ENDOCYTOTIC FUNCTIONS OF AMELOBLASTS AND ODONTOBLASTS - IMMUNOCYTOCHEMICAL AND TRACER STUDIES ON THE UPTAKE OF PLASMA-PROTEINS

Background: Biochemical, (immuno)cytochemical, and radioautographic da ta accumulated over several years have lead to the view that ameloblas ts carry out both secretory and degradative functions throughout amelo genesis, Whereas it has been assumed that maturation stage ameloblasts endocytose aged enamel proteins from the enamel layer, the origin of the newly formed ones detected in the endosomal/lysosomal compartment of ameloblasts from all stages remains to be elucidated. One possible source is from secretory products released ectopically along basolater al surfaces. Methods: To test this hypothesis, we have investigated, u sing colloidal gold immunocytochemistry, whether plasma proteins (albu min and alpha(2)HS-glycoprotein) found in the interstitial fluid are e ndocytosed by rat incisor ameloblasts and other cells from hard and so ft tissues, Rat albumin, tagged with dinitrophenol, was injected intra venously to trace the movement of this protein, Results: Plasma protei ns were immunodetected along the base-lateral surfaces and in multives icular bodies of ameloblasts where enamel proteins were also found, By 2 hours following intravenous administration of dinitrophenylated alb umin, the tracer had left the blood and diffused into the enamel organ and between odontoblasts and osteoblasts, The tracer was also found i n multivesicular bodies of all cells examined, Conclusions: The uptake of albumin by many different cell types suggests that this process is not restricted to ameloblasts and likely occurs in a nonselective man ner. Hence, base-lateral uptake in ameloblasts may play a role not onl y in the continuous removal of plasma proteins leaking from the blood, but also of enamel proteins 'dumped' laterally between these cells, L ikewise, odontoblasts may use the same mechanism to internalize some o f the plasma proteins and any enamel protein that diffuse toward them, (C) 1996 Wiley-Liss, Inc.