Extracellular matrix (ECM) components in a very primitive multicellular animal, the dicyemid mesozoan Kantharella...

One of the most vital synapomorphic properties of metazoans is the presence of an extracellular matrix (ECM), i.e., a complex of proteoglycans, adhesive glycoproteins, and collagens. The genetically controlled ECM mediates between the respective receptors morphogenesis and cell differentiation and is central to gastrulation, i.e., the process which generates the embryonic germ layers, upon which all metazoan body structures are based. However, the primitive metazoans include a phylum, viz. the Dicyemida, which lacks any kind of typical metazoan ECM structures including a basement membrane, and hence does not develop through gastrulation. Since the ECM components fibronectin, laminin, and type IV collagen, all of which are essential constituents of each basement membrane, have been proved to be evolutionary ancient molecules from the lowest metazoans up to vertebrates, antibodies against the respective vertebrate ECM components were employed by electron microscopy to look for these molecules also in the dicyemid mesozoan Kantharella antarctica. As a result, all three protein families showed an immunolabel which was localized intracellularly and intimately associated with the cell membranes as well as with the submembranously arranged delicate filamentous network. The immunolabel was most intense in the fibronectin-like protein, followed by the type IV collagen-like protein and weakest in the laminin-like protein. From an evolutionary point of view, this kind of distribution of ECM components, primarily found in intracellular regions, seems to reflect a very primitive situation of the structures of the respective ECM molecules having not yet reached their definitive position outside the cell, thus generating the complete biological function of typical ECM. Moreover, these results confirm the long-standing presumption that the dicyemid mesozoan body structure might be the missing link between the Protozoa and Metazoa.