Ciliogenesis in sea urchin embryos--a subroutine in the program of development.

One major milestone in the development of the sea urchin embryo is the assembly of a single cilium on each blastomere just before hatching. These cilia are constructed both from pre-existing protein building blocks, such as tubulin and dynein, and from a number of 9 + 2 architectural elements that are synthesized de novo at ciliogenesis. The finite or quantal synthesis of certain key architectural proteins is coincident with ciliary elongation and proportional to ciliary length. Upon deciliation, the synthesis of architectural proteins occurs anew, a new cilium grows, and the stores of various building blocks are replenished. This routine of coordinated ciliary gene expression may be replayed experimentally many times without delaying normal development. The ability to regenerate cilia has allowed elucidation of these various protein synthetic relationships and has led to the discovery of the pathways by which membrane-associated tubulin and axoneme-associated architectural proteins are conveyed into the highly compartmentalized growing cilium. The sea urchin embryo thus provides a very convenient model system for studies of ciliary assembly and maintenance, coordinate gene expression and membrane dynamics.