Electron microscopic observations on healing in dental pulp in the rat

The process of repair in rat incisor dental pulp has been studied in 150 animals with light and electron microscopy over a period of 14 days following pulp exposure and capping with calcium hydroxide. Within the zone of injury created in the pulp by the operative procedure, inflammatory cells were present until the fifth post-operative day. The repair process had commenced by the second day with mitotic proliferation of pulp cells surrounding the wound, and over the next 2 days many of these cells migrated into the wound, displaying considerable morphological differentiation. Collagen fibrils appeared within the wound on the third day, and by the fourth day an extensive meshwork of irregularly arranged fibrils was present in the spaces between the cells. In sections of undecalcified tissue, small clusters of crystals were observed on the third day in intimate relationship to collagen fibrils at various sites throughout the wound. With progressive deposition of crystals, large expanses of calcified material of uniform appearance were formed. Decalcified sections revealed the demineralized substructure of these crystal aggregates. The earliest foci took the form of spherical structures less than 2 μ dia., having an amorphous electron-lucent peripheral zone around an electron-dense central region. By the fourth day large numbers of these foci were present throughout the wound. Growth and fusion of these small globules produced irregular masses which continued to expand by growth of the dark central region, the peripheral light layer remaining approximately the same width. Within the dark central region, collagen fibrils again became visible, and these resembled the collagen of mature dentine.