Cleavage of Sea-urchin Eggs in Colchicine

INTRODUCTION We have recently put forward a theory linking mitosis with cleavage, in which we suggest that cleavage is caused by an expansion of the cell cortex which starts at either pole of the cell and later moves round to the region of the furrow (Mitchison, 1952). There is evidence that this expansion is brought about by substances diffusing from the two groups of daughter chromosomes which, by the end of anaphase, lie near the poles of the cell (Swann 1951a, b). If this theory is correct, the important structure in mitosis is the spindle which is responsible for separating the two groups of chromosomes. The asters, where they exist, should act only as passive guides for the advancing furrow. Some theories of cell division, on the other hand, e.g. Gray (1924), Dan (1943), suppose that the spindle and asters have an active function in cleavage, and that the cell surface is pulled passively into the furrow. It should be possible to distinguish between these opposing views by suppressing the spindle and asters at a point when the chromosomes have already separated, and seeing whether cleavage still continues. The most satisfactory way of doing this is to use colchicine. The action of this poison in stopping cleavage and suppressing the formation of the spindle without affecting the condensation of the chromosomes, has been known for some time, and there is a large literature on the subject (Eigsti, 1947). Its action on sea-urchin eggs has also been studied by a number of workers including Nebel (1937), Wilbur (1940) and Beams & Evans (1940). There is general agreement that colchicine, at concentrations of about 5 x io~* M inhibits mitosis and cleavage in these eggs. It prevents the formation of the spindle and asters, or, if they are already formed, it causes them to solate and disappear. The normal viscosity rise associated with the growth of fibrous material in the mitotic figures is therefore stopped, and viscosity remains low.