Size restriction in the glomerular capillary wall: importance of lamina densa

Cationized monomeric, dimeric and oligomeric ferritins (pI maxima 9.3 to 9.6) were used to study size restriction in the rat glomerular capillary wall. Monomer and dimer ferritins were able to penetrate the lamina densa; trimer and larger aggregates could not, even though they were able to enter the lamina rara interna. This demonstrates that the lamina densa is a size restrictive barrier, cationic molecules with a Stokes-Einstein radius exceeding a value between 6 to 12 nm are excluded. Monomeric and dimeric cationic ferritins were handled differently in the lamina rara externa; monomer accumulated perferentially immediately below the slit membrane whereas dimer was found within the inner regions of the rara externa, nearer to the lamina densa.