Restriction to endogenous plasma proteins by a fenestrated capillary endothelium: an ultrastructural immunocytochemical study of the choriocapillary endothelium.

The choriocapillaris is a fenestrated capillary bed in the eye that restricts the egress of exogenous tracer molecules with Einstein-Stokes radii (ESR) greater than or equal to 3.2 nm. The present study examined the permeability of its endothelium to the endogenous plasma proteins albumin (ESR, 3.5 nm) and IgG (ESR, 5.5 nm) using ultrastructural immunocytochemistry. Reaction product indicative of the localization of albumin and IgG (using Fab-HRP conjugates) was high in the capillary lumen. In contrast, neither protein was localized extravascularly in Bruch's membrane, in endothelial vesicles, or in endothelial channels. The restriction was evident at the luminal side of the diaphragms spanning fenestrae, vesicles, and channels, and at the luminal front of cell junctions. In marked comparison, high levels of reaction product were localized in the extravascular space surrounding mucosal capillaries in the ileo-jejunum. Observations of tissue subjected to postembedment staining using a protein A-gold method were similar. These findings demonstrate for the first time the restriction of endogenous plasma proteins by a capillary endothelium identical in morphology to that of other vascular beds proven to be permeable.