Some Factors Involved in the Control of Microtubule Assembly in Sea Urchins

Some recent evidence is discussed which suggests that the control of microtubule assembly during the formation of the mitotic apparatus in sea urchins involves three factors. These include: nucleation sites such as the kinetochores, a mechanism for regulating intracellular calcium, and a heat stable substance which prevents spontaneous, non-nucleated microtubule polymerization.