Experimental analysis of the reproduction of spindle poles

We have investigated the functional properties of the mechanisms that control the reproduction of spindle poles in fertilized sea-urchin eggs. By prolonging mitosis by three independent means, we show that a spindle pole can split during mitosis into two functional poles of normal appearance. However, these poles have only half the normal reproductive capacity; each daughter cell that receives a split pole, always forms a monopolar spindle at the next division. Each monopolar spindle appears to be exactly half of a spindle because two of them can come together to form a functional bipolar spindle of normal appearance. The poles of such spindles show normal reproduction in subsequent divisions. By following the development of individual cells with monopolar spindles, we show that such a cell can stay in mitosis longer than normal, and the single pole splits into two asters, which move apart to give a functional bipolar spindle. The poles of such a spindle have only half the normal reproductive capacity, because the two daughters of the cell always form monopolar spindles at the next mitosis. This novel cycle of development is often repeated. The occurrence of such phenomena does not depend upon the method used to induce monopolar spindles. These results show that each normal pole has two polar determinants. The results also demonstrate that the reproduction of spindle poles consists of three distinct events: splitting of the polar determinants, physical separation of the two determinants, and duplication of the determinants to return the pole to a duplex state. Splitting and duplication are distinct events because they can be experimentally put out of phase with each other for several cell cycles.