Histopathological Features of Ossification of the Posterior Longitudinal Ligament

Ossification of the posterior longitudinal ligament (OPLL) occurs mainly through endochondral ossification. Although researchers have been attempting to elucidate the pathogenesis of OPLL for many years, it remains unclear which cell types in the ligament have chondrogenic capacity. Furthermore, the contribution of direct bone formation without chondrogenesis remains elusive. To characterize the osteochondrogenic potential of the cells around the ossification area of OPLL, we immunohistochemically evaluated a series of proteins specific to endochondral ossification. We found SOX9/type II collagen-positive chondrocytes in the fibrocartilage area, hypertrophic chondrocytes with type X collagen expression, and osteoblastic cells expressing RUNX2, osterix, and bone sialoprotein in the stromal fibrous area (SFA) between the ossified areas, thereby confirming the concept of an endochondral ossification-like process. In addition, the osteoblastic cells and blood vessel cells in the degenerated ligament, as well as the cells in the SFA outside of the ossified area, expressed both osteoblastic markers and SOX9, suggesting that these cells were bipotential osteochondral progenitor cells. Additionally, we revealed that the blood vessel cells and stromal fibrous cells exhibited bipotential osteochondrogenic features. Our observation suggests that any or all of these osteochondral mesenchymal cells may be initiators of OPLL.