Allograft rejection, autograft fusion and inflammatory responses to injury in Callyspongia diffusa (Porifera; Demospongia).

Sponges exhibit a variety of swift, cellular defence responses to protect self integrity. The sponge Callyspongia diffusa has been used to characterize the cytological changes that occur during allograft rejection, autograft fusion, and inflammation. Allogeneic contact results in fusion of the two exopinacoderms followed by an infiltration of mesohyl cells into the graft zone. As mesohyl cells accumulate, they form tissue bridges that span the graft interface. After a few days, the tissue bridges and the nearby cellular infiltrate become necrotic and slough off, which separates the allogeneic tissues. Autograft fusion begins similarly but cellular infiltration does not follow exopinacoderm fusion. Contacted exopinacocytes are redistributed, the endopinacoderms and choanosomes come into contact, and the grafted sponge tissues merge. Tissue damage exposes internal regions of the sponge to the external environment. In many areas of injury, exposed choanosome is sealed by infiltrating mesohyl cells. In other areas, exposed endopinacoderm appears to serve as new exopinacoderm. Cellular debris is removed by phagocytic archaeocytes and new exopinacoderm is regenerated over the damaged choanosome. No scars remain once the inflammatory infiltrate has dispersed. In general, mesohyl cells are involved in defence responses without an observed enrichment of any specific cell type. However, archaeocytes from rejecting sponges appear to line both sides of the allogeneic interface.