TISSUES (CONNECTIVE) 



lites is also unrelated to the orientation of 

 the neighboring collagen fibrils. The align- 

 ment observed in mature tissues is appar- 

 ently a secondary rearrangement caused by 

 physical stresses. 



Tissues 



Reticulin. This is the tissue that holds 

 the various parts of the body in place. 

 Spread underneath the epidermis is a base- 

 ment membrane. Various structures and 

 organs, and sometimes even individual 

 cells, are surrounded by similar membranes. 

 The thickness varies according to the me- 

 chanical strength required at any given site. 

 The thinnest membranes found in such or- 

 gans as the kidney, spleen and liver are of 

 the order of lOOA, while at the other end of 

 the scale they may be several thousand 

 Angstrom units thick. Several workers con- 

 sider that there are distinct forms of reti- 

 culin. E. L. Benedetti and R. Tiribelli have 

 described two in the glomeruli of kidneys 

 (Arch. Ital. Anat. IstoL, 27, 1954) and A. 

 Bairati and B. Pernis (Boll. Soc. Ital. Biol. 

 Sper. 34(6), 250, 1958) suggest that there is 

 a different form for each tissue with which 

 the reticulin is associated. 



In sections, reticulin membranes are seen 

 to be sometimes single and sometimes mul- 

 tiple, with the material of an apparently 

 more or less uniform texture. When frag- 

 mented material is treated with alkali, the 

 reticulin is observed to be very resistant 

 chemically, as compared with most other 

 tissue components. Consequently it can be 

 isolated, and individual membranes viewed 

 using transmitted electrons. When this is 

 done, the finer fragments (e.g., those from 

 liver, kidney, spleen, adrenal and lung) 

 which are sufficiently thin to allow electron 

 penetration are seen to contain an almost 

 random two-dimensional network of colla- 

 gen fibrils (H. Kramer and K. Little "Nature 

 and Structure of Collagen", p. 33, 1953). 

 The proportion of collagen to the polysac- 

 charide-containing ground substance varies 



Fig. 11. Fragment of reticulin from human 

 liver. The positions of some of the collagen bands 

 suggest a non-fibrillar structure. Metal shadowed. 

 X5000. 



from one site to another. A fragment of 

 reticulin isolated from liver is shown in Fig. 

 11. 



Capillary walls consist of a reticulin mem- 

 brane, with cells arranged at intervals on its 

 surface, and occasionally on the inner sur- 

 face. Capillaries may be distinguished from 

 veins or arteries by the fact that cells are 

 not a necessary component part of the wall 

 (that is, after it has been manufactured). 

 There is as yet no definite evidence whether 

 or not capillary walls contain collagen. In 

 Fig. 12 is seen a portion of a capillary wall, 

 the lumen being clearly demonstrated by the 

 presence of the barium sulfate particles used 

 as an injection medium. This injection 

 method is particularly useful when the 

 capillary is passing through loose tissue. 

 The one shown in Fig. 12 is taken from a 

 specimen of the metaphysis of a growing 

 bone. Its wall can be seen to consist of more 

 than one layer. D. C. Pease (./. Histchem. 



283 



