Are morphogenetic tissue interactions mediated by transmissible signal substances or through cell contacts?

Abstract

TRANSMISSION of morphogenetic signals between embryonic cells is a special form of intercellular communication which may involve diffusible signal substances^1,2 or intercellular contacts^3,4. We have previously presented evidence that the induction of metanephric kidney tubules is dependent on cell contacts between the interacting tissues rather than on diffusible signals^6,7. Several embryonic and neoplastic tissues possess an inductive effect when tested against metanephric mesenchyme^8–11. If different inductors release an extracellular, transmissible signal substance, the restriction of induction by interposed filters with varying pore sizes should be independent of the type of inductor tissue, assuming that there is no major difference in size between signal substances. If cell contacts and thus ingrowth of cytoplasmic processes into the filter are required, however, there may be differences between the transfilter inductive capacity of various inductor tissues depending on their ability to send cytoplasmic processes through the filter. That is now shown to be the case, in a comparison of the spinal cord and the mesenchyme of embryonic salivary gland as inducers of metanephric mesenchyme.