Effect of length of aggregation time upon sorting-out behavior of cells from chick embryo tissues

The timing hypothesis of Curtis proposes that cells which go through a sequence of types of behavior at different rates sort out from one another in aggregates. In order to further test this hypothesis we have given cells from one chick embryo tissue a head start in aggregating before adding cells from a second tissue. By such experimental manipulation the normal position of cells in an aggregate should be reversed, according to predictions from the timing hypothesis. When heart ventricle cells were allowed to aggregate 6,12, 20, or 22 hours before addition of neural retina cells, the aggregates all showed internal heart cells surrounded by neural retina cells. The same final positions of heart and neural retina were found in aggregates in which neural retina cells started aggregating 4, 6, or 22 hours before addition of heart cells. Control aggregates, with heart and neural retina dissociated and co-aggregated simultaneously, also showed heart internal and neural retina external. No effect of length of aggregation time could be detected with this pair of tissues. When pigmented retina cells were allowed to aggregate 6 or 20 hours before addition of heart cells, the cells were in the same final positions as in control aggregates, namely heart external and most pigmented retina cells internal. The only position reversal occurred when heart cells were given 6 or 20 hours to aggregate before addition of pigmented retina cells, which now took up all external positions. This position reversal could result from the heart cells becoming more adhesive with time in culture.