Figure 4.

(A) Clinically healthy chicks were euthanized at 2 weeks of age, eviscerated and fixed by immersion in 10% buffered formalin for histological evaluation. Care was taken throughout to avoid pulling on or twisting the legs. After 5 days of fixation, the proximal half of each femur was removed with the pelvic socket still attached. The bones were decalcified, embedding in paraffin, sectioned at 5 μM thickness, and stained with hematoxylin and eosin. In two of the eight legs evaluated, narrow osteochondrotic clefts or voids (thick arrows) were evident at the boundary between the epiphysis (e) and the growth plate (physis, p) and occasionally between cartilage columns near the calcifying zone (cz) of the metaphysis (m). (B) Magnification of the clefts (thick arrows) at the junction of the epiphysis (e) and growth plate or physis (p). A penetrating epiphyseal vessel (pev) is threatened with transection. There is no evidence of inflammation or cellular debris adjacent to or within the clefts. The pre-mortem existence of these clefts cannot be proven, but clearly this key boundary zone is structurally fragile. Osteochondrotic clefts at the epiphyseal-physeal boundary and within the physeal cartilage are considered to predispose broilers to epiphyseolysis, which in turn is considered an early or initial macroscopic manifestation of BCO (Duff, 1984b, 1989a, 1990b; Duff and Randall, 1987; Thorp et al., 1993b; Bradshaw et al., 2002). Also shown are epiphyseal vascular canals (ec) and a metaphyseal vascular plexus (mvp).