Ampulla Morphogenesis in Anural and Urodele Molgulid Ascidians: (ascidians/ampulla morphogenesis/contraction rings).

Vascular ampulla morphogenesis was studied in two anural-developing molgulids and one urodeledeveloping molgulid. In all three species, each juvenile developed one longer ampulla (termed the primary ampulla) and several shorter ampullae (termed secondary ampullae). Ampulla growth was accompanied by the formation of contraction rings that moved in a proximal to distal direction. Contraction ring formation was initiated by the epidermal cells situated in the proximal region of each ampulla. The formation of an expanding bubble-like region within each lumen preceded ring formation. These rings moved at approximately 120 μm/min. Rings were produced for several days until the ampullae retracted towards the body wall. In all three species, hemoblast cells were evident at day 4 within the developing mantle sinus (hemocoel). Ampulla growth was studied by positioning chalk particles on the tunic surface near the tips of ampullae. During the growth phase, chalk particles moved towards the body of the zooid. This report is the first to describe contraction ring formation and the polarized transport of tunic components during ampulla morphogenesis in two anural ascidian species.