Fig. 1. Schematic representation of possible routes for Nodal signal transmission from the node to LPM in the mouse embryo. (A) Schematic transverse section of an E8.2 embryo showing endoderm (green), mesoderm (gray), ectoderm (orange), node (yellow) and Nodal-expressing (dark blue) cells. Internal (red arrow) or external (blue arrow) potential routes of Nodal signal transmission are indicated. (B,C) Two hypothetical mechanisms of Nodal signaling from the node (blue square) to LPM (blue hexagon). Nodal, red; type I and type II Activin receptors, green and purple; Cryptic, pink. In the direct transport model (B), Nodal produced at the node travels directly to LPM, where it is captured by Cryptic and induces Nodal expression. In the indirect signal-relay model (C), Nodal produced at the node binds to Cryptic and induces the expression of downstream gene products (X₁, X₂, X_n) that relay the Nodal signal to induce Nodal expression in LPM.