Early postnatal development of the arcuate nucleus in normal and sexually reversed male and female rats.

The ultrastructural characteristics of the arcuate nucleus in the rat were examined at days 2, 5, 10 and 15 postpartum in order to evaluate the cytological development of the nucleus during sexual differentiation of the hypothalamus. In an effort to accentuate potential steroid-dependent structural features, the nucleus in normal male and female rats was compared with that of rats in which sexual differentiation had been reversed, by subcutaneous injection of testosterone proprionate in neonatal female rats, and by bilateral castration of neonatal male rats. The arcuate nucleus did not reveal any evidence of sexual dimorphism nor any effects of neonatal castration or androgenisation. The nuclei of any animals exhibited an increase, with age, in morphologically mature neurons and synapses. At day 15, however, the nuclei still possessed features indicative of an immature neural system, including organelle-poor profiles of young neurons, growth cones, focal points of cellular degeneration, and a lack of myelinated axons and morphologically mature macroglial cells. Of particular interest was the observation that neonatally-castrated male rats failed to exhibit a quantitative increase in whorl bodies by day 15 post partum, in contrast to the characteristic increase in whorl bodies observed in male rats that were castrated in adulthood. The results indicate that the arcuate nucleus undergoes progressive structural maturation during sexual differentiation of the hypothalamus and that this development is not altered ultrastructurally by changes in the steroid environment. At day 15 postpartum, an age at which sexual differentiation is irreversibly fixed, the nucleus remains a partially developed neural system. Further structural maturation, both neuronal and glial, will undoubtedly contribute to changes in hypothalamic regulation of the pituitary-gonad axis which occur with age. The lack of whorl body proliferation in neonatally-castrated males indicates that the arcuate nucleus of developing animals does not respond morphologically to negative steroid feedback mechanisms in a manner comparable to that of adults. The proliferation of whorl bodies in response to steroid withdrawal is thus an age-dependent phenomenon.