate hormonal and chemosensory cues. These cues are detected and integrated within a highly conserved pathway that converges on a small nuclear group in the lateral aspect of the medial preoptic area, the magnocellular subdivision of the medial preoptic nucleus (MPN mag). The MPN mag plays a critical role in the regulation of male mating behavior--bilateral ablation of the MPN mag eliminates copulation. The MPN mag is sexually differentiated in both neuron number and density, but not in overall volume or volume of individual neurons. The current study used unbiased stereological methods to determine when the MPN mag becomes sexually differentiated. Our data indicate that the MPN mag becomes sexually dimorphic in volume and cell number after the critical period when steroid treatment induces male sexual behavior.