ELECTRON MICROSCOPE IN BIOLOGY — WYCKOFF 255 



cations of electron microscopy. Filaments display a striking tendency 

 to assume parallel arrays. Thus, the secondary walls of plants consist 

 of stacked sheets of parallel-aligned filaments of cellulose. Striated 

 muscle has a parallel alignment of molecular threads as a sort of 

 backbone for its other components. Sections cut at right angles to 

 a muscle fiber demonstrate many fine details of an ordered packing 

 of these threads that in some places is almost crystalline in its per- 

 fection. Evidently the kind of knowledge that will result from such 

 observations permits an entirely new approach to the problem of mus- 

 cular contraction. 



An equally good but very different type of molecular order prevails 

 in tendon and other forms of connective tissue. Collagen, for exam- 

 ple an Achilles tendon from the heel, has been examined as shredded 

 fragments and in sections cut in various directions through an intact 

 tendon. At low magnifications not much above those used in optical 

 microscopy, one sees the tiny fibrils that are associated together in 

 bundles and sheets to form the macroscopic tendons. Higher mag- 

 nifications reveal the fine structure of each component fibril. The 

 examination of transverse sections suggests that a fibril is enveloped 

 by a tube of collagen; preliminary swelling shreds the collagen and 

 shows how its macromolccular particles are knitted together to pro- 

 duce such a tube (pi. 1, fig. 2). Collagen dissolves in very dilute acids 

 and alkalis and can be reprecipitated from its solutions. The electron 

 microscope demonstrates tliat under certain conditions these re-formed 

 fibrils have the molecular fine structure of the original collagen, but 

 that under other conditions of precipitation fibrils of different fine 

 structures are produced. Some of these are found in Nature, others 

 are not. Such investigations of the various forms of collagen are im- 

 portant, not only for their bearing on the nature of paracrystalline 

 order, but also because they permit us to study the changes in connec- 

 tive tissue that accompany and perhaps may sometimes be responsible 

 for arthritis, rheumatism, and similar degenerative diseases of man. 



When we seek to deal with the third broad field of study opened 

 up by electron microscopy — how these macromolccular particles arise — 

 we immediately realize the need first for an adequate picture of the 

 fine structure of the living cells that produce them. Most of what 

 we know about the cells that constitute the higher plants and animals 

 has been gained from optical microscopic studies on thin sections of 

 their tissues. To extend our observations to electron microscopic 

 magnifications it has been necessary to learn how to cut sections that 

 are about fifty times thinner than those required for this optical work. 

 Such wafers of tissue only a few millionths of an inch thick can now 

 be cut as a matter of routine. With them we are begimiing to survey 

 again some of the problems of bacteriology and many problems of 



