Ossification of the longitudinal ligament in the spine is a disorder of unknown cause characterized by ectopic ossification. In the animal models of spinal hyperostosis, the vertebral ligament is gradually replaced by bony tissue forming an osseous bridge around enthesis, and a high turnover osteopenia occurs after the maturation in the trabecular bone of vertebrae. It has been suggested that the recruited vasculature facilitated the filling of the niche with host-derived haematopoietic cells during the ectopic ossification process. Recent data suggest that regulation of hematopoietic and osteogenic stem/progenitor cell populations may contribute to the formation of an ectopic spinal hyperostosis.