Cilia and flagella are important organizing centers for signaling in both development and disease. A key to their function is a poorly characterized barrier at their base that allows the protein and lipid composition of the ciliary membrane to be distinct from that of the plasma membrane. We review current models of ciliary membrane biogenesis, highlighting several structures, including the ciliary necklace and ciliary pocket, that appear during biogenesis and that likely contribute to the barrier. The regulated movement of membrane proteins and lipids across this barrier is central to the sensory function of these organelles.
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