The fate of early embryo tissues transplanted to adult hosts

THE EVIDENCE for and against the presence in cells of embryo and foetal mice of transplantation antigens, the cell-bound substances provoking the rejection of allogeneic tissue homografts, rests on two classes of experiments: those relying upon serological techniques for the detection of the antigens, and those involving some test of transplantation antigen activity in intact animals. Experiments of the former class have indicated that there are no histocompatibility antigens present on the cells of most newborn or late foetal mice, but that these antigens quickly appear during the first 2 days of neo-natal life, rendering cells liable to agglutination or lysis (Pizarro, Hoecker, Rubenstein & Ramus, 1961; Moller, 1961<2, 19616). However, Moller (1961a) was able to show that cells of newborn mice resistant to cytotoxic isoantisera did absorb the antibodies of these sera, suggesting that the tissues of newborn mice contain transplantation antigens, but that these antigens are not present on cells in sufficient numbers to make them susceptible to agglutination or lysis. This suggestion was borne out by later experiments (Moller, 1963) in which transplantation antigens were demonstrated in 13-day embryos by injecting lethally irradiated liver cells of these embryos into allogeneic adults and measuring the injected animals' production of humoral antibodies against the grafted tissues. Experiments in intact animals have demonstrated transplantation antigens in embryo tissue indirectly, in terms of grafted animals' subsequent responses to adult tissue grafts of the embryo donor strain. These responses may either be depressed, when embryo tissues are grafted to neo-natal hosts inducing tolerance (Billingham & Silvers, quoted by Medawar, 1959), or elevated when