The Organum vasculosum laminae terminalis

Summary The Organum vasculosum laminae terminalis (OVLT) of the duck is lined innerly by specialized ependymal cells (tanycytes) and outwardly by a well-developed superficial vascular network, the capillaries of which often show a fenestrated endothelium. The OVLT also includes glial cells, internal non-fenestrated capillaries, bundles of fine nerve fibers and three groups of axonal swellings. One type contains granulations of 1000–1400 Å in diameter as well as 300–500 Å clear vesicles. The second type exhibits granulations and dense core vesicles of 500–800 Å in diameter along with small electron-lucent vesicles having diameters of 300–400 Å. In the third type, exclusively clear vesicles 300–600 Å in diameter are found. Asymmetrical synapses on dendrites and neuronal perikarya are found at every level of the organ. In the most external zone, the interposition of tanycyte endings sometimes allows neurosecretory axons to reach the parenchymal basement membrane (basal lamina).When tritiated molecules (amino acids or monoamines) are administered either in vitro by incubation or in vivo by intraventricular injections, radioautographic grains are observed over the tanycyte perikarya. Although this labeling is observed at every time point following the administration of the tracers, within three minutes only ³H-GABA appears to be concentrated in the cytoplasmic processes of the tanycytes. ³H-noradrenaline and ³H-serotonin are taken up and retained by some axons of the second type described above. Noradrenergic fibers are primarily localized in the inner zone of the OVLT where they display axodendritic synaptic contacts. Serotonergic fibers appear sparsely distributed in the OVLT but are more numerous in the lateral edges of the organ where synaptic differentiations on dendrites or on dendritic spines are also observed.It is concluded that the duck OVLT probably displays a neuroendocrine activity. Uptake and selective transport of exogenous molecules by tanycytes are also suggested by the present radioautographic observations. Finally, monoaminergic innervation is discussed at the OVLT level with special reference to the occurrence of serotonergic synapses.Supported by the Département de Biologie du C.E.A., and the I.N.S.E.R.M. (C.R.A.T., 74.1.438.45)