Fig. 1

Synovial-like morphology of jaw and fin joints in ray-finned fish.

(A) Phylogenetic tree contrasts the old model of synovial joint evolution (grey asterisk) with the new model of synovial joints evolving in a common precursor of all bony vertebrates (black asterisk). (B) Alcian Blue-stained adult zebrafish and accompanying diagrams show the pectoral fin joints, and jaw joints in open and closed positions. (C–E, G–I) Sections of 14 dpf (n = 4), 1 mpf (n = 3), and adult (n = 6) zebrafish jaw joints; ray-scapula joint in the adult zebrafish pectoral fin (n = 4); and stickleback (1 mpf, n = 3) and spotted gar (10.2 cm, n = 3) jaw joints. Sections are stained by H&E (C, D, G–I) or trichrome (E). Articular chondrocytes (black arrowheads) line the cavity. (F) Schematic of adult jaw joint shows bone (red), cartilage (blue, lighter shade indicates articular), and synovial membrane (green). (J, K) Magnifications of (I) show the synovial membrane (arrow), fibrous capsule (asterisk) and multilayered articular cartilage (K). Scale bar in h, 100 μm; all other panels, 50 μm. aa: anguloarticular; q: quadrate; sc: scapula; r: ray; pr: proximal radial; dr: distal radial; M: Meckel’s; pq: palatoquadrate; s: superficial; t: transitional; rd: radial layer; c: calcified cartilage; b: bone.