An immunocytochemical study of the routes of secretion of collagen and phosphophoryn from odontoblasts into dentin

Polyclonal antibodies to rat incisor phosphophoryns and to the amino-telopeptide of the al(I)-chain of type I collagen were used to follow the pathways of movement of collagen I (COLI) and phosphophoryns (PP) from synthesis in the odontoblast to secretion into the mineralized dentin. The antibodies were detected at the transmission electron microscopic level by their reaction with Protein A-colloidal gold conjugates. Special care was given in specimen preparation to retention of maximal antigenicity during fixation while maintaining cellular and extracellular ultrastructure at the mineralization front (MF) in nondemineralized sections. Intracellularly, COLI and PP were detected within the endoplasmic reticulum (ER), the Golgi (G) and secretory granules (SG). However, as determined by double-immunolabeling with different size gold particles the COLI and PP were not found together within the same ER, G or SG compartments. PP was localized within the tubular ER, round-shaped transitional vesicles, the Golgi and in narrow asymmetric SG. These asymmetric SG were found in abundance in the odontoblastic process. PP secretion from these vesicles was near the MF at the predentin-dentin boundary. COLI was localized within rosette form ER compartments, the Golgi and in large, distinctive SG. COLI was deposited at the cell-predentin boundary. No COLI SG were seen within the odontoblastic process near the MF. In the region of the MF, prior to mineralization, the PP was localized along the surfaces of the COLI fibrils of the predentin. The mineral phase etched surfaces revealed both COLI- and abundant mineral-associated PP. These data support the hypotheses that, in dentin, the interaction between COLI and PP may initiate crystal nucleation and that additional interactions between PP and the growing crystals may modulate the crystal growth pattern and crystal size.