The cytoskeletal framework of chick osteoclasts in resin-less sections

The cytoskeletal framework of chick osteoclasts was examined by the resin-less polyethylene glycol (PEG) method. Resin-less thin sections revealed changes in the 3-dimensional organisation of a microtrabecular lattice during different phases of osteoclast activity. This lattice, composed of interconnecting strands, occupied part of the osteoclast cytoplasm including the ruffled border and clear zone areas. In firmly attached, active osteoclasts, the clear zone exhibited a compact and orderly array with a meshwork appearance. This organelle-free area could be clearly distinguished from other cytoplasmic regions by the compactness of its lattice structure. Well developed membrane infoldings of the ruffled border next to the resorption lacunae were composed of interconnected strands, some of which were in direct contact with the ruffled border membrane. Migrating osteoclasts remote from the bone surface lacked these membrane modifications. Their peripheral cytoplasm exhibited a disorganised meshwork of strands without the clear zone. While migrating, osteoclasts which still adhered to the bone surface, appeared to form loci within the compact lattice structure referred to as podosomes. These results demonstrate the dynamic morphological changes in the organisation of the trabecular lattice that occur within attached and migrating osteoclasts.