These rings may provide a scaffold for nucleation of microtubule assembly. Diagram in L p. 801. Each centriole is a cylinder with a wall made of 9 triplet microtubules. Two centriole cylinders are oriented at right angles. Diagrams in L p. 799-800.
Microtubules exhibit dynamic instability .
They may grow steadily, and then shrink rapidly, at the plus end. In vitro , the tendency to grow or shrink may be a function of tubulin concentration. As microtubules grow, tubulin dimers are depleted. Below a critical concentration of tubulin dimers, microtubules shrink. A rapidly growing microtubule may accumulate a few layers of tubulin-GTP at the plus end.