SH compounds in mitosis: II. The effect of mercaptoethanol on the structure of the mitotic apparatus in sea urchin eggs

Abstract

1. 1. The cleavage of fertilized eggs of Strongylocentrotus purpuratus may be blocked reversibly by mercaptoethanol (monothioethylene glycol). 2. 2. The critical concentration for blockage of the next division is about 0.075 M. At lower concentrations, the eggs may be blocked after one or more divisions. 3. 3. The divisions are blocked, at the critical concentration, if mercaptoethanol is introduced at any time up to metaphase. If it is introduced shortly after metaphase, cleavage proceeds without delay and the cells are blocked in the following division. 4. 4. Superficially, the blockage has the character of a simple arrest. That is, the total time from fertilization to division, minus the time spent in mercaptoethanol, is equal to the division time of the controls. However, the following observations indicate that the evident arrest is not merely a “freezing” of the mitotic apparatus at a given stage. 5. 5. Nuclear reproduction and possibly the reproduction of the centrioles proceed during the mercaptoethanol block. If the eggs are removed from mercaptoethanol after the time when the controls have made their second division, they divide directly into four cells, from which a normal embryo develops. 6. 6. Isolation of the mitotic apparatus from the blocked cells shows that their structure at the microscopic level is very much disorganized by mercaptoethanol. 7. 7. It is concluded that the apparent disorganization at the microscopic level does not imply an equally drastic disruption of the underlying organization. The effect of mercaptoethanol is analogized to a “relaxation” or “loosening” of structure that can be reversed almost instantaneously.