Don W. Fawcett, D. M. Phillips (2011) CIL:11597, Mammalia, spermatid. CIL. Dataset

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.

图314来自Don Fawcett在第12章（中心体）中的描述。在哺乳动物精子细胞的鞭毛底部，近核中心体与具有相似构型的中心体附加结构相连（箭头所示）。在后者末端，存在一种松散质地的物质积累，似乎正在凝结以促进该细胞器的延长。中心体附加结构的腔室比中心体本身的腔室更为狭窄。在左下角，通过中心体的横切面（位于上图中的白色线水平处）展示了闭合的三联微管的典型涡旋排列。在右下角，通过附加结构的横切面（位于上图中的黑色线水平处）揭示了某些独特的结构差异。三联微管亚单位a是典型的闭合微管，但b和c通常为开放状态，呈现出未能与相邻微管壁融合的自由边缘。此外，中心体附加结构具有缺乏于中心体的复杂超结构衬里层。这解释了它们中央腔室直径的差异。在精子细胞中心体附加结构中形成的非典型三联微管证明了中心体三联微管亚单位c作为微管蛋白成核位点的能力。为何其在轴突生成过程中保持不活跃，从而只形成双联微管，尚无合理解释。本章的副本可在ASCB的BioEDUCATE网站上查阅。