The formation of mesoderm and mesectoderm in presomite rat embryos cultured in vitro, using WGA-Au as a marker.

Presomite rat embryos cultured in vitro were injected with the cell marker wheat germ agglutinin-gold in order to find out whether the ectoderm already formed mesodermal cells. These labelled so-called mesectodermal cells were found in all embryos studied, ranging in age from 8.7 to 9.3 days post coitum. In embryos younger than 9.0 days, the entire head fold ectoderm produced mesectodermal cells. From 9.0 days onwards, the neural crest and surface ectoderm placodes were recognizable as separate entities, both producing mesectodermal cells. The early onset of mesectodermal cell formation and the numerous and continuous manufacture led us to the conclusion that mesectodermal cells are deposited at their definitive location and that subsequent long-distance migration is unnecessary.