Fine morphology of bony dysplasia of the murine ear: comparisons with otosclerosis.

Dysplastic bony lesions that bear certain similarities to otosclerosis occur spontaneously in LP/J inbred mice. These lesions develop after the fifth week of life and progressively enlarge; they are present in 90% of animals by 90 weeks of age. In the present study, the fine morphology of these lesions was investigated. Dysplastic lesions in the middle ear of LP/J mice appeared to begin as subperiosteal accumulations of amorphous material. These accumulations occurred adjacent to the bone surface below the epithelium of the middle ear. This amorphous material was often associated with macrophages on the adjacent epithelial surface. The material appeared to be replaced with collagen fibers in a disorganized manner, forming a homogeneous base (osteoid) on which calcification occurred. The fine morphology and periodicity of the collagen appeared normal. The lesions then calcified by forming calcospherites that became confluent. During this period, transformed fibroblasts (osteoblasts) appeared in the calcifying matrix, resulting in a lesion made of bone. These lesions could sometimes be seen replacing normal bony contours but most often were exophytic. The lesions at all stages of development may be associated with macrophages and effusions in the middle ear. The lesions in the middle ear of LP/J mice appeared to develop by an active process of bone formation; in contrast, otosclerosis is an active process of bone resorption and formation.