ELECTFtON IVIICROSCOPY 



9 * ' *'i 



Fig. 2. Section of human epidermis, showing 

 portion of prickle cell. Tonofibrils pass through 

 the intercellular bridges from one cell to another. 

 Formalin fixed. Embedding medium removed. 

 X 10,000. 



cellular bridges. What is normally regarded 

 as inadequate fixation has been found useful, 

 since with other cytoplasmic components de- 

 graded and removed, individual fibrils can 

 easily be traced through a series of cells. 

 When entering a cell near the nucleus the 

 fibrils are seen to turn through almost a 

 right angle as soon as they are clear of the 

 cell wall. There has been no suggestion that 

 these fibrils are associated with ground sub- 

 stance, but there is a limited amount of evi- 

 dence suggesting that they might be kerati- 

 nous. 



Ground Substance. Ground substance 

 is ubiquitous, and may be regarded as the 

 glue that holds all the extra-cellular tissues 

 together. Its characteristic components are 

 polysaccharides. The texture of the ground 

 substance varies from thin tough sheets, as 



in reticulin (Fig. 3), to a three-dimensional 

 gel, as in articular cartilage. Fig. 4 is a sec- 

 tion of cartilage, necessarily dried out, where 

 the spaces are caused by loss of fluid from 

 the gel. Collagen is never found without 

 ground substance, though the proportion of 

 the two may vary considerably, both be- 

 tween different types of tissue and within 

 any one type. Their association is fundamen- 

 tally that of a mixture and not a chemical 

 compound. The ground substance adheres 

 to the outside of the collagen fibrils with 

 particular tenacity at the bands. In tissue 

 such as the epiphyseal growth cartilage, 

 where the banded structure of the collagen 

 is not very apparent, the 640A spacing is 

 most clearly demonstrated by the ground 

 substance stretching between fibrils (see Fig. 

 16). 



Collagen. While the ground substance 

 holds it together, collagen fibers provide the 

 mechanical strength for the framework of the 



Fig. 3. Fragment of reticulin from human liver. 

 Uranium shadowed. X5000. Specimen prepared by 

 Dr. H. Kramer. 



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