Fig. 7

Proposed Models and Illustrations. a Maturation of ES cell aggregates coincides with the formation of a surface actin cap prior to differentiation. We discovered that ES cell aggregates mature with further culture and form a polarized surface that can be observed with a polarized actin cap, prior to the initiation of endoderm differentiation. b Mechanism for the reversion of cell positioning with adhesive affinity. When a highly adhesive cell type (such as wild-type ES cells) is intermixed with a less adhesive cell type (such as the E-cadherin deficient ES cells), the highly adhesive cells are sorted to the interior, enveloped by the less adhesive cells. The pattern is predicted by Steinberg’s differential adhesive affinity hypothesis. However, we observed that upon maturation, the highly adhesive cells subsequently form an outer shell, surrounding the less adhesive cells, a pattern contradictory to the differential adhesive affinity hypothesis. The ability for highly adhesive cells to form a polarized surface layer is a likely explanation for the observed cell sorting behavior that contradicts the differential adhesive affinity hypothesis. The process of cell sorting and positioning is illustrated, and 3D depictions of the different sorting patterns are generated based on optical sectioning of the representative spheroids