utocentrum development and chordoblast positioning in the intervertebral space. Lateral view of (a) Toluidine blue stained section shows bone (bo) deposition around the chordacentrum (chc), forming the autocentrum in “ring” vertebrae. (b) TEM micrograph showing the intervertebral region (IV). Osteoblasts (ob) are clearly associated with bone deposition. (c) Autocentrum formation also follows chordacentrum development in caudal fin vertebrae although with apparent less amount of bone and osteoblasts due to the presence of the cartilaginous arches (white arrowheads). (d) TEM micrograph of the intervertebral (IV) region between two caudal fin centra showing the bone associated osteoblasts. (e) Sagittal section through the notochord at the level of the IV space. Once chordacentra are fully defined, as shown by von Kossa staining, chordoblasts (cb) tend to accumulate in the intervertebral region, acquiring an oblong shape, larger size, and positioning in a dorsal-ventral direction. (f) Sagittal section through the notochord at the level of the IV space. At this stage chordoblasts become highly proliferative, as confirmed by PCNA positive staining. Sections from panels a-d are from 11 mm TL fish, while e-f represent an earlier developmental stage with 6 mm and 6.2 mm TL fish, respectively. Further abbreviations: HPU2-3 - Haemal arch of Preural 2 and 3; Hy2 – Hypural 2; nsh – notochord sheath. Scale bars (a, c): 50 μm; (b, d): 10 μm; (e, f): 30 μm.