囊肿Cyst · 骨 · 骨骼肌肉系统 · NTP ntp-0571

Comment：
Cortical bone cysts (aneurysms) are characterized by expansion of the cortex and/or marrow cavity by an expansile cystic space composed of a thin wall of cortical bone ( Figure 1 ). The expansile nature of these lesions may induce pressure atrophy of the surrounding bone, with loss of bony trabeculae or cortical bone and replacement by connective tissue ( Figure 2 ). Bone cysts may be unilocular ( Figure 3 ) or multilocular and may contain variable amounts of erythrocytes, fibrin, or proteinic material ( Figure 4 ). Solitary bone cysts are rarely observed in the F344 rat, and their significance and pathogenesis are not known. These lesions may occur anywhere along the diaphysis; however, cysts may occur within the subchondral bone as a component of joint degeneration or osteochondrosis in the rat.

Recommendation：
Solitary bone cysts should be diagnosed when observed. However, when a cyst occurs as a component of a primary lesion of osteochondrosis (seen as a developmental defect in Sprague-Dawley rats) or degeneration of the joint, it should not be diagnosed. These lesions represent sporadic background processes in the B6C3F1 mouse and F344 rat; increases in bone cysts have not been associated with chemical exposures. Secondary lesions, such as atrophy, should not be diagnosed separately unless warranted by severity.