Neural Induction in the Mouse Embryo.
A. The epiblast of the early embryo exists in a pre-neural state. As gastrulation initiates, the early primitive streak forms in the posterior embryo, with the early gastrula organizer (GO) anterior to the primitive streak. The GO, together with the anterior visceral endoderm (AVE), induces the expression of anterior neural markers in the distal region of the epiblast (ectoderm region).
B. By mid-streak stages, the AVE has migrated anteriorly, the primitive streak has elongated, and early neural markers are expressed in the anterior half of the embryo. These markers represent specification of the neural tissue.
C-D. By E7.5, the anterior neural precursor cells migrate anteriorly, to become juxtaposed with the AVE that prevents them from acquiring posterior character. In addition, the prechordal plate, a node derivative, migrates anteriorly from the node to underline the midline of the anterior neural plate and maintains the anterior neural identity. The transformation to the more posterior types of the neural tissue occurs in response to signals provided by the notochord.
E. By E8.5, neural induction is mostly complete and the neural plate begins to form a tube, in a process called neurulation. The neural tube is comprised of the forebrain (FB), midbrain (MB), hindbrain (HB) and spinal cord (SC). The prechordal plate and the notochord underlie the neural tube and are required for its maintenance.
F. By E10.5, the neural tube is completely closed.
Abbreviations: AVE, Anterior Visceral Endoderm; GO, Gastrula Organizer; FB, Forebrain; MB, Midbrain; HB, Hindbrain; SC, Spinal Cord.