Does RNA pass from mesenchyme to epithelium during an embryonic tissue interaction?

Chick and mouse embryonic lung mesenchyme were incubated in vitro with an equimolar mixture of (12)C- and (15)N-labeled adenosine, guanosine, cytidine, and uridine, and trace amounts of the four tritiated ribonucleosides. This permits an unambiguous method for detecting transfer of macromolecular RNA from such a pre-incubated mesenchyme to responding lung epithelium across a Millipore filter, and for purifying any RNA transferred for further characterization. During the important period of mesenchymal support of epithelial morphogenesis, no detectable transfer of RNA was found. The level of sensitivity of these experiments was such that less than 0.01% of the labeled RNA in the mesenchyme could have been detected in the epithelium, corresponding to less than 75 labeled RNA molecules transferred to each epithelial cell. No detectable transfer of DNA from mesenchyme to epithelium was found during the inductive period, using the same approach, although with much lower sensitivity.