Ultrastructural evidence of sexual dimorphism in supraoptic neurons: a morphometric study

We have recently shown that in spite of the absence of receptors for gonadal steroids in the supraoptic nucleus (SON) of the rat hypothalamus, 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 female rats are correlated with body weight and dependent on the vasopressinergic 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 structures 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 endoplasmic reticulum and neurosecretory granules were analysed and we estimated their volume or surface densities and the total volume of nucleoli and rough endoplasmic reticulum, and total surface area of rough endoplasmic reticulum. We found that, with the exception of neurosecretory granules, the densities of the organelles did not differ among the groups studied, but total values were higher in males. These differences 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-dependent sexual dimorphism observed in this nucleus reflects greater synthetic activity of its vasopressinergic neurons associated with the need to maintain water balance in larger bodies.