Fetally and Neonatally Induced Immunologic Tolerance

For embryologists working in the first half of the twentieth century, the most challenging problem was to differentiate between the embryo and its component parts. It was understood that the transplantation of tissues to genetically different adults was a failure, and because it was thought important for the tissue to mature in a developmentally similar environment. The hosts were either amphibian larvae or chick embryos atthe same level of development as the graft. Although, the importance of the immune system in graft rejection was not appreciated at that time, with hindsight these studies clearly demonstrated the immunologic immaturity of both amphibian embryos and larvae and avian embryos suggested that a small proportion of such grafts could survive into adulthood. Evidence from the transplantation of embryonic chick tissues to chick embryos is equally challenging; it was a widely used method for studying limb and feather differentiation, the migration of pigment cells from the neural crest, and the formation of feather color patterns. Others were bold enough to transplant xenogeneic tissues to chick embryoswith remarkable success.