If the sandwich explant is properly made, it should allow only planar signaling. Dixon and Kintner (77) tested the effect of vertical signals in addition to planar signals by making standard "Keller" explants and adding head/PM to their inner, anterior surfaces (see Fig. 5 in ref. 77). In this case, they found that neural tissue, assayed by expression of several markers, was induced poorly when only vertical apposition of tissues was allowed. Planar apposition was more effective, but the strongest response came when vertical apposition was coupled with planar apposition, implying that a synergistic effect of the two signaling routes exists.
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