Figure 9

Differentiation of tentacle longitudinal muscle cells in a muscle specific transgenic (MHC::mCherry) reporter line. F-actin was stained with phalloidin (green), mCherry with an α-RFP antibody (red). A-G Optical longitudinal sections of mid-planula stages (4–5 d) in the future tentacle bulb region. A Overview. B-D Maximum projection of six layers. An early undifferentiated tentacle muscle cell (labelled by the α-RFP antibody) spans throughout the ectodermal epithelium and already shows a weak accumulation of actin filaments (D, inlet). E-G Maximum projection of seven layers. Tentacle muscle cells lose connection to the apex of the epithelium. Actin filament accumulation at the base of the epithelium increases in several cells (G, arrows). H-N Optical longitudinal sections of early primary polyp stages (7 d). Maximum projection of 21 layers. Different stages of detachment of tentacle longitudinal muscles from the epithelium co-occur at the same stage (J, long arrow: epithelial muscle cell, short arrow: submerged muscle cell). Actin filaments accumulate in several cells before mCherry becomes visible (K, arrows). L-N Columnar retractor muscle cells expressing mCherry. O-Q Optical longitudinal sections of primary polyps (13 d). Maximum projection of 53 layers. Numerous actin filaments are enriched in the outgrowing tentacle bulbs (Q, arrow). Tentacle longitudinal muscles have lost their connection to the apex of the epithelium and become situated at the base of the ectoderm, increasing their length with tentacle elongation (P, arrow).