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The community effect is an interaction among a group of many nearby cells that is necessary for them to maintain tissue-specific gene expression and differentiate co-ordinately. A community interaction is required for the muscle precursor cells of the Xenopus embryo to develop into terminally differentiated muscle, but exactly when and where the community effect acts during myogenesis has not been determined. Here, we ask whether dependence on the community effect varies with the developmental age of the muscle precursor cells. We find that dependence on the community signal changes with time through the muscle precursor cell population. During neurulation muscle precursor cells that are still in the vicinity of the blastopore and that are fated to form posteriormuscle continue to require interactions with their neighbours, while differentiation of the anterior paraxial mesoderm,which gastrulated earlier, is independent of cell contact at this time. Thus the time during which a particular sub-population of muscle precursor cells requires a community interaction is related to their final destination along the anterior-posterior axis. In addition we show that this later acting community interaction around the blastopore involves FGF signalling.
Fig. 1. Anterior and posterior paraxial mesoderm contain
muscle precursor cells that give rise to terminally
differentiated muscle. (A) Posterior dorsal view of stage
15 embryo, with anterior and posterior (circumblastoporal)
tissue dissections outlined in red and blue respectively.
Drawing from Nieuwkoop and Faber (1967). (B) Quantified
data showing the percentage of anterior and posterior cells
expressing XMyoD when analysed by immunohistochemistry
at stages 15 and 20. The pieces were cultured intact
throughout, and only disaggregated immediately prior to
fixation, so that individual cells could be counted. (C) Whole
embryo, anterior and posteriortissue pieces assayed with
the 12/101 antibody at stage 28. All dissected pieces
contain terminally differentiated muscle.
Fig. 2. Community interactions
persist in posteriormesoderm.
(A) Bright field views of cells dissected
at stage 15 and assayed at
stage 20 with an antibody against
XMyoD. On the left are posterior
cells, on the right are anterior
cells. The top row shows cells
that were cultured dispersed from
stage 15, below are late dispersal
controls (dispersed at stage 20,
immediately prior to fixation).
Anterior cells can maintain expression of XMyoD when dispersed from stage 15, while posterior cells cannot. Thus community interactions persist in
the posterior paraxial mesoderm after anterior cells have become independent of their neighbours. (B) Quantified data from one representative assay.
Fig. 3. (Left) eFGF is expressed in the posteriormesoderm between stages 15 and 19. Anterior (A) and posterior (P) explants were analysed by RNase
protection. The expression level of eFGF is shown as a percentage of the cognate FGF-R loading control value beneath each lane. WE 15/20; whole embryo
control, stage 15/20. Posterior pieces express a higher level of eFGF than anterior pieces. eFGF is present in the posteriormesoderm at the stage when
these cells are still dependent on a community interaction if they are to express myogenic genes.
Fig. 4. (Right) FGF signalling is required for posterior cells to express XMyoD. (A) Posterior cell reaggregates express XMyoD. The percentage of XMyoDexpressing
cells is reduced on treatment with 15 μM SU5402. Quantified data from one experiment analysed by immunohistochemistry. Posterior cells are
subject to a community effect for XMyoD expression, and FGF signalling is an essential component of this community interaction. (B) The percentage of
dispersed cells expressing XMyoD is increased on addition of 100 ng/ml eFGF protein to the culture medium, but does not reach control levels. Quantified
data from one experiment analysed by immunohistochemistry.
