TISSUES (CONNECTIVE) 



Fig. 4. Section of articular cartilage from rabbit. The thickness of this section is less than the 

 diameter of the thicker collagen fibrils. Two or three are shadowed on their inner surfaces. Em- 

 bedding medium removed. Uranium shadowed. X 10,000. 



body. In the preceding paragraph it was im- 

 pHed that the ground substance is composed 

 of a group of related polysaccharides. Simi- 

 larly, it is most probable that collagen is 

 made up of a group of related proteins, with 

 the 2.86A spacing of X-ray diffraction pat- 

 terns and a periodicity of the order of (340 A 

 along the fibrils as the common factors. 

 Solubility, shape of fiber and mode of for- 

 mation can vary. The evidence at present 

 available suggests that, like ground sub- 

 stance, collagen is polymerized in the cyto- 

 plasm of cells. It is then precipitated in a 

 fibrillar form either at the surface of the cell, 

 or away from the cell in a medium of ground 

 substance. 



So far two types of collagen have been dis- 

 tinguished in the human. The form which 

 can be more easily brought into solution oc- 

 curs as very fine fibrils, is most abundant in 

 the infant, and is the predominant form of 

 collagen in tissues such as primitive anlage 

 cartilage and growth cartilage. The second 

 form is the more commonly illustrated one, 

 less soluble, with a tubular appearance of its 

 fibrils and more prominant r)40A spacings. 

 It is the major component of mature tendon. 

 Most tissues such as articular cartilage and 



osteoid matrix contain a mixture of the two. 

 When specimens of the two types are sepa- 

 rated in a relatively pure form, by making 

 use of the solubility differences, minor dif- 

 ferences are noticed in the wide-angle X-ray 

 diffraction pattern. 



One type of cell which produces collagen 

 directly is the fibroblast. Various workers 

 have studied it with the electron microscope, 

 using thin sectioning techniques. Fig. 5, 

 taken by Dr. J. A. Chapman and Dr. R. 

 Peach of Manchester University, is a photo- 

 graph of regenerating tendon at 6 weeks. 

 This type of specimen is in^-aluable for show- 

 ing cytoplasmic detail, but is not so well 

 suited for demonstrating the surrounding 

 matrix. Thus, Chapman and Peach do not 

 think that there are collagen fibrils within 

 the cytoplasm, while Fitton-Jackson and 

 others think there may be. Fig. 6 is from a 

 thick section of fibrocartilage containing a 

 cell in which the cytoplasm has disintegrated 

 as a result of delayed fixation. This treat- 

 ment does not affect collagen. Here there is 

 no sign of collagen fibrils crossing the cell 

 wall. At one point this wall is expanded into 

 a very fine net -like structure. This observa- 

 tion is common to fibroblasts from a number 



279 



