TISSUES (CONNECTIVE) 



Fig. 7. Section of rabbit aorta. Smooth muscle 

 cells and elastic tissue alternating. Note the in- 

 creased density of the elastic tissue at its surface. 

 Osmium tetro.xide fixation. Embedding medium re- 

 moved. No shadowing. X 10,000. 



sections viewed stereoscopically. In sites 

 such as developing arteries, elastin can be 

 found well away from fibroblasts or any simi- 

 lar cells. For instance, in Fig. 7 elastin is 

 seen on either side of a smooth muscle cell, 

 with no different components in the imme- 

 diate vicinity. D. C. Pease is of the opinion 

 that ground substance is necessary for the 

 formation of elastin, and this view is sup- 

 ported by the fact that in areas such as that 

 sho^^^l in Fig. 8, taken from a section of 

 elastic cartilage, the elastin is confined to 

 one type of surrounding matrix. Fig. 9 is 

 from the loose connective tissue of skin, and 

 here it can be seen that the elastin has tended 

 to engulf some of the surrounding collagen 

 fibrils — a very common occurrence. 



Calcium Phosphates. The mineral 

 which imparts rigidity to the hard tissues is 



hydroxyapatite. As far as the characteristics 

 of the individual crystallites are concerned, 

 a great deal of work has produced remark- 

 ably few uneciuivocal results. This is partly 

 because some of the difficulties inherent in 

 the method of examination could not be 

 appreciated until the appearance of such 

 phenomena as dislocations, twin boundaries 

 and total reflection were commonly recog- 

 nized. The crystallites in bone and dentine 

 are very small compared with those in 

 enamel. They are rod-shaped, with diameters 

 of approximately 40-50A, and it is now gen- 

 erally agreed that their average length is of 

 the order of GOOA (T. W. Speckman and W. 

 P. Norris, Science, 126, 753, 1957). The 

 X-ray diffraction results of D. Carlstrom 

 and A. Engstrom are in agreement with 

 these dimensions. Experimentally, enamel 

 crystallites are easier to observe. Even with 





Fig. 8. Section of luunan epiglottis. Elastin in 

 cartilage matrix. The shadowing metal on the cut 

 surface gives a false impression of density. Osmium 

 tetroxide fixation. Embedding medium removed. 

 Uranium shadowed. X 10,000. 



281 



