FREEZE-FRACTURE STUDIES OF THE DISTAL PLASMA-MEMBRANE OF RAT ODONTOBLASTS DURING THEIR DIFFERENTIATION AND POLARIZATION

We have examined freeze-fracture replicas of maxillary first molar too th germs of newborn rat at early stages of dentinogenesis to study the development of tight junctions in the distal plasma membrane of diffe rentiating odontoblasts. In addition, freeze-fracture was combined wit h filipin to observe the distribution of cholesterol on the distal pla sma membrane of odontoblasts during differentiation. Only gap junction s were present in early differentiating odontoblasts. The distal plasm a membrane exhibited low cholesterol content, which might indicate hig h fluidity. With the beginning of mineral deposition in matrix-vesicle s, the first sign of tight junction formation were observed. Further d evelopment revealed increasingly complex focal tight junctions. In lat er stages, when mineralisation is observed progressing to the fibrilla r and non-fibrillar constituents of the matrix, well developed focal t ight junctions were detected. Concomitantly, cholesterol is distal por tions of the odontoblast plasma membrane increased, indicating, probab ly, a higher rigidity. Thus, a distal plasma membrane domain is establ ished, odontoblasts become fully differentiated, and partial compartme ntalisation of matrix occurs. At this stage, odontoblasts may be able to secrete specific matrix molecules to ensure the progression of mine ralisation.