FIGURE 13.

Skeletogenesis and myogenesis in the Paracentrotus lividus adult rudiment. Developmental stages are as follows: (A) primary podia twisting stage (PTwist); (B–D,F–I,K,L,Q) primary podia papilla stage (PPap); (E,J,N,S) mature rudiment stage (MRud); (M,O,R) juvenile spine stage (JuvS); (P) primary podia stage (PPodia). In (A–D,F–M,O), images were acquired using bright-field light microscopy. In (E,N), images were obtained using polarized light. In (P–S), images are maximum intensity projections of confocal z-stacks of larvae co-labeled for F-actin (muscles; cyan) and DNA (nuclei; blue), and they correspond to projections of the entire rudiment. (A–E) Close-ups of the skeletal element associated with the definitive spines in top, oral view in (A–D) and in lateral view in (E). (C) Close-up of the region outlined by the yellow box in (B). (F–H) Close-ups of primary podia, in lateral view in (F,G) and in papilla view in (H). (I,J) Close-up of the anlage of the future adult skeletal shell at the surface of the developing adult rudiment, which is in side view. (K–N) Close-up of the skeletal element associated with the juvenile spines in top, oral view in (K–M) and in lateral view in (N). (L) Close-up of the region highlighted by the yellow box in (K). (O) Close-up of the ring canal and of a dental sac in an adult rudiment in side view. ((O) inset) Close-up of the dental sac. (P–S) Close-ups of the developing adult rudiment in oral, deep view in (P–R) and in side view in (S). In (A,B), white arrowheads highlight the developing skeletal spicules of definitive spines. In (A,B,F,O), white asterisks mark the primary podia. In (D,E), the yellow arrowhead indicates the hexaradiate spicule of a definitive spine, and the blue arrowhead marks the hexagonal ring. In (E), green arrows highlight additional skeletal branches emerging from either the hexaradiate spicule or the hexagonal ring. In (F–H), black arrowheads highlight developing primary podia skeletal disks. In (I,J), white dotted lines delineate the anlage of the future adult skeletal shell in (I), and the developing ocular and interambulacral plates in (J). In (K,L), green arrowheads mark the developing triradiate spicules of juvenile spines. In ((O), (O) inset), orange arrowheads highlight a primordium of a developing tooth located within a dental sac. In (O,R,S), white dotted lines outline the ring canal. In (R,S), white arrowheads mark the muscles associated with the radial canals. Scale bar: (A,B,F) 50 μm; (C,D,L,M) 12.5 µm; (E,K,N,P–S) 25 μm; (G,H) 20 μm; (I,J,O) 15 μm; ((O) inset) 7.5 µm. DenS: dental sac; DS: definitive spine; IAmbP: interambulacral plate; JS: juvenile spine; Lp: longitudinal processes; Mes: mesentery; OcP: ocular plate; PP: primary podium; RiC: ring canal; RM: ring muscle; Sb: stereomic bridge.