Brain barrier systems in the lamprey. I. Ultrastructure and permeability of cerebral blood vessels.

It is unclear whether the lamprey (class: cyclostomes) has a blood-brain barrier as in other vertebrates. Therefore, the present study re-examined the lamprey blood-brain barrier. Brain capillaries in the lamprey (Lampetra fluviatilis), and their permeability to horseradish peroxidase (HRP; molecular weight 40,000) and microperoxidase (MP; molecular weight 2000) were studied in the electron microscope. All brain vessels were capillaries. Each capillary formed a single hairpin-like loop. Their endothelia contained vesicles and tubules which often opened to the surface, preferentially the abluminal. Tubules creating transendothelial channels were not observed. Adjacent endothelial cells interdigitated extensively. The intercellular cleft was obliterated by 2-7 punctate appositions or fusions of the membranes. Intravenously injected HRP and MP, which were allowed to circulate for up to 30 and 20 min, respectively, did not permeate the brain endothelium. Few endothelial vesicles or vacuoles were labelled by the tracers. In the intercellular clefts, reaction product was only observed in their luminal part. HRP injected into the brain ventricles permeated the ependyma and diffused into the brain intersitium. The tracer permeated the pericapillary sheath of glial cells and the endothelial basal lamina of the capillaries within this diffusion profile. Labelling of the cleft between adjacent endothelial cells was confined to their abluminal part. It is concluded that the lamprey has an endothelial blood-brain barrier to macromolecules.