Hydroxyapatite applied as direct pulp capping medicine substitutes for osteodentin.

Tissue changes in rat incisors after direct pulp capping with hydroxyapatite (OH-AP) have been investigated using light and electron microscopy. Immediately after direct pulp capping, necrotic cells and cell debris were scattered only in the superficial zone of an exposed site. At day 1, both neutrophils having many vacuoles and macrophages phagocytizing numerous OH-AP granules appeared in the lower part of the OH-AP layer. At day 3, the cells surrounding the basal part of the OH-AP layer had large nuclei and well-developed rough-surfaced endoplasmic reticulum. At day 5, the newly formed mineralized tissue consisting of needle-like crystals was observed directly adjacent to the deepest zone of the OH-AP layer. However, fibroblast-like cells phagocytosing applied OH-AP granules were seen even at this period. These findings suggest that the applied OH-AP could directly substitute for osteodentin.