Appearance of tyrosine hydroxylase immunoreactivity in the human embryo

Tyrosine hydroxylase was detected immunohistochemically in 3 human fetuses aged 5 1/2-6, 9-10 and 13-14 weeks. In the youngest fetus only the developing sympathetic chains were stained. By 9-10 weeks sympathetic terminals, adrenal medulla, carotid bodies, locus ceruleus, substantia nigra and basal ganglia showed positive reactions. In the oldest fetus a dense sympathetic innervation was present in the ductus venosus. Catecholaminergic neuronal systems, including their terminal axons, are well developed by 2 1/2 months of human gestation. PAP immunohistochemistry has the advantage of being applicable to archival formalin-fixed, paraffin-embedded tissue.