Ultrastructural abnormalities in bone and calcifying cartilage in two siblings with a newly described recessive...

Ultrastructural abnormalities in bone and calcifying cartilage are presented for a recently identified lethal chondrodysplasia. Two siblings, aged 20 and 30 weeks of gestation, showed severe short-limb dwarfism and histologically distinct, highly disorganized masses of cartilage, bone, and mesenchymal tissue in the long bones. Regions of inappropriate cartilage calcification showed unusual, electron-dense, amorphous islands of mineralization and larger, less dense, layered calcified masses that occasionally entrapped chondrocytes. Bone abnormalities included abnormal cartilage-bone transition at the growth plate, general bone matrix disorganization due to irregularly oriented bundles of collagen, mineral crystals on the osteocyte lacunar rim, and accumulations of thickened collagen fibrils, also along the osteocyte lacunar rim. These findings point to abnormal calcification and mineralization distinct from those seen in other reported skeletal dysplasias. These abnormalities are associated with an anarchic distribution of mesenchymelike tissue infiltrating the cartilage and bone.