Development of the area postrema: an immunohistochemical study in the macaque.

The organization and chemical development of the area postrema (AP) in the macaque monkey was studied by immunohistochemistry imaged with conventional and confocal microscopy from day 40 of gestation to adulthood. The thin ependyma of the adult was found to develop from a thick continuous structure beginning in the second trimester. It was later invaded by tyrosine hydroxylase immunoreactive (TH+) and dopamine beta-hydroxylase immunoreactive (DBH+) cells and fibers, suggesting a possible route for release of neurotransmitter directly into ventricular cerebrospinal fluid. Other TH+ and/or DBH+ fibers were found in close approximation to blood vessels. Prominent vascularity of the parenchyma of AP was present late in the first trimester (fetal day (Fd)57 in the macaque) and increased further until birth. By contrast, the underlying solitary nucleus was hypervascular at Fd57, but its vascularity rapidly declined by late in the second trimester. TH+ neurons first appeared late in the first trimester, and DBH+ neurons appeared in the second trimester; these findings are consistent with the view that catecholaminergic cells in AP are the earliest members of the A2 noradrenergic group. Catecholaminergic cells or fibers in AP contained little labeling for synaptic vesicular proteins, suggesting that the release of neurotransmitter there may not involve a synaptic mechanism. Synapses were first observed in mid-second trimester, and most were associated with GABA+ fibers.