Scanning Electron Microscopy of the Osteoclast-Bone Interface In Vivo

Rat femoral bones were studied by scanning electron microscopy to demonstrate the morphology of osteoclast-bone matrix interfacial relationships. Two general morphological types of actively resorbing osteoclasts were observed . One cell type was approximately ovoid with highly fimbriated 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 approximately 2-3 μm in length and 150 nm in diameter. They were found to penetrate the bone matrix to a depth of 1 μm 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 bone-contacting face of pseudopodia formed a network of irregular membranous branches apposed closely to the underlying bone matrix. The results revealed the three dimensional ultrastructure of in situ relationship of osteoclast to bone matrix and necessitate a reinterpretation of the geometry of the secondary lysosome of these cells.