Sponge cell motility: A model system for the study of morphogenetic processes

Sponges offer a wide repertory of motile phenomena linked to morphogenetic processes. Owing to cell aggregation properties, these organisms have been used for in vitro studies of cell behaviour. Investigations on cytoskeletal structures showed that the actomyosin system is the main component controlling cell shape and motility. After sponge cell coalescence, a functional organism is built anew through cell differentiation and sorting out depending on differences in the motility and adhesiveness of cells. In vitro nonrandom cell motion gives insights into the mechanisms regulating cell positioning in tissue reorganizations. The role of extracellular matrix in cell displacement and positioning is emphasized by the close correlation between cell migration and collagen synthesis. This is particularly evident in such processes as gemmulogenesis, larval settlement, somatic growth and graft rejection. Tissue contractions and cell coupling allow a certain degree of functional integration compensating for the absence of a neuromuscular system. Sponges never achieve a full development as seen in other Metazoa, since differentiated cells maintain potentials typical of embryonic stages. Such a dynamic pluricellular organization could be related to the development of anatomical structures in the absence of a definite body plan.