Stereological evaluation and Golgi study of the sexual dimorphisms in the volume, cell numbers, and cell size in the medial preoptic nucleus of the rat

The medial preoptic nucleus (MPN) and the sexually dimorphic nucleus of the preoptic area (SDN-POA) stand out as prominent sexually dimorphic cell gro ups of the rat brain. However, quantitative data on sex-related differences in these nuclei in the adult rat are confined to their volume. We have use d stereological methods and Golgi-impregnated material to examine whether, in young adult rats, the sexual dimorphism in the volume of the MPN, includ ing its divisions, and of the SDN-POA, reflect similar differences in the n umber and size of their neurons. We found that the total number of neurons in all MPN divisions is higher and the mean somatic volume larger in males than in females. In addition, the total dendritic length of MPN neurons is greater, but the dendritic spine density is smaller, in males than in femal es. Likewise, in the SDN-POA the total number and size of its neurons is gr eater in males than in females. The sex differences in all quantitative par ameters evaluated accounted for the larger volume of the MPN and SDN-POA in males relative to females. In addition, the MPN neuropil also displays sex -related differences in its volume, and these differences closely match tho se detected for the volume of each MPN division. It deserves to be emphasis ed that the numerical density of neurons was the only parameter found to be significantly higher in females than in males in all MPN divisions and in the SDN-POA. Our results show that the MPN and the SDN-POA display sex diff erences in the volume, total number of neurons, and size of neuronal cell b odies and dendritic trees. Furthermore, they also indicate that the neuropi l is critical for the establishment of sexual dimorphism in the size of the MPN.