Quantitation of blood-brain barrier ultrastructure

This paper describes a quantitative approach to evaluating the ultrastructural features of brain capillaries that relate to the low non-specific permeability of the blood-brain barrier (BBB). Critical features in this approach include examination of large numbers of tissue samples and consistent, objective means of measuring features of interest. Junctional clefts, i.e., continuous channels between tight junctional regions correlate well with the know vascular permeability, being low in normal adult blood-brain barrier, high in fetal brain, and high in tumours, both human and rat. Endothelial vesicles do not always correlate with vascular permeability. They have a low density in normal adult BBB, but are also low in fetal BBB and low in some intracranial tumour vessels. However, they have a high density in muscle capillaries, and others have shown that they increase in BBB vessels damaged by hypertension. Fenestrations are consistently high in leaky vessels, but not all leaky vessels have fenestrations. The density of mitochondria in endothelial cells is high in BBB vessels of some species but not in others. Glut-1, the glucose transporter of the BBB is asymmetrically distributed between the luminal and abluminal membranes of BBB capillaries, being almost four times as numerous on the abluminal face. A large intracellular pool of glucose transporter may provide a means for rapid upregulation of the surface transporters.