Self/non-self recognition in sponges

A variety of procedures have been utilized to study the sponge ability to d iscriminate self- from non-self. In adult sponges, the histocompatibility r eaction has traditionally been tested by transplantation techniques or para biosis. These consist of pushing into contact different species (xenograft) or conspecific individuals (allograft) with their outer pinacodermal layer s intact. A fast reaction can be observed when fragments of the inner parr of the sponge are tied together. Lack of compatibility generates signals su ppressing or activating both exopinacocytes bounding the contact surface an d discrete mesohyl cell types (archaeocytes, collencytes, gray and spherulo us cells). These cells are involved to different extents according to the s pecies (active migration along the contact surface, cytotoxicity, phagocyto sis, synthesis of a collagen barrier). In sponge cell aggregates, species-s pecific sorting-out expresses the recognition reaction of the mixed cells. Glycoconjugates are key molecules for self/non-self cell discrimination. In the model derived from Geodia cydonium cells, galectin molecules in the pr esence of Ca2+ farm a bridge between the intercellular multiprotein complex aggregation factor (AF) and the cell surface-associated aggregation recept or (AR). The AF is a soluble and diffusible macromolecule that may play an important role also in self recognition mechanisms occurring in sponges in toro. A new paradigm has recently been suggested for cell recognition: inte ractions between the most peripheral cell surface specific proteoglycans (g lyconectins). These molecules have been purified from three marine sponge s pecies (Microciona prolifera, Halichondria panicea, and Cliona celata). Thi s mechanism of self recognition could represent a remarkable event for the appearance of multicellularity.