An electron microscopic study of Araldite-embedded, undecalcified human woven and chick lamellar bone is presented. The fine structure of the cells of bone in their normal milieu is described. Active osteoblasts possess abundant granular endoplasmic reticulum, numerous small vesicles, and a few secretion droplets. Their long cytoplasmic processes penetrate the osteoid. The transition of osteoblasts into osteoid osteocytes and then into osteocytes is traced and found to involve a progressive

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... ction of cytoplasmic organelles. Adjoining the osteocytes and their processes is a layer of amorphous material which is interposed between the cell surfaces and the bone walls of their respective cavities. Osteoclasts contain numerous non-membrane-associated ribosomes, abundant mitochondria, and little granular endoplasmic reticulum, thus differing markedly from other bone cells. The brush border is a complex of cytoplasmic processes adjacent to a resorption zone in bone. No unmineralized collagen is seen at resorption sites and it appears that collagen is removed before or at the time of mineral solution. All bone surfaces are covered by cells, some of which lack distinctive qualities and are designated endosteal lining cells. The structure of osteoid, bone, and early mineralization sites is illustrated and discussed. The ultrastructure of bone has not been extensively studied owing to the many technical obstacles. Some of the structural features of osteoblasts and osteoclasts have been described as they appeared in embryonic fowl and in kittens (7, 23) . Previous studies have shown that the bone lesions in patients with polyostotic fibrous dysplasia contain coarse woven bone in small trabeculae and that this tissue can be cut easily without prior decalcification (12). The purpose of this paper is to present results of a study of the ultrastructure of bone cells as they appear in coarse fiber bone and in newly formed lamellar cancellous bone. MATERIALS AND METHODS Osseous tissues obtained from four biopsies of three patients with polyostotic fibrous dysplasia were * Deceased. selected to allow study of coarse woven bone. Cancellous bone specimens from the sternum of ten-weekold white Leghorn chicks were selected for study of newly formed lamellar bone. The fresh specimens were immediately placed in buffered 1 per cent osmium tetroxide with sucrose added at 4°C (2), and while immersed they were cut into smaller pieces. After 4 hours' fixation at 4°C, dehydration at room temperature was accomplished in 50, 70, 90, and 95 per cent acetone solutions for ~ hour each, followed by three changes, 1 hour each, of absolute acetone. Infiltration in an Araldite medium with the proportions of 5 ml resin, 5 ml hardener, and 0.2 ml dibutyl phthalate was carried out at room temperature for 18 hours (26). Two changes of the infiltrating medium for 2 additional hours each were followed by embedding in an Araldite mixture identical with the infiltrating mixture except that it contained also 0.35 ml of accelerator. After the specimens had remained for 2 to 5 days in an oven at 60°C, thick sections of approxi-627