Rethinking Embryonic Germ Layers

Abstract

One of the central concepts we teach in developmental biology is that a major early decision made by the embryo is how to allocate cells to the three germ layers: ectoderm (epidermis, neural), mesoderm (muscle, cardiovascular), and endoderm (gut, liver). Using an in vivo lineage-labeling strategy based on random activation of a nonfunctional lacZ (β-galactosidase) gene in the mouse embryo, Tzouanacou et al. show that while the surface ectoderm (epidermis) is set aside early, a bipotential neuromesodermal cell population produces both neural and muscle cells as the body continues to grow out during the somitogenesis stages. This nicely challenges our simple assumptions about how the germ layers are established in early development while providing the basis for a mechanism that coordinates allocation of cells to the spinal cord and the muscles as the embryonic body elongates. This PaperPick refers to Redefining the Progression of Lineage Segregations during Mammalian Embryogenesis by Clonal Analysis by E. Tzouanacou, A. Wegener, F.J. Wymeersch, V. Wilson, and J.F. Nicolas, published in September 2009. Video Abstract Elena Tzouanacou and Val Wilson discuss their interests in mammalian cell lineages and how they came to identify one such lineage that violates a strict assumption of germ-layer segregation, as described in their Developmental Cell paper. They also mention recent work that suggests how the behavior of this cell lineage may be regulated to give rise to both mesodermal and neural cell types.