In various mesenchymal tissue alterations in lathyrism, as the lesions of cartilage and connective tissue and dissecting aneurysm of the aorta, a defect in formation of collagen fibers and mucopolysaccharide has been considered as an essential basic change. The present electron microscopic study was made on the ultrastructures of the bone matrix forming cells and the newly formed bone matrices in the aminoacetonitrile induced lathyrism, in order to see if the defect mentioned above is also found in the periosteal hyperostosis. The present investigation revealed following points: (1) The basic changes in the lathyric periosteal hyperostosis consist of a failure in development of the cytoplasmic organellae of the bone matrix forming cells, and a defect in formation of the collagen fibers as well as the ground substance of the newly formed bone matrix, resulting in characteristic mineralization disturbance in mottled pattern. (2) Of these basic changes the defect in formation of the collagen fibers as well as the ground substance, probably mucopolysaccharide, seems to be the same as those in the other mesenchymal tissue. (3) The failure in development of the cytoplasmic organellae, especially the granular endoplasmic reticulum of the bone matrix forming cells may be related to the defect in formation of the collagen fibers and the ground substance. (4) The defect in formation of the bone matrix may be responsible for the characteristic disturbance of mineralization in the newly formed bone matrix
