Distribution and morphological variation of septoclasts during mouse cranial base development

Septoclasts are mononuclear cartilage-resorbing cells derived from pericytes and are involved in the formation of the primary ossification center and resorption of the growth plate of long bones. In this study, we examined the chronological immunolocalization of the cell markers, epidermal fatty acid-binding protein (E-FABP, FABP5) and cathepsin B of septoclasts, platelet-derived growth factor receptor beta (PDGFRβ) of pericytes, and endomucin of vascular endothelial cells in the midline of a mouse cranial base during the embryonic and neonatal periods. We found that septoclasts expressing FABP5, cathepsin B, and PDGFRβ were adjacent to endomucin-positive capillary endothelial cells. At the neonatal period, septoclasts were localized on the surface of the spheno-occipital synchondrosis (SOS) and the intersphenoidal synchondrosis (ISS) of neonatal mice. Similar to the growth plates of long bones, septoclasts were distributed along the longitudinal septa of the cartilage. However, their length was shorter than that of the growth plates. The mature septoclasts predominantly exhibited morphological features reflecting high cartilage resorptive activity, such as spindle-shaped cell bodies with elongated cytoplasmic processes. During development, septoclasts were distributed adjacent to invasive capillaries within the ossification centers of the cartilaginous primordia and converged on the resorption surfaces of the synchondroses when the ossification centers were completely developed. The immature septoclasts predominantly exhibited morphological features reflecting low cartilage resorptive activity, specifically spindle-shaped cell bodies with short processes. Our findings suggest that septoclasts play a role in the distinct morphogenesis of the cranial base by participating in the cartilage resorption of synchondroses and the formation of ossification centers.