Fig 6.

Atrial primordium fates determined by caged fluorescence. (A) Cartoon of a Ciona juvenile indicating its main anatomical features, including the atrial complex (shaded green). Side view is the orientation depicted for panels C and D. Dashed square on side view indicates approximate region shown in E-G. Dashed rectangle from the front view indicates view for panel H. aa, atrial aperture; gs, gill slits; eso, esophagus; st, stomach; sto, stolon; es, endostyle; os, oral siphon. (B) Cartoon detail of atrial complex showing salient features. Primary gill slits are positioned at the base of the atrium (represented as area within dotted line) and connect the atrium to the underlying pharyngeal cavity. (C) Live juvenile after uncaging in the right atrial primordium at the larval stage. The entire atrium shows fluorescence, including the bright ring marking the atrial aperture (aa). (D) Similar view of another individual after uncaging showing fluorescent label in the atrium walls, aperture, and gill slits. (E) Detailed view of a labeled atrium with punctate fluorescence in the atrium walls. Towards the top, the atrium is constricted at its aperture. (F) A labeled atrium shows that all surfaces enclosing the volume of the cavity are positive for fluorescence (G) Same animal as F, focused on the gill slits/ciliary epithelia (ce) and the tongue bar (tb) at the base of the atrial cavity. (H) Front view of an atrium shows signal in both inner and outer atrial walls with an area of intense fluorescence at the atriopore. Arrowheads indicate the outer trunk epidermis, visible through low-level background fluorescence. (I, J) Schematic showing contributions to the atrium by the placodal epidermis based on lineage tracing. Green depicts lineal contributions of the atrial primordium. Cross section of a simplified atrium (J) depicts topologies at or shortly after gill slit formation.