The technique of back-transplantation was used to investigate the developmental potential of neural crest-derived cells that have migrated to and colonized the avian bowel. Segments of quail bowel (removed at E4) were grafted between the somites and neural tube of younger (E2) chick host embryos. Grafts were placed at a truncal level, adjacent to somites 14–24. Initial experiments, done in vitro, confirmed that crest-derived cells are capable of migrating out of segments of foregut explanted at E4. The foregut, which at E4 has been colonized by cells derived from the vagal crest, served as the donor tissue. Comparative observations were made following grafts of control tissues, which included hindgut, lung primordia, mesonephros and limb bud. Additional experiments were done with chimeric bowel in which only the crest-derived cells were of quail origin. Targets in the host embryos colonized by crest-derived cells from the foregut grafts included the neural tube, spinal roots and ganglia, peripheral nerves, sympathetic ganglia and the adrenals, but not the gut. Donor cells in these target organs were immunostained by the monoclonal antibody, NC-1, indicating that they were crest-derived and developing along neural or glial lineages. Some of the crest-derived cells (NC-1-immunoreactive) that left the bowel and reached sympathetic ganglia, but not peripheral nerves or dorsal root ganglia, co-expressed tyrosine hydroxylase immunoreactivity, a neural characteristic never expressed by crest-derived cells in the avian gut. None of the cells leaving enteric back-grafts produced pigment. Cells of mesodermal origin were also found to leave donor explants and aggregate in dermis and feather germs near the grafts. These observations indicate that crest-derived cells, having previously migrated to the bowel, retain the ability to migrate to distant sites in a younger embryo. The routes taken by these cells appear to reflect, not their previous migratory experience, but the level of the host embryo into which the graft is placed. Some of the population of crest-derived cells that leave the back-transplanted gut remain capable of expressing phenotypes that they do not express within the bowel in situ, but which are appropriate for the site in the host embryo to which they migrate.
Self-organized morphogenesis of a human neural tube in vitro by geometric constraints