Cell sorting out according to species in aggregates containing mouse and chick embryonic limb mesoblast cells

Aggregates were prepared containing mouse and chick embryonic limb mesoblast cells. After 21 to 33 hours in culture, these aggregates showed prominent sorting out of the mouse cells from the chick cells. This sorting out was even more extensive after 46 to 54 hours. This sorting out is contrary to previous reports for mouse‐chick limb aggregates. After three to four days of culture, chondrification became quite prominent within the aggregates. Accompanying this event, the segregation of mouse and chick cells became less extensive within the chondrified zones. This rescrambling seems to be due at least in part to the interpenetration of neighboring chondrifying regions. Such interpenetration would be expected, due to the expansion which occurs within chondrifying areas. This reshuffling is the probable reason why sorting out was not observed in earlier experiments with mouse‐chick limb aggregates. In these earlier studies, the reports were limited to aggregates which had extensive matrix secretion. The finding that mouse and chick limb mesoblast cells sort out from each other, together with recent reports that similar sorting out occurs in mouse‐chick heart ventricle aggregates and in mouse‐chick mesonephric aggregates, invalidates the generalization that cell sorting properties for any one tissue are constant from species to species among warm‐blooded vertebrate embryos.