Mesenchymal stem cells (MSC) and multipotent adult progenitor cells (MAPC) copurify from the bone marrow. Past studies have shown that bone marrow-derived cells transplanted into the periodontium obtain characteristics of several periodontal cells types, such as alveolar bone, periodontal fibroblasts, and cementoblasts. Bone marrow-derived cell populations are heterogeneous mixtures of different cell types. Characterization of each individual cell type in the bone marrow has not been completed. In this study, the MAPC were isolated from the bone marrow of 3-week-old rats by selecting attached cells that did not express leukocyte (CD45), erythroid (HIS49), or the MSC marker CD44. DiI-labeled male MAPC were injected into the periodontal ligament (PDL) of female adult rats. After 3 weeks, the morphology of the DiI-labeled cells was determined. Periodontal tissues sections that included alveolar bone, PDL, cementum, and dentin were also immunostained for collagen I and III after 3 days, 3 weeks, and 6 weeks following injection of male MAPC into the female PDL. Male and female cells were differentiated by fluorescent in situ hybridization tagging of the X and Y chromosomes. Our results show that DiI-labeled MAPC were spherical in shape before injection but after 3 weeks a portion of the injected MAPC obtained the characteristic spindle shape of PDL cells with an orientation parallel to the surrounding PDL cells. Consistent with the DiI results, male MAPC had a spherical shape on immunostained sections 3 days after injection and expressed collagen I and III at significantly reduced levels as compared with the endogenous female PDL. Six weeks after injection, the MAPC expressed both collagen I and collagen III at levels similar to the endogenous surrounding PDL and obtain a morphology characteristic of PDL. In conclusion, MAPC obtain PDL-like characteristics over a 6-week period after injection into the PDL.