CENTRIOLES AND BASAL BODIES 49 



basal bodies of ciliary organelles, as found in recent electron 

 microscope studies, leaves little doubt that these two structures 

 are very closley related. In fact, centrioles and basal bodies may 

 be interchangeable, so that perhaps the name basal body should 

 be replaced by centriole, as Gatenby (1961) suggested. 



Although leucocytes do not show a normal mitosis, one or two 

 centrioles are present in each cell, and these, as well as centrioles 

 from other cells of vertebrates, have been studied with the electron 

 microscope by de Harven and Bernhard (1956), Amano (1957), and 

 Bessis et al. (1957, 1958). They found that the centriole is a 

 cylinder about 0-15 fx in diameter and 0-3 to 0-5 ja long, with a 

 less dense centre and opaque walls, in which are embedded nine 

 groups of three longitudinal tubular fibrils (PL Xlla, b). Each 

 fibril is 150 to 200 A in diameter and, in each group the three 

 fibrils are in a line tilted at about 40° to the tangent to the centriole 

 exactly as in the cihary base described on p. 24. Bessis et al. (1958) 

 described additional club-shaped appendages, with a head 

 (" massule ") about 700 A in diameter at the end of a stalk 

 C' pont ") 600 to 900 A in length and 200 A thick, which were 

 attached to the longitudinal fibrils, about two to each fibril group. 

 Structures of the same shape and size were found on the basal 

 bodies of mammalian retinal rod cilia by Tokuyasu and Yamada 

 (1959) (see PL Vic). Bernhard and de Harven (1960) have called 

 these pericentriolar structures " satellites". 



Frequently the centriole appears to taper slightly towards one 

 end, which, since it is the end continuous with the flagellum in 

 many sperm tails, is regarded as the posterior end and is equivalent 

 to the distal end of the ciliary basal body. The centriole may be 

 open at both ends, or closed at the posterior end by a cross- 

 membrane, or a granule, or both membrane and granule. Peri- 

 pheral fibrils of cilia and flagella are continuous with the fibrils in 

 the centriole wall, while the central fibrils are associated with the 

 posterior granule or membrane. 



In normal somatic cells there may- be a single centriole or two 

 centrioles characteristically orientated with their axes at right 

 angles. The tubular spindle fibres (150 to 200 A in diameter) of 

 mitotic cells are found to radiate from the centriole region, but 

 their exact relationship to the centriole is uncertain. There are 

 frequently more than 9, or even more than 27 spindle fibres, so 



