Ontogenesis of tyrosine hydroxylase-immunopositive structures in the rat hypothalamus. An atlas of neuronal cell bodies.

The development of the catecholaminergic system in the hypothalamus and in the septal region was studied in rats from the 12th fetal day until the 9th postnatal day. Catecholaminergic structures were visualized with pre-embedding immunocytochemistry using antiserum to tyrosine hydroxylase. An intensification of diaminobenzidine product with silver and gold was additionally applied to make the immunocytochemical technique more sensitive. In this paper only the data on the appearance and distribution of the tyrosine hydroxylase-immunopositive neurons (cell bodies) are presented, whereas the catecholaminergic innervation of the hypothalamus with the tyrosine hydroxylase-immunopositive fibers is the topic of an accompanying paper. Sparse tyrosine hydroxylase-immunopositive neurons were first observed in the anlage of the hypothalamus and septal region on the 13th fetal day. Their number increased progressively with age and by the 15th fetal day they already gave rise to a large dorsal accumulation. From the 18th fetal day on, tyrosine hydroxylase immunopositive neurons began to occupy their definitive positions, mainly concentrating within the hypothalamus: in the zona incerta, periventricular and arcuate nuclei. To a lesser extent, they were concentrated in the medial preoptic area, suprachiasmatic, supraoptic, paraventricular, dorsomedial, and anterior hypothalamic nuclei. The data on the distribution of the tyrosine hydroxylase-immunopositive neurons both in the hypothalamus and in the septal region during ontogenesis are summarized in the precise atlas. Primarily small bi- and unipolar catecholaminergic neurons first observed in the youngest fetuses undergo cytodifferentiation during ontogenesis, giving rise to at least two different populations localized ventrally, mainly in the arcuate nucleus, and dorsally, in the zona incerta. The neurons of the former population remain similar to those of the youngest fetuses, whereas the neurons of the latter increase significantly in size, forming several long, highly ramified processes.