On the origin of the mesoderm in the Mexican axolotl, Ambystoma mexicanum

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Abstract
The origin of the mesoderm in the Mexican axolotl, Ambystoma mexicanum, has been the subject of discussion in the literature for some time. Løvtrup proposed that the mesoderm originated from cells that were displaced from the blastocoel roof to the ring-shaped groove surrounding the arching endodermal primordium. Smith and Malacinski later showed that cells which could contribute to the mesoderm were located on the surface of the embryo at the onset of gastrulation, whereas Lundmark showed that both surface-layer and deep cells from late blastula embryos could contribute to the mesoderm. However, neither of the latter studies addressed Løvtrup's proposal that deep cells contributing to the mesoderm originated from the blastocoel roof. In this study, we tested this idea by injecting an lacZ-fusion construct into the upper tier of cells of 32-cell embryos and following the fate of these cells. As a control, third-tier cells, which should include both deep and superficial marginal-zone cells, were also injected with the lacZ construct. Our results show that third-tier cells contribute to the mesoderm, as expected; however, first-tier cells do not, although we do observe labelled cells in the process of dropping into the blastocoel.

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