Heterotopic bone formation (HBF), an ill-defined phenomenon, refers to the formation of bone in tissue that normally does not ossify. Two existing theories to explain HBF are that two cellular entities, one from the bone and the other from the muscle or fascia (two cell types) are involved, and that stem cell responds to a factor induced by trauma (one cell + factor). In our report, the HBF in a patient's vertical abdominal wound was possibly due to IEL, a stem cell, which is stimulated by a factor from xiphoid when it is traumatized by surgical incision. After 28 days' culture rat tissue specimens from the xiphoid, upper gastrointestinal tract, pubis and bladder exhibited macroscopic mineralization with cellular infiltration, a paradigm of 2-dimensional BF. Characteristically, pubis + bladder, xiphoid + ileum and xiphoid + duodenum showed 2-dimensional BF by as early as 5 days. Thus, it appears that both theories of HBF may be valid.