Hoops H J, Witman G B
J Cell Biol. 1983 Sep;97(3):902-8. doi: 10.1083/jcb.97.3.902.
Analysis of serial cross-sections of the Chlamydomonas flagellum reveals several structural asymmetries in the axoneme. One doublet lacks the outer dynein arm, has a beak-like projection in its B-tubule, and bears a two-part bridge that extends from the A-tubule of this doublet to the B-tubule of the adjacent doublet. The two doublets directly opposite the doublet lacking the arm have beak-like projections in their B-tubules. These asymmetries always occur in the same doublets from section to section, indicating that certain doublets have consistent morphological specializations. These unique doublets give the axoneme an inherent structural polarity. All three specializations are present in the proximal portion of the axoneme; based on their frequency in random cross-sections of isolated axonemes, the two-part bridge and the beak-like projections are present in the proximal one quarter and one half of the axoneme, respectively, and the outer arm is absent from the one doublet greater than 90% of the axoneme's length. The outer arm-less doublet of each flagellum faces the other flagellum, indicating that each axoneme has the same rotational orientation relative to the direction of its effective stroke. This strongly suggests that the direction of the effective stroke is controlled by a structural component within the axoneme. The striated fibers are associated with specific triplets in a manner suggesting that they play a role in setting up or maintaining the 180 degrees rotational symmetry of the two flagella.
对衣藻鞭毛连续横切面的分析揭示了轴丝中的几种结构不对称性。有一个双联体缺少外动力蛋白臂，其B微管中有一个喙状突起，并带有一个双部分桥，该桥从这个双联体的A微管延伸到相邻双联体的B微管。与缺少该臂的双联体直接相对的两个双联体在其B微管中有喙状突起。这些不对称性在不同切片的相同双联体中总是出现，表明某些双联体具有一致的形态特化。这些独特的双联体赋予轴丝一种固有的结构极性。所有这三种特化都存在于轴丝的近端部分；根据它们在分离轴丝随机横切面上的出现频率，双部分桥和喙状突起分别存在于轴丝近端的四分之一和二分之一处，并且在轴丝长度超过90%的一个双联体上没有外臂。每个鞭毛中没有外臂的双联体面向另一个鞭毛，表明每个轴丝相对于其有效冲程方向具有相同的旋转方向。这有力地表明有效冲程的方向由轴丝内的一个结构成分控制。横纹纤维以一种表明它们在建立或维持两个鞭毛180度旋转对称性方面起作用的方式与特定的三联体相关联。
