Ultrastructure of fenestrations in endothelial choriocapillaries of the rabbit - a freeze-fracturing study

Replicas of freeze-fractured endothelial cells of normal rabbits' choriocapillaries were studied. Differences in fenestral appearance on the E face and P face could be detected. E face pores appeared as circular craters containing particulate material arranged at the pores' rim and a central diaphragm. Mean pore diameter was 78.5 nm, average diaphragm size 30 nm, and peripheral particle size 12.3 nm. In several pores possible radiating connections between diaphragm and peripheral particles could be observed. Pores in the P face appeared as vallate papillae, with a circular, particulate, elevated rim surrounding a shallow surface and a central accumulation of particles resembling the diaphragm. The diameter of P face pores was found to be 76.5 nm, with a diaphragm size of 23.7 nm, and peripheral particle size of 12.7 nm. The calculated space between peripheral particles and diaphragm was 12.0 nm for the E face and 12.7 nm for the P face. With the Markham method a regular pattern of 8 peripheral particles at the pores' rim was observed on both E face and P face. A possible 3-dimensional model of the choriocapillary endothelial fenestration is presented. This model consists of 8 peripheral particles which are connected with a central diaphragm creating a space of 12.0-12.7 nm between them. This sieve-like structure and the calculated passage size fit well with the 'small pore' theory of molecular permeability.