The peptide growth factor Activin A has been shown to induce complete axial structures in explanted blastula animal caps. However, it is not understood how much this response to activin depends upon early signals that prepattern the ectoderm. We have therefore asked what tissues can be induced in blastula animal caps by activin in the absence of early dorsal signals. Using whole-mount in situ hybridization, we compare the expression of three neural markers, N-CAM, En-2 and Krox-20 in activin-treated ectoderm from control and ventralized embryos. In response to activin, both normal and ventralized animal caps frequently form neural tissue (and express N-CAM) and express the hindbrain marker Krox-20. However, the more anterior marker, En-2, is expressed in only a small fraction of normal animal caps and rarely in ventralized animal caps; the frequency of expression does not increase with higher doses of activin. In all cases En-2 and Krox-20 are expressed in coherent patches or stripes in the induced caps. Although mesoderm is induced in both control and ventralized animal caps, notochord is found in response to activin at moderate frequency in control caps, but rarely in ventralized animal caps. These results support the idea that in the absence of other signals, activin treatment elicits hindbrain but not notochord or anterior neural tissue; and thus, the anterior and dorsal extent of tissues formed in response to activin depends on a prior prepatterning or previous inductions.

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