The lack of support for the articular cartilage results in collapse into the epiphysis. In reptiles, hyperparathyroidism is mostly due to an imbalanced feeding regime. The nutritional imbalance leading to the development of fibrous osteodystrophy is a deficiency of calcium, vitamin D, or an excess of phosphorus that causes increased secretion of parathyroid hormone. Parathyroid hormone is a potent systemic stimulator of osteoclastic bone resorption. Osteoclasts, however, lack receptors for parathyroid hormone.
Osteoblasts express receptors for parathyroid hormone and control the initiation of bone resorption.
In response to parathyroid hormone, osteoblasts contract and leave exposed bone surfaces on which the osteoclasts can attach. The activated osteoblasts secrete or activate collagenases that remove a thin layer of normal, unmineralized fibers from the bone spicules so that the osteoclast can attach to a fully mineralized surface. Osteoclasts are unable to resorb unmineralized matrix. In addition, osteoblasts release cytokines that attract and stimulate inactive osteoclasts within the local environment. Calcitonin is a systemic inhibitor of osteoclasts.