Patterning in hydra cell aggregates without the sorting of cells from different axial origins.

Aggregates of Hydra cells were studied to find out how the primary centers that form new heads are generated in a system of cells in which the original pattern has been destroyed. Since cells that originate near the heads (apical cells) temporarily maintain a high level of head activation potential during aggregate formation and may contribute to pattern formation, their distribution in the aggregates was investigated. The mutual distances between labeled epithelial cells were followed by vitally staining apical cells with DAPI. The distribution was random during early regeneration stages (3, 6, 24 hr). These results show that epithelial cells originating from apical regions do not sort. That is, dynamic cell movement to generate rudiments of new heads is not necessary for head formation in aggregates. A possible explanation of the mechanism is discussed.