Perichondrium origin of pericyte‐septoclast lineage in endochondral bone development

During endochondral bone development, pericytes differentiate into septoclasts, a mononuclear cartilage-resorbing cells, and contribute to the forming primary ossification center. To clarify the origin of the pericyte-septoclast lineage, the present study investigated the chronological localization of pericytes, septoclasts, and perichondrial cells during the early developmental stage of mouse tibiae using specific histochemical markers: platelet-derived growth factor receptor beta (PDGFRβ) for pericytes, epidermal-type fatty acid-binding protein (E-FABP, FABP5) for septoclasts, distal-less homeobox 5 (DLX5) for perichondrial cells, and von Kossa method and periostin for the periosteum. Before blood vessel invasion and septoclast appearance, PDGFRβ and DLX5 were commonly expressed in pericytes and in the cells of the outer layer of the perichondrium (OPC). Moreover, DLX5 positive OPC cells projected inward and were continuous with pericytes in the mid-portion of the cartilaginous templates. After the onset of blood vessel invasion and septoclast appearance, DLX5 was localized in both pericytes and septoclasts, suggesting that pericytes originated from OPC cells and differentiated into DLX5-expressing septoclasts. Based on the localization of von Kossa-positive calcified substrates and periostin immunoreactivity, OPC cells were transformed into the periosteum. Immunoreactivity of PDGFRβ, which mediates pericyte migration or recruitment, was relatively weak in the perichondrium compared to that observed before blood vessel invasion. In conclusion, the present results suggest that the pericyte-septoclast lineage originates from perichondrial cells during the initial developmental stage of endochondral bone formation.