Sulphation-desulphation of a membrane component proposed to be involved in control of differentiation in Volvox carteri.

In the developing asexual embryo of Y&XX carteri the differentiation into somatic and reproductive cells is seen at the division from 32-64 cells. At this stage, 16 cells of the 32celled embryo undergo unequal cleavage, forming 16 large reproductive cells [ 1,2]. In sexual embryos developing under the influence of an inducer the differentiating cleavage is delayed in the male strain up to the fmal division, which usually occurs at the 128or 256celled stage ]2]. We have proposed a model that explains the counting mechanism which tells a cell that the embryo is in the 2-, 4-, 8-, 16., 32-or 64celled stage [3]. This model is able to predict correctly the very regular spatial arrangement of the reproductive cells (gonidia) within the developing V&ox embryo. A necessary assumption in the model is the existence of a membrane component which mediates celltocell contacts. The pool of this membrane component would be exhausted at a sharply defined stage of division since the number of cell contacts increases exponentially during cell division. This event was assumed to signal differentiation. A first set of experiments supporting this model appeared in [4]. We have studied the developing embryo for components meeting the fo~o~ng criteria : 1. The component in question should be a membranebound (glyco-)protein. 2. Its cellular level should correlate with the developmental program. 3. Its production should be (reversibly) inhibited by chemicah known to (reversibly) disturb the mechanism of gonidial differentiation. Here we report the existence of a sulphated membrane glycoprotein with an extremely high turnover rate. We postulate that this ~ycoprotein is involved in the control of differentiation.