Formation of the Anterior-Posterior Axis in Mammals
Résumé
Reciprocal interactions between extra-embryonic and embryonic tissues are required to establish the anterior-posterior axis of the mouse embryo. A clear morphological manifestation of anterior-posterior asymmetry in the embryo is the onset of gastrulation in the posterior region. Induction of the primitive streak depends on Nodal and Wnt signaling, and is modulated by the Anterior Visceral Endoderm. Before gastrulation, migration of the Distal Visceral Endoderm is the symmetry-breaking event, which defines the anterior pole of the mouse conceptus. Proximal-distal regionalization of the visceral endoderm precedes migration of the Distal Visceral Endoderm. The Distal Visceral Endoderm, which constitutes a heterogeneous population of cells, is induced by Nodal and restricted by Bmp signaling. The movement of the Distal Visceral Endoderm is driven by active cell migration and cell intercalation, accompanied by a global movement of visceral endoderm (VE) cells. The direction of the movement is controlled by Nodal and Wnt signaling. The observations in the mouse model may not fully apply to other mammalian species. The anterior visceral endoderm (AVE) has an equivalent in the rabbit and other vertebrate species. However, the equivalent of the extra-embryonic ectoderm is unclear.