Vitro Studies of Scleral Permeability

The ease with which a molecule diffuses across a tissue can be characterized by its permeability, measured in cm/sec. This value can be conceived of as the velocity of the molecule across the tissue. 

Several investigators have used a two-chamber diffusion cell apparatus to characterize in vitro scleral permeability (3-5,22-25) to radioactively—or fluores-cently—labeled compounds. 

The common element of most such studies is the dissection and isolation of scleral tissue, followed by placement of the sclera between two chambers representing the episcleral surface and the uveal surface. One chamber contains the labeled compound, and the other chamber is sampled periodically after steady-state flux is attained. Some studies have used an apparatus where the chambers are constantly stirred. This may yield a higher apparent permeability by utilizing an unmixed depot on the tissue surface where static boundary layers may exist. However the impact of boundary layers on high molecular weight tracers is not expected to be significant, especially when temperature fluctuations are minimal (26,27). 

Bovine sclera is permeable to molecules as large as albumin (69kDa) (3). Human sclera is permeable to 70 kDa dextran (4), while rabbit sclera is permeable 

We measured the permeability of rabbit sclera to a series of fluorescently labeled hydrophilic compounds with a wide range of molecular weights and radii. Sodium fluorescein, the smallest compound, had the highest permeability coefficient, whereas 150 kDa dextran, which had the largest molecular radius, had the lowest permeability coefficient. Our rabbit permeability data was quite similar to the reported permeability of human sclera. Bovine scleral permeability was less than that of the rabbit or human (Table 1). 

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