Cytokinesis: Cleavage furrow establishment in cylindrical sand dollar eggs

Abstract

AbstractBy confining fertilized, denuded sand dollar (Echinarachnius parma) eggs in short lengths of capillary tubing so that they were reshaped into elongate cylinders it was possible to control the orientation of both the mitotic apparatus and the subsequent cleavage furrow as well as the distance from the mitotic apparatus to the cylindrical cell surface. The technique left the polar regions accessible to experimentation and was used before first and second cleavages.The time when the equatorial surface becomes capable of forming a complete, permanent furrow which can function independently of the mitotic apparatus was determined by aspirating the mitotic apparatus from one of a pair of cylindrical cells at second cleavage and timing the interval between the operation and the appearance of the furrow in the control cell. When the mitotic apparatus is removed 4 minutes or less before control furrowing, the operated cell cleaves, but when the same operation is accomplished 5 minutes or more before control division, furrows do not appear in the operated cell. The results imply that the necessary interaction between the mitotic apparatus and the surface is not completed until 4 minutes before the furrow initiates function, and that thereafter the mitotic apparatus is dispensable. The distance between astral centers and the extent of their radiate structure continue growing after the position of the furrow is established.Experiments performed before the mitotic apparatus becomes dispensable, during the period of its interaction with the surface, revealed that agitation of the poles by vibration and repeated manipulation did not interfere with division when imposed continuously until the furrow appeared. Changing the shape of the poles did not affect cleavage; neither did aspiration of polar cytoplasm and polar portions of asters before both first and second cleavage. Cleavage also occurred despite sequestration of the polar portions of both asters in large‐bore pipettes. Repeated skewering of the mitotic apparatus along the length of its midline often altered its appearance but did not prevent division.