ELECTRON MICROSCOPY 



Fig. 18. Sfclion through osteocyte in poorly 

 calcified trabeculum. Processes from the cell can 

 be seen to pass through canaliculi in the as yet un- 

 calcified matrix. Formalin fixed. Embedding me- 

 dium removed. 



in channels in the matrix — the canaHculi. 

 Observation of sections of areas of active 

 bone formation seem to show that the 

 osteoblasts bury themselves within the 

 matrix they manufacture, whereupon they 

 are referred to as osteocytes. The osteocyte 

 in Fig. 18 has several canaliculi cut in the 

 plane of the section, and processes from the 

 cell can be seen to lie in these. Normally by 

 this stage the matrix would be completely 

 calcified, but the cell chosen for illustration 

 was from an experimental animal with defi- 

 cient calcification, so that only about a 

 quarter of the matrix in the photograph was 

 calcified. The matrix contains collagen fibrils 

 of assorted diameters. 



R. A. Robinson and D. A. Cameron (/. 

 Biophys. Biochem. Cytol. Suppl. 2(4), 253, 

 1956) have pointed out that age in itself is 

 not responsible for the character of different 

 types of fibril. They found the fibrils from 

 osteoblasts to have 2-5 times the diameter 

 of those seen in growth cartilage, and to 

 possess obvious periodic structure, both 

 types of collagen having been laid down at 

 about the same time. 



In the normal formation of bone, the 

 stages appear to be the laying down of colla- 

 gen and ground substance, followed by the 

 deposition of apatite crystaUites in the gelati- 

 nous ground substance. R. Frank (Thesis, 

 Strasbourg, 1957) has examined a case of 

 post-traumatic osteoporosis. X-ray difTraction 



showed no change in the mineral composi- 

 tion, while in the electron microscope the 

 changes were consistent with the disappear- 

 ance of first the crystallites and ground sub- 

 stance, leaving collagen fibrils prominent, 

 followed by the gradual breakdown of these 

 fibrils. The changes occurred in a patchy 

 and irregular manner. 



Dentine. As compared with bone, a great 

 deal of time and effort has been expended on 

 the electron microscope examination of den- 

 tine, but the conclusions are by no means 

 unequivocal. Many photographs of carious 

 dentine with bacteria within the tubules 

 have been published, particularly by R. W. 

 G. Wyckoff, D. B. Scott and R. Frank; and 

 M. U. Nylen and D. B. Scott have produced 

 a compilation of photographs of early devel- 

 oping dentine (U. S. Public Health Service 

 Publication No. 613). The odontoblasts 

 differ from chondrocytes and osteoblasts, 

 but have some of the characteristics of each. 

 They are found on the border of the develop- 

 ing dentine, and produce matrix, but unlike 

 the osteoblast they never become completely 

 surrounded by it. They have a single large 

 process which passes through the dentinal 

 tubule, and a number of workers have re- 

 ported small branches from these, which are 

 analogous to the canaliculi in bones. Like 

 chondrocytes, they do not produce bundles 

 of fibers on their surfaces, but the collagen 

 is mainly found between the tubules in a 

 manner which has originally only a small 

 degree of ordering. 



Fig. 19 shows the arrangement of collagen 

 fibrils round a tubule near the pulp. Vessels 

 penetrate to the odontoblast layer, and cal- 

 cification of the matrix takes place at the 

 point farthest from cells and vessels. This 

 may be compared with calcified cartilage in 

 which calcification is at the end away from 

 the vessels which supply the cartilage with 

 its nourishment. M. U. Nylen and D. B. 

 Scott report a change in the appearance of 

 the ground substance of the matrix which 

 precedes calcification. It has been known for 



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