Post‐nodal Mesoblast Caudalizes the Host Axis and Inhibits Cell Population Growth, and Induces New Primitive Streaks in Chick Embryos

One to eight post‐nodal fragments (PN) or Hensen’s nodes (HN) from full primitive streak stage chick embryos were transplanted onto the area pellucida or area opaca of stage 4 embryos and cultured for 20 h. Thirteen morphological and numerical parameters were affected in the host embryos and analyzed by multiple logistic regression for parametric hierarchy. In the area pellucida, both PN and HN transplants inhibited cell population growth while only PN caudalized the host axis and induced supernumerary primitive streaks expressing the mesoderm‐specific gene Brachyury. In the area opaca, neither grafts influenced host axis morphogenesis, but PN inhibited the cell population growth significantly. Tracking [3H]TdR labeled grafts showed that PN cells migrated towards the host axis and participated in the formation of supernumerary somites and hearts. When placed near the host axis, PN caudalized it and inhibited cell population growth.