Immunohistochemical study of hard tissue formation in the rat pulp cavity after tooth replantation.

While mineralized tissue is formed in the pulp cavity after tooth replantation or transplantation, little is known of this hard tissue formation. Therefore, we conducted histological and immunohistochemical evaluations of hard tissue formed in the pulp of rat maxillary molars after tooth replantation. At 5 days after replantation, degenerated odontoblasts were lining the pulp cavity. At 14 days, dentin- or bone-like tissue was present in the pulp cavity. Immunoreactivity for osteopontin (OPN) and bone sialoprotein (BSP) was strong in the bone-like tissue, but weak in the dentin-like tissue. Conversely, dentin sialoprotein (DSP) was localized in the dentin-like tissue, but not in the bone-like tissue. Cells positive for BMP4, Smad4, Runx2, and Osterix were found around the blood vessels of the root apex at 5 days. At 14 days, these cells were also localized around the bone-like tissue. Cells expressing alpha-smooth muscle actin (SMA) were seen around the newly formed bone-like tissue, whereas no such cells were found around the newly formed dentin-like tissue. In an experiment involving the transplantation of a green fluorescent protein (GFP)-transgenic rat tooth into a wild-type rat tooth socket, GFP-positive cells were detected on the surface of the bone-like tissue and over all dentin-like tissue. These results indicate that the original pulp cells had the ability to differentiate into osteoblast-like cells as well as into odontoblast-like cells.