The formation of proximal and distal definitive endoderm populations in culture requires p38 MAPK activity

Abstract

Endoderm formation in the mammal is a complex process with two lineages forming during the first weeks of development, the primitive, or extraembryonic, endoderm that is specified in the blastocyst and the definitive endoderm that forms later, at gastrulation, as one of the germ layers of the embryo proper. Fate mapping evidence suggests that definitive endoderm arises as two waves, which potentially reflect two distinct populations. Early primitive ectoderm-like (EPL) cell differentiation has been used successfully to identify and characterise mechanisms regulating molecular gastrulation and lineage choice during differentiation. Using EPL cells and chemical inhibitors of p38 MAPK activity, roles for p38 MAPK in the formation of definitive endoderm have been investigated. These approaches defined a role for p38 MAPK activity in the formation of the primitive streak and a second role in the formation of the definitive endoderm. Characterisation of the definitive endoderm populations formed from EPL cells demonstrated the formation of two distinct populations, defined by gene expression and ontogeny, which were analogous to the proximal and distal definitive endoderm populations of the embryo. Formation of proximal definitive endoderm required p38 MAPK activity and was correlated with molecular gastrulation, defined by the expression of T. Distal definitive endoderm formation also required p38 MAPK activity but could be formed when T expression was inhibited. Understanding lineage complexity will be a prerequisite for the generation of endoderm derivatives for commercial and clinical use.