title: Human retinal molecular weight exclusion limit and estimate of species variation.
abstract: OBJECTIVE
  To determine the maximum size of molecule capable of freely diffusing across human retina, referred to as the retinal exclusion limit (REL), and the location of any sites of high resistance to diffusion. To assess the degree of interspecies variation in the REL of three animals commonly used to model human disease.
  
  
  METHODS
  Trephines of human neuroretina were mounted in a modified Ussing chamber. FITC-dextrans of various molecular weights (MWT) were dissolved in phosphate-buffered saline, and the rate of transretinal diffusion was determined over 24 hours with a spectrophotometer. The theoretical REL was calculated by extrapolating the linear relationship between the rate of diffusion and log(MWT). In separate experiments to determine the sites of barrier to diffusion, FITC-dextrans with a MWT greater than the calculated REL were applied to either the inner or outer retinal surface, processed as frozen sections, and viewed with a fluorescence microscope. Experiments to determine the REL were repeated in bovine, porcine, and rabbit retina.
  
  
  RESULTS
  The REL in human tissue was 76.5+/-1.5 kDa (6.11+/-0.04 nm). The inner and outer plexiform layers formed the sites of highest resistance to diffusion. The REL in pigs, cattle, and rabbits were 60 +/- 11.5, 78.5 +/- 20.5, and 86 +/- 30 kDa, respectively (5.68 +/- 0.45, 6.18 +/- 0.61, and 6.38 +/- 0.88 nm).
  
  
  CONCLUSIONS
  In humans, the inner and outer plexiform layers are sites of high resistance to the diffusion of large molecules, resulting in an REL of 76.5 kDa. There was only moderate interspecies variation in the REL of the animals studied, suggesting that they provide adequate models for the study of human transretinal macromolecular diffusion.
authors.affiliation: Department of Ophthalmology, The Rayne Institute, St Thomas' Hospital, London, United Kingdom. june.krafft@kcl.ac.uk
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Jackson
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Antcliff
authors.forename: Jost
authors.initials: J
authors.lastname: Hillenkamp
authors.forename: John
authors.initials: J
authors.lastname: Marshall
mesh_terms: D000368:Aged; D000818:Animals; D001692:Biological Transport; D002417:Cattle; D003911:Dextrans / Q000378:metabolism*; D004058:Diffusion; D015194:Diffusion Chambers, Culture; D016650:Fluorescein-5-isothiocyanate / Q000031:analogs & derivatives* / Q000378:metabolism*; D006801:Humans; D008856:Microscopy, Fluorescence; D008970:Molecular Weight; D010539:Permeability; D011817:Rabbits; D012160:Retina / Q000378:metabolism*; D013045:Species Specificity; D013552:Swine
chemical_list: D003911:Dextrans; C015219:fluorescein isothiocyanate dextran; D016650:Fluorescein-5-isothiocyanate