ELECTRON ^IICKOSCOPY 



Fig. 12. Capillary wall in metaphysis of rabbit 

 tibia. Blood has been replaced by the fine particles 

 of a barium sulphate injection medium. Formalin 

 fixed. X3000. 



Cijtochem., 3(4), 295, 1955) has studied the 

 glomerular capillaries in kidneys. He found 

 that a fenestrated endothelial sheath was 

 cemented onto the basement membrane of 

 the capillary walls, and in many places found 

 a layer of the cementing material on either 

 side of the membrane. In the sections he 

 examined he found no evidence for a fibrillar 

 structure in the basement membrane. 



Loose Connective Tissue. It is this tis- 

 sue which is mainly responsible for allowing 

 bodily structures to move relative to one 

 another without friction. It contains nerves 

 and blood vessels; too great a mobility, 

 which might damage these, is prevented by 

 a loose interweaving of bundles of collagen 

 fibrils. These bundles are the collagen fibers 

 observed by histologists using the light mi- 

 croscope. The fibroblasts producing these 

 fibers are commonly observed to form bun- 

 dles of fibrils of uniform diameter. This is 

 seen in Fig. 9. In the foetus and infant the 

 fibroblasts of the loose connective tissue pro- 

 duce fibrils of smaller diameter than those 

 found in mature tissue. The tissue contains 

 much fluid, so that there are many spaces 

 in the dried sections used in the electron 

 microscope. In areas where there are greater 

 stresses, notably the subcutaneous tissues, 

 elastin is also found. Such elastic tissue ap- 

 pears to be laid down after the collagen 



bundles, and the edges of these are often 

 embedded hi the adjoining elastin (Fig. 9). 

 The elastic "fibers" are of variable size, 

 shape and arrangement. 



In the Ehlers-Danlos syndrome, in which 

 the skin, and sometimes other tissues, has 

 unnatural mobility, it has been reported (L. 

 H. Jansen, Arch. Belg. derm. syph. 10(3), 

 251, 1954) that collagen formation is defec- 

 tive, with the fibers less entangled than 

 usual. Unless sufficient elastic tissue is pres- 

 ent to compensate for this, the tissues have 

 a very low tensile strength. 



Tendon and Ligament. Tendons are a 

 specialized form of compact connective 

 tissue, designed to take a high tensile load. 

 Bimdles of collagen fibrils are tightly inter- 

 woven, with their orientation predominantly 

 parallel to the long axis of the tendon. Un- 

 like the loose connective tissues, it can be 

 seen that a single collagen bundle contains 

 fibrils of different diameters. It has been ob- 

 served that the greater the forces to which a 

 tendon is habitually subjected, the larger 

 and more robust are the individual fibrils. 

 Thus larger fibrils with more marked 640A 

 banding are found in kangaroo tail tendon 

 than in himian Achilles tendon. The latter, 

 in turn, are larger than those in some of the 

 smaller tendons. These visual observations 

 are in accord with the results of solubility 

 experiments on tendons of experimental ani- 

 mals. Solubility tends to vary from one 

 tendon to another. Considerable variation is 

 also observed in the proportion of polysac- 

 charide present in the tendon or ligament. 

 In identically prepared untreated sections, 

 individual fibrils will be clearly distinguish- 

 able in human Achilles tendon, and almost 

 obscured by ground substance in the liga- 

 mentum nuchae. It was in tendon specimens 

 that the tubular structure of at least some 

 collagen fibrils was first established (J. J. 

 Kennedy, Science, 121: 673, 1955). The 

 methods of preparation developed by R. W. 

 G. Wyckoff are particularly well suited for 

 the examination of this type of tissue. 



284 



