Dependence of oral development and cleavage on cell size in the ciliate Stentor

Abstract

AbstractIn Stentor, the two main cytoplasmic events of cell division are development of the oral primordium that gives rise to the oral apparatus of the posterior daughter cell and cleavage. The present work examines directly the dependence of these two events on the size of the cell body. Cell body size was reduced by various amounts with a microneedle during early stages of cell division and the effects on oral development and cleavage observed. The oral primordium was always resorbed and cleavage never occurred when the cell body was reduced to one quarter of its original size at Stage 3 of cell division; appropriate controls showed that this result was not caused by operation injury. Progressively smaller reductions in cell size yielded correspondingly lower percentages of inhibited cells. These data suggest that the mechanisms underlying the dependence of oral development and cleavage on cell size do not involve accumulation of regulator substances up to a critical concentration during interphase. The fact that oral development can be reversed by reducing cell body size supports the hypothesis that oral development and cell division are initiated when the oral apparatus becomes disproportionately small in relation to cell body size as a result of interphase growth. The size‐dependence of cleavage is probably best understood as a mechanism for regulating cell size in Stentor.