Rickets is a metabolic bone disorder characterized by osteopenic changes resulting from the failure of calcification of the osteoid matrix and absent mineralization of hypertrophic cartilage cells at the epiphyseal growth plates in growing primates, herbivores, swine, carnivores, and birds. The causes of rickets include inadequate dietary provision of calcium, phosphorus, and vitamin D. Osteomalacia in reptiles, simian bone disease in nonhuman primates, and osteodystrophia fibrosa (secondary hyperparathyroidism) or "bran disease" in herbivores are caused by a diet that has a much higher content of phosphorus than calcium, combined with inadequate exposure to direct sunlight. Medullary bone consists of interconnected spicules of bone resembling embryonic bone and is established in relation to the shell formation cycle of laying birds. Hypertrophic osteodystrophy develops in large-breed growing dogs, chickens, and guinea pigs and is possibly caused by vitamin C deficiency. Tibial dyschondroplasia is a defect in endochondral ossification characterized by a widened proximal tibial physis that is not penetrated by metaphyseal vascular sprouts, commonly found in growing broiler chickens, turkeys, and exotic birds.