ABSTRACT
An essential element in the growth of cilia and flagella is an enigmatic structure known as a basal body—a compact barrel of short microtubules that nucleates the assembly of the axoneme. Basal bodies are closely related to the centrioles found in the centrosome and under some circumstances the one can convert into the other. Rapid production of large numbers of cilia or flagella requires an abundant source of axonemal proteins. Newly made axonemal proteins first enter the cytoplasm in an unassembled state, becoming part of a pool of assembly-competent precursors. The development of primary cilia occurs either deep-seated within the cell, from the region of its centrosome, or at the cell periphery, one of the two centrioles having migrated to a position close to the plasma membrane. Cilia come from basal bodies and basal bodies come from centrioles. In most animal cells, new centrioles form close to preexisting centrioles. But there are situations in which centrioles arise de novo.
