Comparison of several types of allografts in Biomphalaria glabrata (Mollusca: Pulmonata).

Interpretation of prior studies on molluscan allografts is complicated by the variety of experimental methods used and recurring reports of acute rejection of digestive gland implants. In this study, allografts of adult digestive gland, saccular kidney, mantle, albumin gland, brain, ovotestis, heart, or amoebocyte-producing organ (APO) were implanted into the hemocoel of Biomphalaria glabrata snails, and their histological condition at 60 days post implantation (DPI) was compared with that of preimplantation tissues. Also, digestive gland tissue, still surrounded by body wall, was implanted from immature donors and examined at 120 DPI. All 60-day allografts maintained some degree of normal cell and tissue structure, and most displayed additional evidence of viability, i.e., contraction (heart), hyperplasia (APO), gametogenesis (ovotestis), secretion (albumin gland, digestive gland, and kidney), shell deposition (mantle), or neuroma formation (brain). Recipient hemocytic reaction was minimal or absent, except that in three of six ovotestis implants and five of seven digestive gland implants, variable numbers of acini were undergoing phagocytosis by hemocytes or had been reduced to granulomas. Among seven implants examined at 120 DPI, digestive gland acini were found in five (with one instance of hemocytic encapsulation). Thus, in this study, acute rejection of allografts did not occur. Although the observed hemocytic reactions could be viewed as evidence of chronic graft rejection, the inconsistent and protracted nature of these responses suggests instead that they represent resorption of degenerating tissue damaged by nonimmunological effects, e.g., mechanical trauma, heterotopic location, or autolysis.