The tiny cell antenna biologists dismissed as useless for a century
Almost every animal cell sprouts a single hair-thin projection called a primary cilium. Found in 1898, it was written off for about a hundred years as a leftover with no real purpose. It turns out to be a cellular antenna, and when it malfunctions the results range from kidney cysts to heart defects and failing retinas.
The name comes from the Latin for eyelash. Cilia are found only in cells with a nucleus, never in bacteria or archaea, and measure one to five micrometres long. Each grows from a basal body, a modified centriole made of nine triplets of microtubules. A gatekeeping transition zone at the base, with Y-shaped links, decides which proteins may enter. Inside runs a core called the axoneme, and motor proteins shuttle cargo along it in both directions, kinesin toward the tip and dynein back toward the cell.
Cilia come in two broad kinds. Motile ones typically have nine outer pairs of microtubules around a central two, the same layout as a sperm's tail, and dynein arms make them beat. Around 200 of them crowd each cell lining the airways, sweeping mucus out. Others push cerebrospinal fluid through the brain's cavities or move eggs down the fallopian tubes. A special set in the early embryo, lacking the central pair, sets up the body's left-right asymmetry.
Sensory cilia usually lack the central pair. Most cells carry just one, blood cells being a notable exception, though smell neurons, where odour receptors sit, have about ten apiece, and the light-sensing cells of the retina use a highly specialised version. Cells build their cilium during the G1 phase and take it apart before dividing, a step that requires an enzyme called aurora kinase A.
Defects in these structures cause a family of disorders called ciliopathies, among them Joubert syndrome and polycystic kidney disease. Faulty transition zones in mammals disrupt Hedgehog signalling, which steers how many fingers and toes form and how the central nervous system is patterned. In the pancreas, cilia on insulin-making beta cells help regulate their function, and removing them can lead to type 2 diabetes.
Source: Cilium
