Animal pole cells (AC) and vegetal pole cells (VC)
dissociated from early Xenopus gastrulae
were intermingled, and the cell sorting process occurring within the aggregate
was analyzed. The overall process of cell sorting was found to morphologically
consist of two steps, “concentrification” and “polarization”, as designated
here. First, AC and VC clusters emerged at random positions in the aggregate,
and the individual clusters gradually assembled themselves by 5 hours in
culture (5 hC), forming a concentric arrangement, in which the AC cluster was
enveloped by the VC cluster. This concentrification step is essentially
consistent with the descriptions in earlier studies. As the next step, the AC
and VC clusters moved up and down from 7.5 to 12 hC, resulting in
the vertical polarization, namely, a serial array just like in vivo. Immunohistochemical analyses
showed that AC expressed both C- and E-cadherins, while VC only expressed
C-cadherin, as in vivo, suggesting
the normal participation of cadherin system. On the other hand, the actin
localization showed that the actin bundles accumulated at the edge of the AC
cluster until the concentrification was completed, and gradually decreased
during the polarization step. Another important finding was that AC cluster
could generate cartilage tissues during the long-term (7 days) culture,
evidence for a healthy inductive interaction between the AC and VC. Taken
together, the present experimental system allows the AC and VC to be viable and
grow into an embryo-like organization.