A molecular marker for cell guidance information in the axolotl embryo

Previous studies from this laboratory suggested that the elongation of the pronephric duct (PND) in the axolotl Ambystoma mexicanum is directed by an adhesion gradient along the migrating cells' substratum. We have also shown that cranial neural crest (CNC) cells are able to follow the PND guidance information, for which these cells serve as useful probes (S.L. Zackson and M.S. Steinberg, (1986) Dev. Biol. 117, 342-353). These experiments allow the construction of a map of the cell guidance information. This map is presumed to reflect a molecular prepattern representing the distribution of a cell guidance associated molecule (CGAM) responsible for the ensuing pattern of cell migration. We refer to this proposal as the molecular prepattern hypothesis. In this paper we describe and identify a candidate CGAM displaying a localization pattern corresponding closely with our map of the PND/CNC guidance information on the embryonic flank. This candidate CGAM is also found to be abundant on the posterior neural tube, an embryonic region not previously explored for PND/CNC guidance information. The latter observation has provided the opportunity for an independent test of the correlation between the presence of this molecule in an embryonic region and the ability of that region to direct cell migration. We have found that grafted CNC cells do indeed migrate upon the strongly labeling posterior neural tube in preference to the neighboring poorly labeling presomitic mesoderm. We identify this candidate CGAM as the cell surface enzyme alkaline phosphatase. Possible roles for alkaline phosphatase in directing embryonic cell migrations are discussed.