Multinucleated osteoclasts, generated upon fusion of multiple mononucleated osteoclasts, are unique bone-resorbing cells that enable vascularization of the forming bone during embryonic development. In the adult, they are involved in ion and protein secretory and vesicular transport in a localized area of bone, and thereby maintain bone homeostasis. Osteoclasts are characterized by a cytoplasm with a high concentration of vesicles and vacuoles, a sparse endoplasmic reticulum, and an extensive Golgi complex. In bone, osteoclasts are found in cavities on the bone surface (resorption bays). The osteoclast forms a specialized, extensively folded cell membrane, called the "ruffled border", which opposes the surface of the bone tissue. Formation of the folds enables a dramatic increase in the cell surface, facilitating bone resorption and serving as the main morphologic characteristic of an active osteoclast.