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 bl ocks, such as tubulin and dynein, and from a number of 9+2 architectur al elements that are synthesized de novo at ciliogenesis. The finite o r quantal synthesis of certain key architectural proteins is coinciden t with ciliary elongation and proportional to ciliary length. Upon dec iliation, the synthesis of architectural proteins occurs anew, a new c ilium 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 abi lity to regenerate cilia has allowed elucidation of these various prot ein synthetic relationships and has led to the discovery of the pathwa ys by which membrane-associated tubulin and axoneme-associated archite ctural proteins are conveyed into the highly compartmentalized growing cilium. The sea urchin embryo thus provides a very convenient model s ystem for studies of ciliary assembly and maintenance, coordinate gene expression and membrane dynamics.