A survey of the binding of polycationic ferritin in several fenestrated capillary beds: indication of heterogeneity in the luminal glycocalyx of fenestral diaphragms.

Polycationic ferritin (PCF) was injected into umbilical veins of 17- and 21-day-gestation rats and saphenous veins of young, male, adult Sprague-Dawley rats and allowed to circulate for 1 min or less followed by excision of tissues and immersion fixation. Fetal liver, intestine, and kidney, as well as adult liver, intestine, pancreas, kidney, adrenal cortex, bone marrow, and pituitary were examined. Nearly all fenestral diaphragms, but no subendothelial structures, were labeled with a tuft of particles in fetal intestine, adult intestine, pancreas, pituitary, and renal peritubular capillaries. A fraction of the diaphragms covering fenestrae in the thyroid, adrenal cortex, and fetal kidney glomerulus, but none in the bone marrow and fetal liver, had associated PCF. In these tissues some diaphragms appeared to be permeable to PCF, because tufts were observed immediately beneath unlabeled fenestral diaphragms either in the basal lamina (fetal kidney, adult thyroid, and adrenal cortex) or on other subendothelial structures (fetal liver, adult bone marrow). Also periodic concentrations of PCF were observed in the lamina rara interna of the adult glomerular basement membrane. PCF binding to the fenestral diaphragms and the basal lamina in the adult intestine and the renal glomerulus, respectively, have been reported by Simionescu et al. ((1981a) J. Cell. Biol. 90, 605-613) and Kanwar and Farquhar ((1979) J. Cell. Biol. 81, 137-153) as indicating accumulations of anionic material. In this study we have demonstrated not only high-specificity binding of PCF to fenestral diaphragms in capillaries of organs other than intestine and pancreas, but also considerable variation of PCF stainability of fenestral diaphragms in other organs.