Extracellular matrix in some microturbellarians

Electron microscopy was used to study the ECM (extracellular matrix) in the epidermis, brain, and parenchyma of the catenulid turbellarians Stenostomum leucops leucops (Dugès) and Stenostomum leucops aquariorum Luther and of the proseriates Archiloa unipunctata (Fabricius) and Promonotus schultzei Meixner. Specimens were fixed with a tannic-acid fixation that enhances staining of extracellular proteins (TARI method) or a conventional electron microscopical method. Differences between species were found in the amount of matrix and extracellular fibrils. In S. leucops leucops the ECM was very sparse, consisting of only a thin electron-dense sheath beneath the epidermis and surrounding neuropil. These sheaths were not found in S. leucops aquariorum which had only rarely encountered extracellular spaces. In P. schultzei and A. unipunctata ECM was more abundant. A. unipunctata had a distinct sheath surrounding the neuropil and a fibrillar subepidermal membrane; the same membranes in P. schultzei appeared only as patches of ECM. In A. unipunctata hemidesmosome-like structures attached the epidermal cells to the underlying membrane, and from these attachments, fibrils projected to the underlying muscle layer. The TARI method revealed exocytotic activity in the brain. The morphology of ECM in these and other flatworms indicate that it could be an internal medium for transport and exchange of substances among cells. Variability in the structure of ECM among species probably does not reflect evolutionary relationships.