Defense system by mesenchyme cells in bipinnaria larvae of the starfish, Asterina pectinifera.

Here we characterize starfish larval mesenchyme cells, in terms of not only their phagocytic behavior, but also their structural and functional properties as a defense system. Our study reveals the following: (1) most mesenchyme cells construct a dynamic network structure beneath the body wall; (2) mesenchyme cells phagocytically respond to almost all foreign materials and form syncytial aggregates to conceal relatively large amounts and large sizes of foreign material; (3) the morphologies of the syncytial aggregates differ from one another depending on the species and the surface configuration of the cellular foreign material; (4) no mesenchyme cells respond to live mesenchyme cells even though they phagocytose chemically fixed cells; (5) mesenchyme cells phagocytose both cellular constituents effluxed from the ectodermal cells and foreign materials taken into the blastocoel through the body wall. Together, these results suggest that mesenchyme cells are equipped with a spectrum of abilities to engage in a defense system in starfish larva.