A COMMUNITY EFFECT IN MUSCLE DEVELOPMENT

Background: Most vertebrate tissues arise by embryonic induction, as a result of which new cell layers are formed. These are subsequently subdivided into discrete groups of homogeneous cell populations, each containing different cell-types with specific gene expression. There is preliminary evidence from previous work that the mesoderm-forming induction in amphibian development may be followed by a further interaction among some of the induced mesoderm cells, and that this could be required for muscle gene activation in uniform cell populations. Results: We have established the existence, time and place of this further cell interaction by transplanting muscle progenitor cells from Xenopus mid-gastrulae into ectoderm sandwiches, and then culturing these constructs until the time of muscle gene activation. We find that cells implanted as reaggregates, but not those implanted as single cells, activate early myogenic genes and later muscle-specific genes. More than 100 cells must be near each other for muscle gene activation. These cells can induce non-muscle mesoderm cells to express muscle genes by emitting a signal that differs from the preceding mesoderm induction signal. Muscle gene activation under these conditions does not require gap junction communication. Conclusion: Cells within the muscle progenitor region of a Xenopus embryo need to interact with each other in order to activate muscle genes in homogeneous cell groups. This exemplifies the 'community effect', which may be a widespread developmental mechanism used to increase the homogeneity within, and demarkation between, embryonic tissues.