Spatial and chronological localization of septoclasts in the mouse Meckel's cartilage

Meckel's cartilage (MC) in the first branchial arch of mammals is a transient structure that disappears before birth, except for the most anterior and posterior portions. Recent studies reported that some congenital abnormalities in craniofacial regions are linked with the persistence or dysplasia of MC. However, the mechanisms underlying the resorption of MC have not been elucidated. Cartilage resorption in endochondral ossification is performed by multinuclear osteoclasts/chondroclasts as well as mononuclear septoclasts, which were newly added to the list of cartilage phagocytes. Septoclasts located exclusively at the chondro-osseous junction of the growth plate resorb the uncalcified cartilage matrix. We hypothesized that septoclasts participate in the resorption of MC and attempted to clarify the localization and roles of septoclasts in MC of mouse using a specific immunohistochemistry marker, epidermal type-fatty acid-binding protein (E-FABP/FABP5). E-FABP-immunopositive septoclasts were detected for the first time at the beginning of MC resorption and localized along the resorption surface. Septoclasts of MC in embryonic mice possessed several processes that elongated toward the uncalcified cartilage matrix, expressed cathepsin B, and exhibited characteristic pericapillary localization. Additionally, they localized between hypertrophied cartilage and osteoclasts/chondroclasts in the resorption surface. Confocal laser-scanning microscopy revealed a decrease in the numbers of septoclasts and their processes with the progression of MC disappearance before birth. The present study showed that E-FABP-immunopositive septoclasts participated in the disappearance of MC through the resorption of the uncalcified cartilage matrix and that they have different roles from osteoclasts/chondroclasts.