Survival of Heterotopic Headfoot Transplants in Biomphalaria glabrata (Mollusca: Pulmonata)

Abstract

The entire headfoot (HF) of small snails, Biomphalaria glabrata, was implanted into the hemocoel of allogeneic adults and examined histologically at 30 and 60 days postimplantation (DPI). Although displaying varying degrees of abnormal morphology, the HF allografts continued feeding motions with the buccal mass and showed muscular contractions of the foot muscle. The transected esophagus fused shut and became distended with amorphous material, possibly ingested and/or secreted substances. The buccal mass grew, and pieces of the exfoliated radular ribbon were found in the esophageal sac. Although most major structures of the HF appeared quite normal at the cell and tissue level, the tentacles and epithelium covering the dorsal and lateral surfaces of the HF apparently were lost when in direct contact with the recipient's hemolymph, but persisted if folded or invaginated below the implant surface. No host hemocytic response was observed at either 30 or 60 DPI. These results demonstrate not only considerable physiological plasticity by the implant but also immunological tolerance to histologically complex allografts by the recipient, and support the hypothesis that allorecognition is poorly developed in molluscs. Additional key words: allografts, histocompatibility, snails, tissue transplantation We recently reported survival, for at least 60 days, of several types of allografts in the pulmonate snail Biomphalaria glabrata (SAY), including digestive gland, kidney, mantle, albumin gland, brain, ovotestis, heart, and amebocyte-producing organ (Sullivan et al. 1998). Although pathological changes were observed in implants, these were hypothesized to result from wound healing reactions and degenerative changes (e.g., due to disuse atrophy, accumulation of secretory products, and the physiological unsuitability of the heterotopic location), rather than from an allograft response by the recipient. In fact, little convincing evidence was found for true allograft rejection in molluscs, and previous reports to the contrary may have been due to experimental technique (Sullivan et al. 1998). In this study we examined the fate of larger and histologically more complex transplants. Specifically, we were interested in determining whether the entire headfoot (HF) of small snails would survive if transplanted into the hemocoel of B. glabrata. If such transplants remained viable, then this method could be used to investigate the longevity of sporocysts of Schistoa Author for correspondence. E-mail: sullivan @universe.uiwtx.edu soma mansoni in resistant snails, i.e., by transplanting the HF from schistosome-resistant snails into susceptible recipients, thus allowing viable sporocysts in the implant to complete development. Because of the numerous tissue types, secretions (e.g., mucus, saliva), and relatively large bulk of the HF, we anticipated that such transplants might be problematic. However, the entire implant, as well as its component tissues, survived remarkably well throughout the observation period, with no evidence of rejection by the recipient.