Ultrastructural evidence of abnormally short and maldistributed actin stress fibers in osteopetrotic (toothless) rat osteoblasts in situ after detergent perfusion.

Osteopetrosis is a heterogenous group of metabolic bone diseases characterized by a generalized increase in skeletal mass, the product of reduced bone resorption and interceptions in the development and/or function of osteoclasts. In one such mutation in the rat, toothless (tl), osteoblasts are absent from older bone surfaces and there is evidence for aberrant osteoblast gene expression and function. Given the emerging appreciation of the role of osteoblasts in the differentiation and activation of osteoclasts, we have examined ultrastructural features of the cytoskeleton of normal and mutant osteoblasts after perfusion fixation with the non-ionic detergent Triton X-100. This procedure reduces the electron density of the cytoplasm, rendering visible the microfilamentous network in osteoblasts. In normal osteoblasts a prominent system of stress fibers (bundles of actin microfilaments) run parallel to the cell membrane adjacent to osteoid surfaces, stretching for 75% of that distance. However, only 50% of mutant (tl) osteoblasts had stress fibers and in these cells stress fibers were either significantly shorter (18% of normal) or distributed intracellularly rather than along the osteoid surface. In mutant osteoblasts without stress fibers, 20% showed ultrastructural signs of cell degeneration. Given the role of stress fibers in cellular attachment ot extracellular matrices, these observations suggest that the reduced number of osteoblasts in tl rats may be related to their inability to organize actin filaments and adhesion plaques for attachment to bone surfaces. We propose that a feature of osteopetrosis in the tl rat is a disruption of the mechanisms that regulate the synthesis, sorting, and/or assembly of actin.