In situ proliferation and differentiation of macrophages in the dental pulp

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It is well recognized that resident macrophages exist in the dental pulp. However, it is uncertain whether these macrophages proliferate and differentiate in the dental pulp in situ, or whether they constantly migrate from the blood stream into the dental pulp. In this study, we examined and compared the development of dental pulp macrophages in an organ culture system by using in vivo tooth organs to clarify the developmental mechanism of these macrophages. The first mandibular molar tooth organs from the ICR strain mice aged between 16-days of gestation(E16) to 5-days postnatal(5dPN) were used for in vivo experiments. The E16 first mandibular molar tooth organs were cultured for up to 14 days with or without 10% fetal bovine serum. Dental pulp tissues were analyzed with immunohistochemistry to detect the macrophages and with RT-PCR for the detection of factors related to the macrophage development. The growth curves for the in vivo-and in vitro-cultured cells revealed similar numbers of F4/80 positive macrophages in the dental pulp. RT-PCR analysis indicated the constant expression of M-CSF in both in vivo-and in vitro-cultured dental pulp tissues. Anti-M-CSF antibodies significantly inhibited the increase in the number of resident macrophages in the dental pulp. These results suggested that (1) most of the dental pulp macrophages proliferated and differentiated in the dental pulp without a supply of precursor cells from the blood stream, (2) M-CSF might be the candidate molecule for the development of dental pulp macrophages, and (3) serum factors might not directly affect the development of the macrophages.