Cellular aspects of allograft rejection in marine sponges of the genus Polymastia

Allograft rejection processes of two taxonomically related sponge species, Polymastia robusta and Polymastia mamillaris, have been studied at the cellular level. Grafts in parabiosis have been made by pairing papillae. They were investigated using histology and scanning and transmission electron microscopy. In Polymatia robusta, allografts undergo progressive rejection with adhesion. A few days after grafting, archaeocytes and collencytes migrate toward the zone of contact and fall into line on either side of this zone. The narrow space which separates the grafted papillae is progressively filled up with collagen fibers thanks to which the parabionts are firmly held together. Allografts of Polymastia mamillaris never fuse to each other; the specimens are completely and directly rejected. In this species, the cell migration to the zone of contact is not followed by their alignment nor by deposition of a collagen layer. Autografts made in similar conditions succeed in all cases in both species. These experiments demonstrate for the first time that sponges, even taxonomically closely related, may develop different but defined processes of rejection.