Multiciliogenesis is a cascading process for generating hundreds of motile cilia in single cells. In vertebrates, this process has been investigated in the ependyma of brain ventricles and the ciliated epithelia of the airway and oviduct. Although the early steps to amplify centrioles have been characterized in molecular detail, subsequent steps to establish multicilia have been relatively overlooked. Here, we focused on unusual cilia-related structures previously observed in wild-type mouse ependyma using transmission electron microscopy (TEM) and analyzed their ultrastructural features and the frequency of their occurrence. In the ependyma, approximately 5% of cilia existed as bundles; while the majority of the bundles were paired, bundles of more than three cilia were also found. Furthermore, apical protrusions harboring multiple sets of axonemes were occasionally observed (0-2 per section), suggesting an unusual mode of ciliogenesis. In trachea and oviduct epithelia, ciliary bundles were absent, but protrusions containing multiple axonemes were observed. At the base of such protrusions, certain axonemes were completely enwrapped by membranes, whereas others remained incompletely enwrapped. These data suggested that the late steps of multiciliogenesis might include a unique process underlying the development of cilia, which is distinct from the ciliogenesis of primary cilia.

中文翻译：

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多纤毛小鼠上皮中纤毛发生异常模式的超微结构证据

多纤毛发生是在单个细胞中产生数百个运动纤毛的级联过程。在脊椎动物中，已经在脑室室管膜和气道和输卵管的纤毛上皮中研究了这一过程。尽管放大中心粒的早期步骤已经在分子细节上进行了表征，但随后建立多纤毛的步骤却相对被忽视了。在这里，我们专注于以前使用透射电子显微镜 (TEM) 在野生型小鼠室管膜中观察到的异常纤毛相关结构，并分析了它们的超微结构特征和它们的发生频率。在室管膜中，大约 5% 的纤毛以束状存在；虽然大多数束是成对的，但也发现了超过三个纤毛的束。此外，偶尔观察到含有多组轴丝的顶端突起（每节 0-2 个），表明纤毛发生的不寻常模式。在气管和输卵管上皮中，没有纤毛束，但观察到含有多个轴丝的突起。在这些突起的基部，某些轴丝被膜完全包裹，而其他轴丝则未完全包裹。这些数据表明，多纤毛发生的后期步骤可能包括纤毛发育的独特过程，这与初级纤毛的纤毛发生不同。某些轴丝完全被膜包裹，而其他轴丝则未完全包裹。这些数据表明，多纤毛发生的后期步骤可能包括纤毛发育的独特过程，这与初级纤毛的纤毛发生不同。某些轴丝完全被膜包裹，而其他轴丝则未完全包裹。这些数据表明，多纤毛发生的后期步骤可能包括纤毛发育的独特过程，这与初级纤毛的纤毛发生不同。