SCANNING ELECTRON-MICROSCOPY OF THE OSTEOCLAST-BONE INTERFACE IN-VIVO

Rat femoral bones were studied by scanning electron microscopy to demo nstrate the morphology of osteoclast-bone matrix interfacial relations hips. Two general morphological types of actively resorbing osteoclast s were observed. One cell type was approximately ovoid with highly fim briated borders, fully attached and closely adapted to the resorption surface. The ruffled border of such cells was composed of a number of individual filopodia which were of uniform, regular shape and approxim ately 2-3 mum in length and 150 nm in diameter. They were found to pen etrate the bone matrix to a depth of 1 mum and interdigitated with the surrounding bone. The other cell type was elongated, covered smaller lining cells, and attached to the bone surface by pseudopodia. The bon e-contacting face of pseudopodia formed a network of irregular membran ous branches apposed closely to the underlying bone matrix. The result s revealed the three dimensional ultrastructure of in situ relationshi p of osteoclast to bone matrix and necessitate a reinterpretation of t he geometry of the secondary lysosome of these cells.