Edge cell migration in the extraembryonic mesoderm of the chick embryo. An experimental and morphological study.

The expansion of the extraembryonic mesoderm was investigated in chick embryos of 2 and 3 days incubation with special regard to the mesodermal edge cells. These cells are lying immediately distal to the sinus terminalis and have the shape of migrating cells. By SEM examination they appear to be linked together to form a uniform edge which extends numerous spike-like filopodia. The shape of these filopodia corresponds to their microtubule pattern, as shown by immunofluorescence staining. Filopodia contain strong bundles of microtubules. By in vivo observation at high magnification, the migration of edge cells was demonstrated, and the results of SEM and immunofluorescence studies could be confirmed. By local application of cytochalasin D, distal to the region of the sinus terminalis, the migration of edge cells was inhibited selectively. Subsequent to the inhibition of migration, the expansion of the mesoderm stopped although the interstitial growth of the mesoderm in drug-treated regions remained unaffected. Thus the edge cells have a promotor function in the expansive growth of the extraembryonic mesoderm. The proliferating mesoderm, located proximally to the edge cells, has no expansive tendency of its own. The selectivity of the cytochalasin effect was checked by examination of the phalloidin stained actin pattern. Furthermore, by in vivo observations at low magnification and by transplantation of endoderm from quail to chick it could be confirmed that the extraembryonic mesoderm spreads out invasively between ectoderm and endoderm separating the two sheets. The promotion of this invasion can be regarded as an additional function of the edge cells. An expansion of the mesoderm can also be observed after endoderm removal. In regions freed from endoderm the mesoderm expands faster than in adjacent regions still covered by endoderm. There is no promoting influence of endoderm on mesodermal expansion. On the contrary, expansion itself is facilitated, when the conditions for invasion are abolished by removing the endoderm.