Which cells are responsible for INTERNAL root resorption?

Odontoclasts (multinucleated giant cells derived from monocyte lineage, also called dentinoclasts or cementoclasts in the root context) are responsible for internal root resorption. They are activated within the pulp tissue and resorb dentin from the inside. Internal root resorption appears as a balloon-like radiolucency within the root on radiograph. It is often associated with chronic pulp inflammation. External root resorption is initiated by cementoclasts from the external surface.