TISSLES (CONNECTIVE) 



However, in all cases mistakes can be 

 avoided when orthodox light microscopy is 

 used as a control. The most obvious example 

 of this has been the work, on elastica, in 

 which many workers who had not used ade- 

 quate controls reported observations on the 

 non-elastic components of the tissues. Other 

 methods which have been used are X-ray 

 and electron diffraction, to identify compo- 

 nents and indicate orientation; injection 

 methods, to trace blood vessels ; microradiog- 

 raphy and direct experiments. 



The use of light microscopy to prevent 

 mistakes is an example of one of the most 

 important aspects of this type of investiga- 

 tion — the recognition of artefacts. Some are 

 comparatively obvious: for example, the fact 

 that both embedding media and many tissue 

 components are polymers of similar density. 

 The divergence of opinion on whether colla- 

 gen fibrils were solid rods or hollow tubes 

 has been enlightening. Those supporting the 

 first theory left their embedding medium in ; 

 those in favor of the hollow tube theory had 

 removed it. Viewing stereoscopically, which 

 has been found a useful method for detecting 

 a number of artifacts, supports the observa- 

 tions that at least those collagen fibrils 

 which are of large diameter are hollow. 



In this account, emphasis will be placed 

 on the electron microscopic appearances of 

 the extra-cellular matrices, since a sui'vey of 

 the literature suggests that the present state 

 of knowledge of the cellular components is 

 still very uncertain. One of the few conclu- 

 sions about these cells for which there is a 

 substantial mass of evidence from a number 

 of lines of approach, including the use of 

 labeled precursors (e.g. I. Karpishka, C. P. 

 Leblond and J. Carneiro, Arch, oral Biol. 1, 

 23, 1959), is that the systems of approxi- 

 mately parallel membranes in the cytoplasm, 

 such as that seen in the cytoplasm of the 

 chondrocyte shown in Fig. 1, are closely 

 concerned with protein formation. 



First, individual components will be dis- 

 cussed, and then the architecture of the main 



tissues concerned. In the available space a 

 comprehensive list of references is not fea- 

 sible, so many of those quoted in detail will 

 be important ones likely to be missed in a 

 casual surve}^ of the literature. Except for 

 Figures 5 and 10, photographs are of a white 

 object on a black background. 



Components 



Tonofibrils. The surface of the skin is 

 covered with a layer of keratin. Immediately 

 under this are the prickle cells, which, when 

 viewed with a high-power light microscope, 

 appear to be connected by fibrils. When 

 reasonably thick sections are viewed in the 

 electron microscope, so that the slightly 

 wavy fibrils remain in the plane of the sec- 

 tion and are not cut away, it is seen that 

 these tonofibrils do, in fact, pass from one 

 cell to another through intercellular bridges. 

 When viewed stereoscopically, the thickness 

 of the section sho^vn in Fig. 2 is seen to be 

 about equivalent to the depth of three inter- 



FiG. 1. Section of chondrocyte from primitive 

 cartilage of rabbit. The system of parallel mem- 

 branes is believed to be the site of production of 

 the matrix. Formalin fixed. Embedding medium 

 removed. X6000. 



277 



