During gastrulation, mesendoderm originates in the primitive streak (PS) where cells undergo an epithelial-mesenchymal transition and an expression switch from E- to N-cadherin. We made measurements of these processes during differentiation of human pluripotent stem cells to PS and downstream mesendoderm subtypes using established protocols and variants in which signaling through key pathways, including activin, BMP and Wnt, were modulated. The anterior-to-posterior identity of cells within the PS had little impact on the subsequent differentiation potential but impacted the degree of cadherin switching. During switching, E-cadherin downregulation and N-cadherin upregulation were uncorrelated and had different dependence on signaling. The exception to the broad potential of cells was the loss of definitive endoderm potential in mid-to-posterior PS. Thus, cells induced to different PS coordinates had similar potential within the mesoderm but differed in cadherin switching. Consistent with this, E-cadherin knockout or overexpression did not alter outcomes during differentiation. Overall, although all processes are regulated by the same set of signaling pathways, the extent of cadherin switching and epithelial-mesenchymal transition can vary substantially within cells adopting the same cell fate.
Zhu et al. (Wed,) studied this question.