Scanning and transmission electron microscopy of tubular structures presumed to be human odontoblast processes

Tubular structures interpreted as being odontoblast processes can be observed with the scanning electron microscope (SEM) on fractured dentin surfaces which have been demineralized and treated with collagenase. To confirm the nature of these structures, SEM preparations exhibiting similar tubular structures were subsequently examined with the transmission electron microscope (TEM). Newly-erupted human third molars were fractured buccolingually with heavy-gauge industrial nippers or sectioned mesiodistally with a Leitz saw microtome and fixed in glutaraldehyde. The exposed dentin surfaces were decalcified to a depth of approximately 500 microns and then treated with bacterial collagenase. Half of the specimens were critical-point-dried and coated for SEM. The other half were post-fixed and processed for TEM. After examination by SEM, the specimens were embedded and thin-sectioned for TEM. SEM observations of both the fractured and cut surfaces of dentin showed tubular structures running from the surface of the pulp to the dentino-enamel junction. When the SEM preparations were examined with TEM, the tubular structures were seen to be the inner sheath of the peritubular matrix, not odontoblast processes. In the specimens directly processed for TEM, the structures lying inside the sheath could be visualized clearly. In the outer two-thirds of the dentin, the tubules were essentially empty. Well-defined odontoblast processes were seen lying inside the sheath only in the inner dentin.