ELECTRON MICROSCOPY 



its early stages it is not subjected to many 

 stresses. When these do arise — in a joint 

 which is beginning to be used — and when 

 these appHed stresses are in \-ariable direc- 

 tions, articular cartilage makes its appear- 

 ance. This is characterized by less prominent 

 cell capsules and the precipitation of fibers 

 of larger diameter in the hitercellular matrix, 

 such as those shown in Fig. 4 and 14. The 

 greater the load the joint has to bear, the 

 higher the proportion of fibrils of large diam- 

 eter. The cartilage is made up of cells and 

 matrix, but it contains no vessels. The nour- 

 ishment for the cells is probably provided 

 by natural movement causing an intermit- 

 tent pvmiping action which circulates syno- 

 vial fluid through the tissue (J. Trueta). All 

 the observed facts are in accord with this 

 hypothesis (for example, articular cartilage 

 wiped dry and then compressed yields syn- 

 ovial fluid), and it also provides a rational 

 explanation of the appearances of the car- 

 tilage in osteoarthritis. 



Osteoarthritis occurs whene\'er the various 

 forces acting across a joint become unbal- 



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"i?t » ' jC '^^ . 



V'f- 



Fig. 14. Section of noriu:il adult human articu- 

 lar cartilage. Embedding medium removed. Metal 

 shadowed. X 10,000. 



Fig. 15. Section of adult human articular car- 

 tilage from a case of osteoarthritis. The section 

 has been cut parallel to the surface of the bone. 

 Embedding medium removed. Metal shadowed. 

 X 10,000. 



anced as a result of injury, unnatural activ- 

 ity, or disease; J. Trueta and his collabo- 

 rators have produced it in experimental 

 animals in an analogous manner, by up- 

 setting local forces acting on the joint. On 

 the microscopic scale the first manifestation 

 is the onset of degradation of the ground sub- 

 stance (an observation which is confirmed 

 by chemical analysis) and a lining up of 

 collagen fibrils normal to the surface of the 

 bone. Fig. 15 shows the appearance of a sec- 

 tion cut parallel to the surface of the bone 

 from a fairly early case where the surface 

 of the cartilage was still almost intact. The 

 amount and direction of orientation is best 

 demonstrated by a combination of X-ray 

 diffraction and electron microscope observa- 

 tions (K. Little, L. H. Pimm and J. Trueta, 

 J. Bone and Joint Snrg., 40B: 123, 1958). In 

 the experimental animals removal of the 

 undesirable forces and tensions was followed 

 by recovery of the cartilage, as judged by 

 both macroscopic and microscopic appear- 

 ances. 



286 



