Fine Structural Observations on the Turbellarians Stenostomum sp. and Microstomum lineare with Special Reference to the Extracellular Matrix and Connective Tissue Systems

Employing transmission electron microscopy, observations were made on epidermis, muscle cells and connective tissue systems, with special emphasis on extracellular matrix components (ECM), in two rather primitive turbellarians: Stenostomum sp. (Catenulida) and Microstomum lineare (Macrostomida). In Stenostomum the only ECM components found are basal laminae, predominantly situated subepidermally. In Microstomum ECM is well developed and connective tissue filaments abundant in conspicuous extracellular spaces. It is uncertain whether basal laminae exist. The finding of basal lamina structures as the only ECM component present in Stenostomum makes it now possible to establish a complete ECM and connective tissue hierarchy in turbellarians, ranging from a purely cellular type with no ECM present to systems dominated by ECM and very similar to loose connective tissue in vertebrates. Comparative aspects of ECM and connective tissue systems in turbellarians are discussed in addition to the difficulties and ambiguities regarding definition and nomenclature of basal matrices as basal laminae and subepidermal membranes.