Osteoclasts are highly polarized cells that form ring-like structure of actin (called “actin ring”) inside cells and ruffled borders (RB) toward the bone surface during bone resorption. However, the precise functions of these structures are unknown. Using CLEM (Nikon: A1, NIS-Elements, HITACHI: FlexSEM 1000) and FIB-SEM (JEOL: JIB-4700F), we analyzed three-dimensional structure of actin ring and ruffled borders of bone-resorbing osteoclasts cultured on dentin slices. Analysis by CLEM revealed that most of actin-ring localized at cell portion of the direction of movement (actin-ring area), which is attached on the undecalcified area and edge of decalcified area (lacunae) (Fig. 1). On the other hand, analysis by FIB-SEM (pitch: 150 nm or 10 nm) indicated that non-actin ring area consists of much more amounts of ruffled boarders and cytosol that contains nuclei, mitochondria and vacuoles compared to the actin ring area. These results suggest a novel concept of the roles of actin ring such as sensing of unmineralized area of bone and towing the whole cell body toward there.
Detection of actin ring in osteoclasts by CLEM.
