Cilia and flagella

Nine doublets around a central pair, with the motor on one side and the signal line running to the middle. The human body carries four kinds of it, not two.

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Four kinds, not two

cells of the human body carry any of four types of cilium: 9+0 motile, 9+0 immotile, 9+2 motile, and 9+2 immotile

human · review · Arora S and colleagues 2023, J Biosci · source

What the radial spokes do

they transmit mechanochemical signals between the central pair and the axonemal dynein arms, which is how ciliary motility is coordinated

mouse · cryo-EM of the radial spoke head-neck complex, plus cryo-ET and sub-tomogram averaging · Meng X and colleagues 2024, Nat Commun · source

Motile cilia are not one design

the cryo-ET map of mouse ependymal cilia lacks inner dynein arms b, c and e, and has no tektin filaments within the A-tubule of the doublets, unlike mammalian respiratory cilia and sperm flagella

mouse · cryo-electron tomography with sub-tomogram averaging · Meng X and colleagues 2024, Nat Commun · source

The primary cilium

a solitary membrane-bound, microtubule-based, non-motile projection of the plasma membrane, whose membrane hosts sensory complexes and receptors, so it acts as a cellular antenna for mechanical, chemical and light signals

general · review · Corbeil D and colleagues 2025, Cell Commun Signal · source

When they move fluid, they work in numbers

a multiciliated cell assembles several hundred motile cilia on its apical surface; in humans these cells line the airways, the brain ventricles, and parts of the reproductive tracts, and their coordinated beating shifts tubular fluid, particles or cells along the epithelium

human · review · Wloga D and colleagues 2024, Cells · source

What failure looks like

mutations related to primary ciliary dyskinesia and to asthenospermia map onto the radial spoke head-neck complex, so the spoke is a point where the architecture fails clinically

mouse structure, human mutations · cryo-EM structure with genetic mutations mapped onto it · Meng X and colleagues 2024, Nat Commun · source

Not shown yet

Length and diameter

no abstract read for this page gives a ciliary length or axonemal diameter with a method. The shaft is shown short enough to see the whole cross-section at once; its proportions are legibility, not measurement.

How many dynein arms and spokes along the shaft

the copy spaces nine dynein arm pairs and six radial spokes up each doublet so the repeat reads as a repeat. The real periodicity is not taken from any source read here and nothing on this page claims it.

The basal body

shown as nine triplets below the axoneme because that is the conventional depiction and it is the same body the centriole page draws. No source read for this page states it, so it is shown and not claimed.

Behind the copy

A cilium is a microtubule-based projection of the cell surface, and the human body carries four kinds of it rather than two: 9+0 motile, 9+0 immotile, 9+2 motile and 9+2 immotile. The motile axoneme shown here is nine outer doublet microtubules around a central pair, with dynein arms and radial spokes between them; the radial spokes transmit mechanochemical signals between the central pair and the axonemal dynein arms to coordinate motility. That architecture is not uniform across the body. A cryo-electron tomography map of mouse ependymal cilia lacks inner dynein arms b, c and e and has no tektin filaments inside the A-tubule of the doublets, unlike mammalian respiratory cilia and sperm flagella, so mammalian motile cilia are structurally diverse rather than one repeated design. The immotile case is a different organelle in practice: the primary cilium is solitary, non-motile, and hosts sensory complexes and receptors in its membrane, acting as a cellular antenna. Where cilia move fluid they work in numbers, a multiciliated cell assembling several hundred of them on its apical surface.

source 1, source 2, source 3, source 4

Real pictures

Gallery and papers in the library

Panel E, "iHTM Control" column: gamma-tubulin (red, basal body), ARL13B (green, ciliary axoneme) and DAPI (blue) in human iPSC-derived trabecular meshwork cells, showing a normal primary cilium. Part of a composite figure on OCRL mutations in Lowe syndrome. Chen, Liu, Wang et al 2026, Advanced Science, CC BY 4.0 · source · immunofluorescence, composite figure

Transmission electron microscope image of a thin section cut through the bronchiolar area of lung (mouse), showing cilia cross-sections. The inside of the cilium contain precisely arranged microtubules, the axoneme. The axoneme has a central unit containing two single microtubules and nine peripheral doublet microtubules (known as the "9+2"). Dynein sidearms project from the A tubule of each doublet. Louisa Howard, Michael Binder (Dartmouth Electron Microscope Facility), Public domain · source · micrograph

High magnification of "Lung cilia#80311-50" image. This image is a thin x-section cut through cilia. The inside of the cilium contain precisely arranged microtubules, the axoneme. The axoneme has a central unit containing two single microtubules and nine peripheral doublet microtubules (known as the "9+2"). Dynein sidearms project from the A tubule of each doublet. Louisa Howard, Michael Binder (Dartmouth Electron Microscope Facility), Public domain · source · micrograph

Performance of AAP purified gamma-Tubulin Antibody. ( A ) Immunofluorescence staining showing AAP purified gamma-Tubulin (#D-0006-100UG) performance in ciliated NIH-3T3 cells. Merged images including gamma-Tubulin (green), anti-acetylated tubulin (blue), and anti TMEM67 (red). The white square outlines a representative ciliary structure used for higher magnification in c-f. Scale bar represents 1 μm. Karapetian, Gerhardt, Lemanska et al 2026, BMC Molecular and Cell Biology, CC BY 4.0 · source · figure from an open-access paper

Decreased transition zone protein localization in ciliated trisomy 21 cells. ( A ) Representative confocal images of RPE1 D21 and T21 cells grown on coverslips and serum depleted for 24 hr. Cells were stained with GT335, PCNT, and the transition zone protein CEP290. Yellow arrows point to the CEP290 transition zone population and cyan arrows point to CEP290 satellites that colocalize with PCNT. ( Jewett, McCurdy, O'Toole et al 2023, eLife, CC BY 4.0 · source · figure from an open-access paper

The GPCRs used in this study localize to the cilium. A, RPE1 cells transiently expressing indicated GFP- or Flag-tagged GPCR were fixed and processed for immunofluorescence. Cilia were identified by immunostaining endogenous ARL13B. In each image, the cilium of interest is enlarged and highlighted in a box at the upper right corner. Flag-HTR7 served as a nonciliary GPCR (negative control). Mahajan, Madugula, Chia et al 2026, The Journal of Biological Chemistry, CC BY 4.0 · source · figure from an open-access paper

Loss of IFT88 in Villin-Cre mice impairs ciliogenesis in the efferent ductules. A Gross anatomy of the reabsorptive mouse efferent ductules showing the proximal, conus, and distal segments. While the proximal segment, adjacent to the rete testis, consists of convoluted tubules, the distal segment converges into a single duct connecting to the epididymis. Its epithelium is composed of two main cell types: multiciliated cells (MCC) and reabsorptive primary ciliated cell (RPCC) that ensure sperm concentration through fluid reabsorption. C Immunofluorescent staining of efferent ductules from adult Ctl and Ift88 cKO mice showing IFT88 (gray), γ-tubulin (γ-tub, green) marking centrioles/basal bodies, and Acetylated α-tubulin (red) labeling axoneme in primary (arrow) and motile cilia. F Ultrastructural components of primary and motile cilia, including the basal body (Bb), axoneme (Ax), daughter centriole (Dc), transition zone (Tz), and basal foot (Bf), in the efferent ductules of control and Ift88 cKO mice by transmission electron microscopy (TEM). Augiere, Hermo, Ferrier-Tarin et al 2026, Cellular and Molecular Life Sciences, CC BY 4.0 · source · figure from an open-access paper

Cross-section of a normal tracheal axoneme by TEM, panel D of a figure comparing wild-type and Armc2-knockout ciliary axoneme structure. Scale bar 50 nm. Giordani, Houdeline, Court et al 2026, Frontiers in Cell and Developmental Biology, CC BY 4.0 · source · transmission electron micrograph, cropped from a comparison figure

Motile cilia covering the apical surface of a bioengineered nasal epithelial construct, by scanning electron microscopy. One panel cropped from a larger figure on ciliary ultrastructure. Scale bar 10 um. Bradbury, Dickson, Curtis et al 2026, Journal of Anatomy, CC BY 4.0 · source · scanning electron micrograph, cropped from a comparison figure

Transverse section of a fertile control sperm flagellum by TEM, labeled with the central pair (CP), outer dense fibers (ODF) and doublet microtubules (DMT), showing the canonical 9+2 axonemal organization. Scale bar 200 nm. Li, Wan, Feng et al 2026, Frontiers in Cell and Developmental Biology, CC BY 4.0 · source · transmission electron micrograph, cropped from a control/proband comparison figure

Transmission electron microscope observation of the body surface cilia in trochophore larva of U. unicinctus . Kong, Wei, Liu et al 2023, International Journal of Molecular Sciences, CC BY 4.0 · source · figure from an open-access paper

The tracheal pseudostratified columnar ciliated epithelium reveals the columnar ciliated cells (C), the more electron-dense goblet cells (G), and basal cells (B). The columnar ciliated cells contain rounded to oval euchromatic nuclei (N) with small patches of peripheral heterochromatin. Regularly arranged cilia (arrows) are present. (rat (trachea); Fig. 9a.) Sarhan, Elekhtiar, Elmowafy et al 2026, Histochemistry and Cell Biology, CC BY 4.0 · source · figure from an open-access paper, transmission electron microscopy (TEM)

TEM images of axonemal cross sections of human sperm flagella. Panel A, right: sperm from M1688, M1344, and M1670; arrows point to examples of outer dynein arms (ODAs). Panel B, right: sperm from M865, with causal DNAH17 variants, clearly show an absence of ODAs as well as axonemal disruption, including missing or supernumerary and misplaced microtubule doublets. Scale bar, 200 nm. (human (sperm flagellum); Figure 2, TEM panels A right and B right.) Koser, Rieck, Aprea et al 2026, Human Reproduction Open, CC BY 4.0 · source · figure from an open-access paper, transmission electron microscopy (TEM)

Loss of zygotene cilia does not overtly affect meiotic chromosome pairing or DNA repair. (A) IF staining of seminiferous tubule squashes from control or cKO mice expressing EGFP-Centrin1 (centriole marker) with antibodies against A-tub and SYCP3 (meiotic chromosomal axis marker). Arrowheads indicate basal bodies extending primary cilia in zygotene cells. Scale bar, 5 um. Adapted: IF panel A only; quantification panel B and chromosome spread panels C-H excluded. (mouse (seminiferous tubule spermatocytes, CEP164-cKO); Figure 3, panel A only (IF of centrioles and primary cilia; quantification panel B and chromosome spread panels C-H excluded; rows 10-710 of full figure, 1600x700).) Chen, Gong, Mak et al 2026, EMBO Reports, CC BY 4.0 · source · figure from an open-access paper, immunofluorescence (EGFP-Centrin1, A-tub, SYCP3)

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