Figure 4

Modifications of the musculature during the blastogenetic cycle. Bud development (A-I; M), take-over (J-L), and larval phases (N, O), shown by: ISH on sections (A-F; J; K; M-O), phalloidin -FITC (H), -TRITC (I), and TEM (L). A) Stage 6; signal in the intersiphonal region (BsMA2; arrowheads). (os), oral siphon. Dotted arrows: epidermis. Scale bar = 50 μm. B) Stage 7; myoblasts (arrows) around the rudiment oforal siphon (os). Scale bar = 50 μm. C-F) Stage 8; mantle fibres between oral (os) and atrial (as) siphons (C, ISH for BsTnT-c). Fibres run along the dorsal lamina (arrows, E, F) and through the recto-oesophageal trabecula (arrows, D, F). Arrowheads (C, E, F): fibres around the siphons. (bs), branchial sac;(st), stomach; (sb), secondary bud. Scale bar = 50 μm (C, D); scale bar = 0.1 mm (E, F). G) Myoblasts adhere to mantle epithelia (e). Asterisks: contractile material. Scale bar = 1.2 μm. H) Stage 8; phalloidin -FITC evidences thin mantle fibres (arrows) and muscle bundles in the adult (arrowheads). Scale bar = 0.1 mm. I) Two zooids approaching the opening of oral siphons (os). Arrows: muscle fibres. Scale bar = 0.1 mm. J, K) Zooids during regression. A decrease (compare Figures 2D-H) in the expression level of BsMA2 (J) and BsTnT-c (K) is recognisable. Fibres (arrows) assume an irregular aspect. Testis (ts; J) is not marked. (os), oral siphon; (en), endostyle. Scale bar = 0.1 mm. L) During regression, muscle fibres (mf)form cytoplasmic protrusions. Arrows: organelles in degeneration. Scale bar = 0.8 μm. M) The heart (arrow) of bud gives a signal only with BsMA2. (st), stomach. Scale bar = 25 μm. N, O) Early larvae. In the striated caudal muscles (arrows) no signal for BsTnT-c (N) and BsMA2 (O) expression. ISH of BsMA2 labels mesenchymal cells (arrowheads). Tunic is around ampullae (am) and cephalenteron and stains non-specifically. Scale bar = 50 μm.