Fine structural changes of bone cells in experimental nutritional osteodystrophy of green iguanas

Lizards, especially iguanids and varanids, frequently develop nutritional osteodystrophy in captivity, but its pathogenesis is incompletely understood. Green iguanas fed an experimental diet low in calcium (0.2%) and adequate in phosphorus (1.1%) developed severe osteoporosis characterized by flattened inactive osteoblasts, marked resorption of endosteal bone by large multinucleated osteoclasts, and pronounced resorption of cortical bone by osteocytic osteolysis. Iguanas fed an experimental diet low in both calcium (0.1%) and phosphorus (0.2%) developed osteoporosis with osteomalacia characterized by large active osteoblasts overlying wide osteoid seams, marked osteoclastic resorption of endosteal bone, and large osteocytic lacunae which had a prominent layer of osteoid. Osteocytes in iguanas fed the low calcium diets were small in relation to the size of their lacuanae and contained abundant microtubules, microfilaments and mitochondria but a paucity of rough endoplasmic reticulum. Mean ash content of tibias from iguanas fed the low calcium diets was lower, particularly in iguanas that developed severe oseomalacia, than in iguanas fed the control diet (2.7% calcium, 1.1% phosphorus). The ultrastructural evidence suggesting active bone formation, osteoclastic resorption, and osteocytic osteolysis seen in control iguanas was consistent with that of a young growing animal. The results of this investigation demonstrated that experimental iguanas fed diets low in calcium developed hypocalcemia and compensatory hyperparathyroidism resulting in progressive osteoporosis due to osteoclastic and osteocytic osteolysis.