ULTRASTRUCTURAL EVIDENCE OF SEXUAL DIMORPHISM IN SUPRAOPTIC NEURONS -A MORPHOMETRIC STUDY

We have recently shown that in spite of the absence of receptors for g onadal steroids in the supraoptic nucleus (SON) of the rat hypothalamu s, the volume of the nucleus and the size of its neurons are larger in males than in females, and that these differences between male and fe male rats are correlated with body weight and dependent on the vasopre ssinergic neurons. As supraoptic neurons and their organelles enlarge when they are engaged in active peptidergic secretion we have carried out a morphometric ultrastructural analysis to determine if cell struc tures involved in the synthesis and storage of neurosecretory material also display weight-dependent sex dimorphism. Groups of six male and six female rats aged 30, 60 and 180 days were used. Nucleoli, rough en doplasmic reticulum and neurosecretory granules were analysed and we e stimated their volume or surface densities and the total volume of nuc leoli and rough endoplasmic reticulum, and total surface area of rough endoplasmic reticulum. We found that, with the exception of neurosecr etory granules, the densities of the organelles did not differ among t he groups studied, but total values were higher in males. These differ ences were found to be weight-dependent. Since the organelles studied are regarded as reliable indicators of the neurosecretory activity of supraoptic neurons, our data fully support the view that the weight-de pendent sexual dimorphism observed in this nucleus reflects greater sy nthetic activity of its vasopressinergic neurons associated with the n eed to maintain water balance in larger bodies.