Micromorphological approaches to bone resorption and formation

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The mouse parietal bone, femur and tibia were examined mainly by scanning electron microscopy (SEM). Osteoblasts, osteocytes, and osteoclasts were exposed for SEM by our EDTA-KOH method to dissolve the bone matrix. The surfaces of the bone matrix or osteoid were exposed by a NaOCl maceration method or water jet. Some of parietal bones were examined by a back scattered method. Femurs and tibias were also examined after estrogen injections or cutting nerves for lower extremities. The results indicate or suggest the followings. 1) Osteocytes and osteoblasts are functionally different cells and differentiated from common osteogenic precursor cells. Osteocytes appears in the layr of osteoblasts. 2) Osteoblasts elongate many, parallel, long and thin cytoplasmic processes in the narrow spaces between the undersurface of the osteoblastic layr and the surface of the osteoid to form parallel narrow gaps for collagen fiber production. Thus, the collagen fibers are produced running parallel for the bone matrix. 3) Osteocytes are buried into the bone matrix by pulling by cytoplasmic processes of osteocytes and form meshwork to detect the stress loading to the bone and inform this information of stress to osteoblasts through the cytoplasmic processes attaching to the osteoblast processes. 4) Osteocytes inform the thickness of the forming bone lamella to the local osteoblasts. 5) Osteoclasts show 2 kinds of ruffled borders of different functional activities. Osteoclasts dissolve the bone matrix in 2 kinds of functional phases. 6) Bone formation and resorption are functionally coupled. 7) Back scattered images of the bone surfaces demonstrate the various stages of calcification of collagen fibers for the bone matrix.