[Electron microscopic study on giant cell tumor of bone, based upon the development of bone in human embryo (author's transl)].

Many reports have been published about giant cell tumor of bone since its initial description in 1818 by Cooper. No theory on its histogenesis has nevertheless been unanimously admitted. The purpose of this paper is to make systemic investigation of differentiation process of osteoblast, osteocyte and osteoclast with spines of 23 human embryos mainly by means of electron microscopic observation and to compare the above results with constituent cells of giant cell tumor of bone, thus to clarify the histogenesis of the latter cells. Results. Enchondral ossification of a vertebral body takes place initially in the upper lumber spines at the beginning of the fourth embryonal month and gradually spreads cranial- and caudalwards. Almost simultaneously blood vessels accompanied by undifferentiated mesenchymal cells around them invade into the vertebral body toward ossification center. These mesenchymal cells differentiate finally into osteocytes passing through the stages of undifferentiated polygonal cell, preosteoblast, and osteoblast. Meanwhile osteoclasts appear approximately when primary marrow cavity is formed. Histogenesis of osteoclast is considered to be morphologically quite different from osteoblast, rather as being derived from macrophage or its precursor cell. The author classified stromal cells of giant cell tumor of bone into three types, of which the type 3 is similar to a giant cell and the latter to an osteoclast. From this reason, giant cell tumor of bone does not appear to be derived from osteoblast or osteocyte, but from macrophage or its precursor cell.