Zebrafish vertebral column development. Lateral view (a) of 4.4 mm TL zebrafish (Danio rerio) vertebral column. In black are the mineralized structures, demonstrating the formation of centrum 1 (C1) and the presence of C3 to C5. (b) At 5.5 mm TL caudal fin vertebrae formation with broad base origin can already be seen, represented by U1, and anterior vertebrae also develop, displaying a ring-shaped pattern of mineralization. In grey are the cartilaginous structures, including the caudal fin modified haemal arches and the anterior C3-C5 neural arches (black arrowhead). (c) At 6.0 mm TL PU3 appears, followed by the last vertebral body to form in the vertebral column, PU2, formed around 6.7 mm TL. At this stage, most anterior vertebral bodies already show bone formation around the notochord sheath. The Weberian apparatus is already well differentiated. Further abbreviations: C – centrum; con – concha scaphium; Hy1-5 – hypurals 1 to 5; in – intercalarium; os – os suspensorium; PHy – parhypural; PU1++U1 – compound centrum preural 1 and ural 1; PU2-3 – preurals 2 and 3; rc – roofing cartilage; sc – scaphium; tr – tripus; U2+ – ural 2; (+) sign indicates vertebral elements that are the product of fusion events. Scale bar: 1 mm.