VESICLE FORMATION AND ECDYSTEROID-INDUCED CELLULAR-DIFFERENTIATION IN THE EPITHELIAL-CELL LINE FROM CHIRONOMUS-TENTANS

The epithelial cell line from the midge Chironomus tentans grows in multicellular vesicles (Wyss, 1982) of 100 up to 1000 mum, depending on the culture conditions. These vesicles are formed by a cell monolayer. In contrast to the cyst formation in vertebrate cells (McAteer et al., 1988) the vesicles originate from single cells by cell division and not by aggregation. The cells are arranged in a polar fashion: microvilli are found only at the outside of the vesicle (apex-out vesicles). The same types of cell junction, which are present in insect epithelia, are observed in Chironomus cells: zonulae adhaerentes, septate junctions, spot desmosomes and gap junctions. The intrusion of heavy dye molecules like Ruthenium red/OsO4 into the lumen of the vesicles is prevented by septate junctions. After addition of 20-OH-ecdysone to preformed vesicles the simple low squamous epithelial cells change into columnar and cuboidal cells which show a stratified or a random arrangement. This is accompanied by an increase of the number of microtubules, which are sometimes arranged in bundles. They are frequently orientated along the main axis of the cells.