DORSOVENTRAL POLARITY AND MESENTOBLAST DETERMINATION AS CONCOMITANT RESULTS OF CELLULAR INTERACTIONS IN THE MOLLUSK PATELLA-VULGATA

In mollusks with an equal four-cell stage, dorsoventral polarity becomes noticeable in the interval between the formation of the third and fourth quartet of micromeres, i.e., between the fifth and sixth cleavage. One of the two macromeres at the vegetal cross-furrow then partly withdraws from the surface and becomes located more toward the center of the embryonic cell mass than the other three macromeres. Only this specific macromere (3D) contacts the micromeres of the animal pole, divides with a delay, and develops into the stem cell of the mesentoblast (4d). After suppression of the normal contacts between micromeres and macromeres either by dissociation of the embryos or by deletion of first quartet cells, the normal differentiation of the macromeres fails to appear. By deleting a decreasing number of first quartet cells, an increasing percentage of embryos shows the normal differentiation pattern. Deletion of one of the cross-furrow macromeres does not preclude formation of the mesentoblast, which then originates by differentiation of an other macromere. It is concluded that initially the embryo is radially symmetrical and that the four quadrants have identical developmental capacities; mesentoblast differentiation from one macromere is induced through the contacts of the first quartet cells and that single macromere. © 1979.