Figure 1. Two modes by which the dorsal mesoderm might signal the dorsal ectoderm to become neural. On the left is a sagittal section of an early gastrula. It is oriented with the animal pole to the left and the vegetal endoderm (v) to the right. The site of involution (star) is indicated by elongated bottle cells (coloured blue). The dorsal mesoderm may signal the adjacent ectoderm (coloured orange) within the plane of the surface tissue (arrow). On the right is a sagittal section of a midgastrula. The involuted mesoderm may signal the overlying ectoderm (coloured orange) in a vertical direction (arrowheads). bc, blastocoele.

Figure 2. Culturing amphibian embryos in a high concentration of salts causes gastrulation movements to proceed outward, thus preventing vertical signalling between the mesoderm and ectoderm (coloured orange). On the top left is a normal gastrulating embryo. The arrow depicts the normal involution movement of the dorsal mesoderm. On the top right is a similar stage embryo in which the mesoderm moves outward in response to high extracellular salt concentrations. On the bottom is the resulting exogastrula, in which an ectodermal cap (coloured orange) forms next to the elongated mesodermal axis and vegetal endoderm (v). bc, blastocoele.

Figure 3. A Keller explant prevents vertical interactions between the dorsal mesoderm and adjacent ectoderm. Left, at early gastrula stages cuts (arrows) are made at the anterior edge of the presumptive neural ectoderm (coloured orange) and at the bottle cells (coloured blue) marking the site of mesoderm involution. Right, this midline piece is placed in culture and covered with a glass coverslip (coloured purple) to keep the tissue flat.

Figure 4. Establishing the regional identity of the neural tube may occur in a two-step process. First (depicted on the left), initial signalling transmitted either through the plane of the surface tissue (purple arrow) or via vertical interactions (red arrow) induces the presumptive neural ectoderm (coloured orange) to express a primarily anterior fate (F, forebrain). Next (depicted on the right), graded signals from the posterior mesoderm (P) establish a gradient of neural fates, with spinal cord (SC) being induced at the high end of the gradient. As tissue lies further from the source of signalling, successively more anterior fates are acquired. H, hindbrain; M, midbrain; F, forebrain.

Figure 5. The molecules involved in the initial step of neural induction are depicted on the surface of the vegetal pole of an early gastrula. BMP on the ventral side of the embryo induces ectoderm to become epidermis. In the organizer region, just dorsal to the dorsal lip of the blastopore (curved line), several BMP-binding proteins are expressed (Ce, cerberus; Ch, chordin; Fs, follistatin; Ng, noggin; Nr, Xnr-3). Their inhibition of BMP-signalling along the dorsal midline allows that ectoderm to express a neural fate. The relative roles of these organizer molecules in distinct aspects of neural induction are under investigation.