The traditional view of neural induction is that the mesoderm comes to lie beneath the prospective dorsal neural ectoderm and sends signals vertically, inducing the ectoderm to realize its neural fate (Fig. 5A). In the past 10 years, there has been a revival of interest in edgewise or "planar" induction. Planar induction is thought to involve a signal passing from the posterior edge of the Organizer, through the plane of the tissue, into the prospective neural region (see refs. 25,50,52,53,65,76-78) (Fig. 4B). This idea is not new, originally favored by Spemann, but discarded because of Holtfreter's work (79), demonstrating that exogastrulae, which supposedly have planar, but not vertical apposition of the inducing and responding tissues, underwent no obvious neural development. With the revival of interest in planar induction, the traditional route of neural induction is now called "vertical" induction.
Several experimental preparations have been used to study the relative roles of planar and vertical signaling. We will describe these preparations and examine their usefulness and limitations, including difficulties in making them, problems of interpretation, and unknown factors that might make them misleading.
Fig. 6. The method of making a sandwich ("Keller") explant of the dorsal sector of the gastrula is shown (A,B). The dorsal sector of the early gastrula (A) is excised by cutting 45-60° on both sides of the dorsal midline, and across the AC (heavy dashed lines, #1 and #2, respectively). This flap of tissue is then peeled outward, away from any involuted material (open arrow), and cut off where it joins the VE, above or below the BC (#3). Two such explants are trimmed at the edges to match one another, and sandwiched, with their inner, deep surfaces together (B) by placing them between a coverslip fragment and the bottom of the dish. The prospective areas of such an explant are shown, including prospective fore- and midbrain (F, M), hindbrain (RH), SC, endoderm (E), notochord (N), and somite (S). Prospective PM may or may not be included, depending on whether the explant was made early or late in stage 10. After 15 min or so, of healing, the explant is taken from under the coverslip and allowed to develop. Such an explant undergoes convergent extension (C,D) of both the involuting marginal zone (solid arrows) and the prospective posterior neural tissue (broken arrows). The involuting marginal zone differentiates into notochord and somitic tissue, whereas the neural region differentiates into regions that express early markers of the SC, RH, and F and M (12,25,78) (D). If the explant is large enough in the animal direction, cement gland is also formed. In the intact embryo (E), the neural and mesodermal tissues converge and extend in parallel (arrows, E), the later beneath the former, rather than serially, as in the explant (C).
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