"Rediscovery" of a forgotten organelle, the primary cilium: the root cause of a plethora of disorders

The primary cilium was recognised in the late 19th century. Conclusive evidence of its existence required the advent of the electron microscope (1950s-1960s), after which its comparison with motile cilia of the (9 + 2) variety was made by Sorokin. Although a small group of devotees researched the primary cilium from this period until the late 1990s, its function as a sensor (previously advocated by Tony Poole) was established because it produced Ca 2+ transients in intracellular signalling. The pathobiological consequences of ciliary agenesis or dysfunction was emphasised in the mid 1990s. But it was only after the recognition that agenesis could be due to mutations in intraflagellar transport proteins several years later that the pathological sequelae were appreciated. Since the early 2000s, the primary cilium has now been implicated as having many functions in cellular behaviour and development, such that disorder in this almost ubiquitous organelle in many tissues of the body leads to an astonishingly wide range of symptoms, from polycystic kidney disease to Alzheimer's. This organelle, dismissed as vestigial or rudimentary by most cell biologists for well over a century, can no longer be ignored in almost any medical and development condition. There is also very much more to learn about the biology of this fascinating organelle. Biomedical Reviews 2013; 24: 1-7.