Ultrastructural and immunofluorescence features of the epidermal cells and its secretory granules in the amphioxus Branchiostoma lanceolatum L.

Background and purpose: In the vertebrate family, the epidermis of terrestrial animals is keratinized while in aquatic forms and amphibians can produce mucus. Amphioxus, a cephalochordate, is an important animal model in the study of chordate phylogeny. Major cytoskeletal and extracellular matrix proteins in epidermal cells of amphioxus (Branchiostoma lanceolatum L.) were investigated using transmission electron microscopy and immunofluorescence. Materials and methods: Amphioxus specimens were fresh caught in the Adriatic Sea. Tissue samples were fixed and prepared for transmission electron microscopy and immunofluorescence techniques. Results: The epidermis consisted of one-layered columnar epithelium. Epithelial cells contained voluminous nucleus, a well-developed supranuclear Golgi apparatus, abundant vesicles and apical secretory vesicles surrounded by cytokeratin filaments. A single row of smaller vesicles also appeared close to the basal cell membrane. Strong positive immunolocalization of hyaluronic acid and collagen type I was observed in the apical domain of the plasma membrane of amphioxus, respectively. The positive signal of hyaluronan and collagen proteins referred to the secretory granules in the apical domain of the epidermal cell. Strong positive labelling of aggrecan was seen around the cell nucleus, deep below the apical domain, suggesting that aggrecan is not located in the secretory granules. A strong signal of the cytokeratin 10 protein was seen throughout the cytoplasm of the epidermal cells. Conclusion: Due to its morphology and histochemical composition, epidermal cells of amphioxus may contribute to course surface protection and the mucous layer in amphioxus may serve as a physical barrier between the body and its environment.