Lack of blood-brain barrier properties in microvessels of the prelaminar optic nerve head

PURPOSE. TO define the blood-brain barrier (BBB) characteristics of microve ssels in the optic nerve head (ONH). METHODS. Immunohistochemical staining of different regions of the ONH, retro-laminar optic nerve, and retina of h uman and monkey eyes was carried out, using antibodies against BBB markers (glucose transporter 1. transferrin receptor, and P-glycoprotein), the non- BBB marker PAL-E, and against plasma proteins fibrinogen and IgG, which ser ve as endogenous markers of nonspecific microvascular permeability. In the ONH of monkey eyes, the number of transport-related endothelial pinocytotic vesicles and their cellular distribution within the microvessels were dete rmined by electron microscopy. RESULTS. In both human and monkey eyes. only microvessels in the prelaminar region of the ONH were positive for the PAL-E antigen. The prelaminar regi on microvessels showed either no or weak expression of the transferrin rece ptor and P-glycoprotein but stained positive for glucose transporter 1. In human ONH, fibrinogen and IgG were present around microvessels in the prela minar region but not in other parts of the optic nerve or retina. By electr on microscopy, endothelial cells of prelaminar region microvessels containe d a higher number of pinocytotic vesicles, located at the luminal and ablum inal side of the endothelial cell membrane, in contrast to a mainly ablumin al localization in microvessels of the retina and other parts of the optic nerve. CONCLUSIONS. Microvessels in the prelaminar region of the ONH lack classica l BBB characteristics and display nonspecific permeability, possibly mediat ed by vesicular transport.