Cathy Fisch

Eukaryotic cilia and flagella perform motility and sensory functions which are essential for cell survival in protozoans, and to organism development and homoeostasis in metazoans. Their ultrastructure has been studied from the early beginnings of electron microscopy, and these studies continue to contribute to much of our understanding about ciliary biology. In the light of the progress made in the visualization of cellular structures over the last decade, we revisit the ultrastructure of cilia and flagella. We briefly describe the typical features of a 9+2 axoneme before focusing extensively on the transition zone, the ciliary necklace, the singlet zone, the ciliary cap and the ciliary crown. We discuss how the singlet zone is linked to sensory and/or motile function, the contribution of the ciliary crown to ovocyte and mucosal propulsion, and the relationship between the ciliary cap and microtubule growth and shortening, and its relation to ciliary beat. We further examine the involvement of the transition zone/the ciliary necklace in axonemal stabilization, autotomy and as a diffusion barrier.

Cilia are specialized organelles that exert critical functions in numerous organisms, including that of cell motility, fluid transport and protozoan locomotion. Ciliary architecture and function strictly depend on basal body formation, migration and axoneme elongation. Numerous ultrastructural studies have been undertaken in different species to elucidate the process of ciliogenesis. Recent analyses have led to identification of genes specifically expressed in ciliated organisms, but most proteins involved in ciliogenesis remain uncharacterized. Using human nasal epithelial cells capable of ciliary differentiation in vitro, differential display was carried out to identify new proteins associated with ciliogenesis. We isolated a new gene, ICIS-1 (Involved in CIlia Stability-1), upregulated during mucociliary differentiation. This gene is localized within the TGF-beta1 promoter and is ubiquitously expressed in human tissues. Functional analyses of gene expression inhibition by RNA interference in Paramecium tetraurelia indicated that the ICIS-1 homologue interfered with cilia stability or formation. These findings demonstrate that ICIS-1 is a new protein associated with ciliated cells and potentially related to cilia stability.