Cilia axoneme

Some specialized eukaryotic cells have cilia that show a whiplike motion. Sperm cells move with one flagella, which is much longer than a cilium but has a nearly identical internal structure called axoneme. It is composed of nine doublet MTs that form a ring around a pair of single MTs. Numerous proteins bind to the MTs. Ciliary dynein motors generate the force by which MTs slide along each other to cause the bending of the axoneme necessary for motion. 

Figure 19-23 (A) Diagram of a cross-sectional view of the outer portion of a lamellibranch gill cilium. This has the 9+2 axoneme structure as shown in Fig. 1-8 and in (B). The viewing direction is from base to tip. From M. A. Sleigh.329 (B, C) Thin-section electron micrographs of transverse (B) and longitudinal (C) sections of wild-type Chlamydomonas axonemes. In transverse section labels A and B mark A and B subtubules of microtubule doublets oa, ia, outer and inner dynein arms, respectively sp, spokes cpp, central pair projections bk, beaks. From Smith and Sale.329a...

Fig. S-36 Representations of ccntrioles. basal bodies and axonemes within a cilium. la) Ccntriole/basal body showing the nine sets of triplet microtubules, (h) Cross-section of a cilium/flagellum showing the 9 + 2" array of microtubules and accessory proteins of the axoneme. (c) View of a cell indicating the organizing role of ccntrioles and basal bodies.

Figure I. Structure of a ciliated photoreceptor cell in vertebrates. (A) Scheme of a rod photoreceptor cell. (B) Transmission electronmicroscopy image of a part of a mouse rod photoreceptor cell. The apical extensions of cells of the retinal pigment epithelium (RPE) evolve the tips of photoreceptors light-sensitive outer segments (OS). The OS is linked via a connecting cilium (CC) to an inner segment (IS) which bears the basal body complex (BB) in its apical region. Synaptic terminals (S) link the photoreceptor cell and the 2nd-order neurons, bipolar and horizontal cells. N = nucleus in B, arrow point to axonemal microtubules projecting into the OS. Bar in B = 0.2 p,m...

Figure 2. Schematic representations of a prototypic cilium and the photoreceptor cilium in comparison. (A) Scheme of a prototypic cilium, in longitudinal extension and cross sections through subciliary compartments axoneme (9x2 + 2 microtubule arrangement), transition zone (9x2 + 0 microtubule arrangement) and centriole (9x3 + 0 microtubule arrangement) of the basal body. (B) Scheme of the ciliary part of a rod photoreceptor cell. Axonemal microtubules (MX) project into the outer segment (OS). The OS is linked via the connecting cilium (CC) to the inner segment (IS). The CC corresponds to the transition zone of a prototypic cilium. The basal body complex (BB) is localized in the apical region of the IS. The calycal process (CP) of the IS is linked by extracellular fibers with the membrane of the CC.

Recent proteomic analysis indicated that the cilia of mammalian photoreceptor cells are significantly more complex than other eukaryotic cilium (Liu et al. 2006). Over 1200 different polypeptides have been identified by the quantitative analysis of the proteome of photoreceptor outer segment compared with the proteome of the axoneme/ciliary fraction of mouse photoreceptor cells. This data set contains all previously identified protein components of the photoreceptor cilium (e.g. Schmitt and Wolfrum 2001). An understanding how the identified proteins function in their native environment of the diverse compartments of the photoreceptor cilia will require increased knowledge of their molecular interaction and networking. Insights into the organization and composition of diverse protein complexes may also provide novel information on how functional modules of the cell, recently proposed by Hofmann et al. 2006, are connected. 

The outer segment of vertebrate photoreceptor cells can be considered as the highly modified distal part of an immotile cilium (Rohlich 1975 Besharse and Horst 1990) and contains only a small number of axonemal microtubules (see Introduction). The RPl protein is specifically associated with these microtubules and is required for... 

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