CLATHRIN SHEETS AND CYTOSKELETAL STRCTURES ON THE PROTOPLASMIC SURAFCE OF THE VENTRAL MEMBRANES OF OSTEOCLASTS IN CULTURE

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Physical cell shearing resulted in various degrees of disruption of the basolateral (upper) membranes, cytoskeletons or cell organelles, and exposed the protoplasmic surface of vental (adhesion) membranes of steoclasts that were attached to the underlying substratum such as cover slips, mica or synthetic apatite plates. Freeze-dried replicas of the ventral membranes left behind on the substratum after cell-shearing provided 3-dimensional information on the ultrastructure of the protoplasmic membrane surface of cultured osteoclasts. An extensive area of the protoplasmic surface and various amounts of cytoskeletal structures attached to the adherent vental surface of the plasma membrane were visible. In particular, the most characteristic finding of the present study is that numerous clathrin sheets displaying various sizes, shapes and curvature were revealed on the ventral membrane. The polygon substructures of the clathrin lattices appeared to be composed of hexagons with a few pentago … More ns interspersed. They were seen at the paramarginal area where they were situated at the sites of close contact with the underlying substratum. In addition, clathrin lattices were never observed on the basolateral (upper) membranes. In favorable stereo views, most of cytoskeletons were not in direct contact with the clathrin sheets. However, a few observations indicated that possible remnants of cytoskeletons attached to clathrin latticesAlthough it is generally accepted that the cytoskeletal podosomes in motile cells such as osteoclasts play a major role in cell adhesion, the present study indicates that membrane-associated clathrin may also function during the attachment to the substrate. In this regard, clathrin is thought to be required for receptor-mediated endocytosis, but whether it may also function in cell attachment is still a matter for debate. This type of clathrin-related adhesion appears to be a previously unrecognized site of cell / substrate adhesion in osteoclasts. To assess this possible function, we focused on clathrin and related cytoskeletal elements on the ventral membranes of cultured osteoclasts Less