Osteoclasts tunneling

Fig. 10.1 Osteoclasts tunneling. Osteoclasts differentiate at cracks in the bone and form a tunnel (lacuna) containing a central capillary surrounded by connective tissue. Fibroblast-like precursors of osteoblasts are activated to become osteoblasts and new bone is formed (Modified from Fig. 22.54 in The Molecular Biology of the Cell. B. Alberts et al., 4th Ed. 2002. Garland Science, Taylor Francis Group, New York)...

Figure 1. The cellular activity during bone remodelling. At the tip (cutting cone) multi-nucleated osteoclasts (OCLs) excavate the mineralised bone tissue. At some distance, after the resting zone, osteoblasts (OBLs) refill the tunnel with (osteoid) that is subsequently mineralised. Osteocytes (OCYs) are former osteoblasts that were entombed within the bone matrix, but remained connected to the bone surface by numerous long slender protrusions (not visible). Courtesy R. Schenk.

Vitamin D deficiency in adults cannot affect the epiphyseal plate, as it has disappeaced, but it can prevent normal mineralization of the osteoid layer in bone that turns over. In vitamin D deficiency the osteoclasts continue to create tunnels and pits in the bone. The osteoblasts continue to synthesize the protein matrix however, complete mineralization of the osteoid may not occur. The result is osteomalacia - This disease may present as bone pain about the hips. Osteomalacia can be diagnosed using a bone biopsy, A sample is taken from the iliac crest — the hip bone. An abnormally wide osteoid is indicative of the disease. X-rays can also be used to diagnose osteomalacia, which is characterized by arrays or zones of tiny fractures in sucli bones as the pelvis and femur. 

In osteoclasts, a tunnel or lacuna in the bone develops at a rate of 50 pm/day (Fig. 10.1). The bone dissolution occurs in an acidic (demineralizing) compartment that is separated from the rest of the cell by a ruffled membrane whose outer periphery is sealed to the bone... 

---