Intraflagellar transport.

Cells need to be able to sense different types of signals, such as chemical and mechanical stimuli, from the extracellular environment in order to properly function. Most eukaryotic cells sense these signals in part through a specialized hair-like organelle, the cilium, that extends from the cell body as a sort of antenna. The signaling and sensory functions of cilia are fundamental during the early stages of embryonic development, when cilia coordinate the establishment of the internal left-right asymmetry that is typical of the vertebrate body. Later, cilia continue to be required for the correct development and function of specific tissues and organs, such as the brain, heart, kidney, liver, and pancreas. Sensory cilia allow us to sense the environment that surrounds us; for instance, we see as a result of the connecting cilia of photoreceptors in our retina, we smell through the sensory cilia at the tips of our olfactory neurons, and we hear thanks to the kinocilia of our sensory hair cells. Motile cilia, which themselves have sensory functions, also work as propeller-like extensions that allow us to breathe because they keep our lungs clean, to reproduce because they propel sperm cells, and even to properly reason because they contribute to the flow of cerebrospinal fluid in our brain ventricles. Not surprisingly, defects in the assembly and function of these tiny organelles result in devastating pathologies, collectively known as ciliopathies (Box 1). Thus, the proper function of cilia is fundamental for human health.