Osteopetrosis fetalis. Report on a case, with special reference to ultrastructure.

The clinical and pathological findings concerning the skeletal abnormalities in a case of osteopetrosis fetalis have been reported. The principal data can be summarized as follows. The areas of endochondral ossification have a rickety appearance because of excessive number of hypertrophic and degenerate chondrocytes. These cells are highly vacuolated and the vacuoles, which are of mitochondrial origin, contain beaded filaments which are exocytosed and become part of the matrix. The calcification process is delayed, probably in consequence of a reduced number of matrix vesicles. Abnormal collagen fibrils are sometimes present in the cartilage. The osteoclasts have a very low reabsorbing activity and appear structurally abnormal. The combined effect of all these abnormalities leads to excessive development of osteocartilaginous trabeculae in marrow spaces. These trabeculae have a Ca/P ratio of 1.79 and their mineral substance appears qualitatively normal under the electron microscope.