Osteoclast

In the process of bone resorption, the ground substance of bone appears to become modified prior to resorption by osteoclasts. Resorption pits or Howships lacunae form in which the osteoclasts are embedded. Active osteoclasts often have a finely striated brush or ruffled border where they are in contact with the bone.

In bone modelling, osteoclastic resorption with concomitant osteoblastic bone formation leads to reduction of the metaphysis. Later, the marrow cavity is enlarged in part through the process of osteoclastic resorption, as a result of which the overall diameter of the shaft is increased.

Remodelling of bone likewise requires the coordinated activity of a group of highly specialized cells, including osteoclasts. In the endosteal and periosteal surfaces of the cortex, osteoclastic resorption leads to a tube-shaped tunnel, designated a resorption canal. The osteoclasts and osteoblasts together create cylinders of bone about linear vascular channels, representing the basic component of a haversian system or osteon.

Bone-regulating hormones have a dramatic effect on the appearance and function of osteoclasts. Parathyroid hormone causes the rapid development of ruffled borders indicative of active osteoclast function, whereas calcitonin leads to an equally rapid disappearance of these borders. Vitamin D also increases the differentiation of precursor cells into osteoclasts.

In Pagets disease, giant osteoclasts with large numbers of nuclei and intranuclear inclusion bodies, possibly of viral nature, are observed. An intense wave of osteoclastic activity with resorption of normal bone by giant multinucleated cells occurs in this disease.

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