Expression of tenascin mRNA in mesoderm during Xenopus laevis embryogenesis: the potential role of mesoderm patterning in tenascin regionalization.

In Xenopus embryos, the extracellular matrix (ECM) protein tenascin (TN) is expressed dorsally in a very restricted pattern. We have studied the spatial and temporal expression of TN mRNA in tailbud-stage embryos by RNAase protection and in situ hybridization using a cDNA probe for Xenopus TN obtained by PCR amplification. We report that TN transcripts are principally expressed in cells dispersed around the neural tube and notochord as well as in myotome and sclerotome cells. No TN mRNA could be detected in lateral plate mesoderm, but expression was detectable beneath tail fin epidermis. In a second series of experiments, we studied the expression of TN mRNA and protein in combinations between animal and vegetal stage-6 blastomeres and in stage-8 blastula animal caps treated with activin A or basic fibroblastic growth factor (b-FGF). Isolated animal cap tissue cultured alone differentiates into epidermis, which expresses neither TN protein nor TN mRNA. TN expression is, however, elicited in response to isolated dorsal vegetal blastomeres and in response to high concentrations of activin, both of which treatments lead to formation of muscle and/or notochord. Low concentrations of activin, and ventral vegetal blastomeres, treatments that induce mesoderm of ventral character, are poor inducers of TN. However, b-FGF, which also induces ventral mesoderm, elicits strong expression. These results indicate that TN regionalization is a complex process, dependent both on the pattern of differentiation of mesodermal tissues and on the agent with which they are induced. The data further show that "ventral mesoderm" induced by low concentrations of activin is distinct from that induced by b-FGF, and imply that activin induces ventral mesoderm of the trunk while b-FGF induces posterior mesoderm of the tailbud.