Periodontal ligament plays important roles in the occlusal function therefore, its maintenance/regeneration is essential for achieving healthy longevity of life. It has been speculated that the growth factors and extracellular matrix proteins secreted by periodontal ligament-constituting cells regulate the maintenance/differentiation of periodontal ligament in a site-specific manner. However, the site-specificity of the periodontal ligament-constituting cells are still not well characterized. The present study revealed that not only the cells existing from the developmental stage but also the hematogenously supplied bone marrow-derived cells are present in the periodontal ligament. These cells contribute to the maintenance of periodontal ligament homeostasis.