Differential cell adhesion, a suggested guiding force for tissue rearrangement during embryogenesis, could be affected by desmosome frequency. A model system for studying embryonic tissue-positioning behaviour involves combining different tissues and following their rearrangements. We have previously shown that for one tissue, embryonic chick heart ventricle, direction of tissue positioning can be altered experimentally. Heart tissue precultured for 2.5 days tends to segregate internally, while tissue pre-cultured for just half a day tends to segregate externally. Also, intact fragments of tissue tend to segregate internally, while reaggregates of trypsin-disaggregated tissues tend to segregate externally. We show here that treatments that increase the tendency to internalize also increase the frequency of adherens junctions and treatments that increase the tendency to externalize decrease the frequency of junctions. An identical hierarchical ordering of the 4 experimental tissues occurs with respect to positioning behaviour and desmosome frequency. In the hierarchy, 2.5-day-cultured fragments greater than 2.5-day-cultured reaggregates greater than 0.5-day-cultured fragments 0.5-day-cultured reaggregates, tissues to the left tend to segregate internally and to have more desmosomes. Tissues to the right segregate externally and have fewer desmosome. This is what is expected if desmosome are organelles for adhesion and if differential adhesion is a factor in tissue-positioning behaviour.

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