Visualizing Undercoat Structures Associated with the Adherent Membrane of Cultured Osteoclasts by Unroofing Freeze-Etching Techniques

Abstract

Osteoclasts are specialized in cells that play a central role in bone remodeling. Polarized osteoclasts are equipped with specialized structures called podosomes, dynamic actin rich adhesions, which distribute over the entire ventral membrane. To explore their cytoplasmic and membrane cytoskeletal organizations, we used a three-dimensional (3D) ultrastructural approach to visualize employed a method of “unroofing”. Podosomal cytoskeletons are incorporated into a dense network of actin cytoskeletal organization in the lamellipodium. On the ventral adherent membranes, an extensive flat clathrin lattice has been shown. Although clathrin-dependent endocytosis is the major pathway for the uptake of nutrients in eukaryotic cells, recent evidence shows that some flat clathrin plaques are apparently restricted to the adherent membrane which may reflect cell adhesion to the substrate. We will review the potential role of podosomes as an adhesion structure, summarize current understanding about functional roles of podosomes and discuss further research on these unique structures.