Figs. 314 from Don Fawcett's Chapter 12 (Centrioles). At the base of the flagellum of a mammalian spermatid, the juxtanuclear proximal centriole is continuous (at the arrow) with a structure of similar configuration, the centriolar adjunct. At the end of the latter, there is an accumulation of loose textured material that appears to be condensing to contribute to elongation of this organelle. The cavity in the centriolar adjunct is narrower than that of the centriole proper. At the lower left a transverse section through the centriole (at the level of the white line in the upper figure) shows the familiar pinwheel arrangement of closed triplet microtubules. At the lower right a comparable section through the adjunct (at the level of the black line in the upper figure) reveals some distinctive structural differences. Subunit a of the triplets is a typical closed microtubule, but b and c are usually open, presenting a free edge that has failed to fuse with the wall of the adjacent microtubule. In addition, the centriolar adjunct has a lining layer of complex ultrastructure which is lacking in the centriole. This accounts for the differences in diameter of their central cavities. The formation of atypical triplet microtubules in the centriolar adjunct of spermatids attests to the capacity of subunit c of the centriolar triplets to serve as a site for nucleation of microtubule protein. Why it is inactive during generation of an axoneme so that only doublets are formed remains unexplained. A copy of the chapter is available on the ASCB's BioEDUCATE website.