Development of sexual dimorphism in synaptic organization in the ventromedial nucleus of the hypothalamus in rats.

The ventromedial nuclei of the hypothalamus (VMN) were examined ultrastructurally in both male and female rats at 5, 20, 45 and 100 days of age. Synaptic number in the VMN was increased to 70-80% of that at 45 days of age during the first 20 days. At 5 days of age, sexual difference in the numerical density of dendritic shaft and spine synapses was not detected. Synaptic sexual difference developed by 20 days of age in the ventrolateral VMN where receptors for sex steroid were abundant. This tendency persisted until adulthood. However, no sexual difference in synaptic pattern was recognized in the dorsomedial VMN. From these results, it is suggested that sexual dimorphism in synaptic organization in the VMN develops from the early prepubertal period.