Electron Microscopy of the Microtubule Framework in Primary Cilia

Primary cilia of kidney cells are microtubule-based sensory organelles extending into extracellular space. They detect environmental signals and influence physiological activities of the cells. Defects of primary cilium assembly or dysfunctions of sensory proteins of primary cilia are linked to many developmental defects and clinical disorders (ciliopathies) [1]. Accurate structural information of primary cilia is fundamental for understanding the assembly, maintenance and sensory functions of primary cilia. There are only a handful reports with conventional electron microscopic (EM) data about primary cilium structure. In contrast, structural data using both conventional and cryo-EM is abundant for motile cilia. Therefore, much of our structural understanding of primary cilia is based upon, or borrowed from, the knowledge obtained with motile cilia. The frameworks (axoneme) of both motile and primary cilia are microtubule-based complexes (MtCs). Microtubules present in singlet [2-5], doublet [6-8], and triplet [9] forms in the axoneme. The 3D structure of primary cilium axoneme has been described as a cylindrical array of nine microtubule doublets (the 9+0 paradigm), which is the same as or similar to the cylindrical array of the 9 peripheral doublets in motile cilia. The 9+0 paradigm is currently the accepted structural baseline of primary cilium research. However, there are several EM reports where the data conflict with the 9+0 paradigm. Lack of direct 3D structural data of primary cilia is due to technical challenges in EM specimen preparation.