Distribution of anionic binding sites in extravascular space of skeletal muscle demonstrated with polycationized ferritin

Polycationized ferritin (PCF) and native ferritin (NF) were microinjected into rat muscle interstitium. The injection site was fixed in situ and processed for electron microscopy. PCF decorated the external aspect of the lamina densa of pericapillary and muscular basal lamina as quasi-regular aggregates, up to ∼200 nm wide, separated with spaces >20 nm wide. The internal aspect of the lamina densa possessed negative binding sites with a similar distribution, whereas PCF was never retained within the lamina densa. PCF bound very closely to the sarcolemma but did not approximate intimately the endothelial plasmalemma including vesicles. PCF also interacted with strict regularity with collagen fibrils (at an axial repeat of >60 nm) forming transverse bands of PCF across collagen bundles. No ordered interaction between NF and extracellular structures was observed. The ordered distribution of negative binding sites in interstitium, as revealed with PCF, should affect the blood-lymph movement of macromolecules.