Histopathology of skeletal dysplasia in a Japanese Black calf with marked epiphyseal enlargement.

A 16-day-old male Japanese Black calf affected by skeletal dysplasia, a recessive genetic disorder characterized by marked enlargement of the limb joints due to epiphyseal enlargement. Histological examination revealed chondrocyte proliferation in both the humeral and femoral growth plates, with disorganized chondrocyte alignment and fewer bone trabeculae observed in the humerus. These findings suggest that the morphological changes of chondrocytes may be imbalanced during endochondral ossification at the growth plate. Joint enlargement was evident in the epiphyseal region at birth, despite the absence of articular chondrocyte hyperplasia. Although the details of the pathological mechanism remain unclear, cartilaginous epiphysis overgrowth during embryonic development or secondary ossification center expansion during endochondral ossification may cause the joint enlargement observed in this genetic disorder.