The supraoptic nucleus in the pig hypothalamus: postnatal development and the effect of gonadal steroids.

Postnatal development of the supraoptic nucleus (SON) in the pig hypothalamus was studied morphometrically. The volume of the SON increased from 6.2 +/- 0.45 (S.E.M.) mm3 at 7 weeks postnatally to 18.5 +/- 1.35 mm3 at 2.5 years of age. A sex difference was found at the development point when the SON volume increased, with earlier SON enlargement in males. This sex difference was 30% at 30 weeks and 50% at 1 year of age. At 2.5 years of age no difference in volume was apparent between the sexes. The number of SON neurones was similar for all age groups concerned (43,500 +/- 1475), except for the 2.5-year-old females where 40% more were found (55,500 +/- 3285). No significant difference was found in neurone number between gonadectomized and sham-operated animals, but the operation caused a 30% reduction in the number of neurones and SON volume. Testosterone supplementation following gonadectomy, during the first 4 weeks postnatally, resulted in sexual dimorphism, the males having more SON neurones than the females. The volume showed only a trend in the same direction. Testosterone supplementation at other ages did not result in any difference when compared with controls. The results of this study show that the postnatal development of the SON of the pig is sexually dimorphic, and that it continues after puberty in females. In contrast to the vasopressin- and oxytocin-containing nucleus, the development of the SON was not influenced by gonadectomy and only slightly by gonadal steroids.