CHAPTER 2 



THE STRUCTURE OF CILIA 



I. Introduction 



The observations of earlier workers using the Hght microscope 

 led them to put forward a variety of opinions concerning the 

 structure of ciHa and their bases. Among these workers were those 

 who beUeved that sperm tails (Ballowitz, 1888) and cilia (Koltzoff, 

 1903) had an organization of internal filaments. The similarity of 

 ciliary bases to centrosomes was noted by Henneguy (1898) and 

 von Lenhossek (1898), who in fact suggested that the basal bodies 

 of cilia were centrosomes, an idea that has since been extended, 

 especially by protozoologists. Many studies of the protozoa (e.g. 

 Schuberg, 1891; Maier, 1903; Sharp, 1914), as well as some 

 studies of metazoa (e.g. Grave and Schmitt, 1925), indicated the 

 presence in the cytoplasm of fibrils connected to the ciliary basal 

 bodies. Many of these early observations involved objects so near 

 the limits of resolution of the light microscope that it is not 

 surprising that they did not always agree; indeed, in many 

 cases it is surprising that the observations could be made 

 at all. 



The use of the electron microscope allows more detailed and 

 accurate information to be obtained about the structure of cilia. 

 From a large number of electron microscope studies it is evident 

 that the shafts of cilia, flagella, sperm tails and certain structures 

 derived from these have a common structural plan. Earlier electron 

 micrographs showed that cilia fray into a number of longitudinal 

 fibrils ; there were usually eleven of these fibrils, two of which were 

 rather different from the other nine. The thin-sectioning tech- 

 nique was used to show that the fibrils are arranged in a particular 

 way, with a ring of nine fibrils around two at the centre, and that 

 the whole bundle (axoneme) is surrounded by a membrane that is 

 continuous with the cell membrane. Similarly, the structure of the 



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