Expression of amino acid decarboxylase in proliferating cells of the neural tube and notochord of developing rat embryo

In vertebrate embryos, the catecholamine (CA) neurons of brain and periphery arise from undifferentiated precursors in the ventral neural tube and neural crest to migrate to their final location where they first express tyrosine hydroxylase (TH). We sought to determine by immunocytochemistry and autoradiography whether the next enzyme of the CA pathway, aromatic L-amino acid decarboxylase (AADC) is expressed by CA precursors before or, as TH, after reaching their final site and is present in proliferating cells. AADC was first observed on day 12 of development (E 12) in all cells of the notochord along its entire length and in neuroepithelial cells of the ventral neural tube from the mesencephalic to the lumbar region. At E 14 the cells of the sacral basal plate of the neural tube also contained AADC. At this stage, some cells of the notochord and neural tube containing AADC were also labeled with tritiated thymidine. At E 16 the cells of the notochord lacked AADC immunoreactivity, and at E 21 the enzyme was no longer detected in the ventricular layer of the developing brain. At this stage and in the adult stage, however, a few cells containing AADC were found scattered in the spinal cord. AADC was not detected in migrating neural crest cells. The studies indicate that in rat embryo: AADC is expressed by cells of the ventral neural tube and notochord from E 12 until late gestation; the onset of expression of AADC occurs prior to the last cell division; and at least in brain, the precursors of CA neurons express AADC while still in their site of origin and TH after arriving at their final location.