Abstract
Cardiac tissue in the bird is derived from paired regions of lateral mesoderm within the anterior half of the embryo (Rawles [1943] Physiol. Zool. 16:22-42; Stalsberg and DeHaan [1969] Dev. Biol. 19:128-159). Previously, we reported that WNT11 is expressed in early avian mesoderm in a pattern that overlaps with the precardiac regions. To examine whether this molecule may play a role in promoting cardiogenesis, we cultured tissue explants from microdissected HH stage 4, 5, and 6 quail embryos. The isolated tissue consisted of both the mesoderm and endoderm layers from either anterior precardiac or posterior noncardiogenic regions of the embryo. As a necessary control for examining the ability of WNT11 to convert noncardiogenic mesoderm to cardiac tissue, we compared the cardiogenic potential of anterior and posterior regions. For stages 5 and 6, our results were consistent with what has been previously reported (Rawles [1943] Physiol. Zool. 16:22-42; Sugi and Lough [1994] Dev. Dyn. 200:155-162); as anterior mesoderm becomes contractile, while posterior mesoderm does not produce cardiac tissue. Surprisingly, when we examined stage 4 embryos both anterior and posterior regions gave rise to cardiac tissue in culture. To determine whether WNT11 could promote cardiac differentiation in tissue that was noncardiogenic, this molecule was ectopically expressed or added to mesoderm/endoderm explants obtained from stage 5 or 6 posterior tissue. Transfection of stage 5 posterior tissue with a WNT11 expression plasmid provoked the appearance of cardiomyocytes in 33% of the explants; half of which were contractile. Similarly transfected stage 6 posterior explants did not demonstrate cardiac differentiation. More dramatic results were obtained when noncardiogenic tissue was exposed to conditioned media containing soluble WNT11; as 63% and 33% of posterior stage 5- or stage 6-derived explants underwent cardiac differentiation. Together, these results indicate that WNT11 can promote cardiac development within noncardiac tissue. The expression of WNT11 in anterior mesoderm of early gastrula stage embryos suggests it may play a role in the formation of the vertebrate heart. Dev Dyn 1999;216:45-58.
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