Fig. 2. Developmental sequence and regional character of zebrafish axial skeleton. Zebrafish skeletal matrix stained with Alcian Blue and/or Alizarin Red at representative stages between 7 d and adult. Anterior towards the left; dorsal towards the top. (A) Matrix foci (arrows) of the anterior-most centra dorsal to the notochord at 7 d. (B) At 8 d matrix of the developing centra form rings around the notochord in anterior trunk. (C) Matrix surrounds chondrocytes (arrows) at the base of the neural arches of vertebrae (V) 3 and 4 visualized here at 13 d. (D-J) The vertebral column is composed of classes of vertebrae based on developmental and morphological features characteristic of AP position. Examples of each category are visualized at 16 d (F,I,J), 21 d (D,E), or 2 years (G,H). (D,F) Cervical vertebrae comprise two unique and relatively small vertebrae with neural arches (arrowheads in D) that are lost in adults. (E-G) The number of rib-bearing vertebrae (arrows indicate ribs in G) was variable. The first two (V3, V4) had distinctly large neural arches (arrowheads in E) and modified ribs. V3-V5, in conjunction with cervical vertebrae (V1, V2) formed the series (F) that later contributes to the Weberian ossicles. Rib- and hemal arch-bearing vertebrae were found in most but not all fish; after 20 d the final one or two ribs sometimes had hemal arch attachment sites (H, arrows). Hemal arch-bearing vertebrae began with a series of three or four progressively longer hemal arches (arrows in I). The tail set (J) included four vertebrae with unique morphology such as a dorsal bend of the penultimate centrum (arrow) and no neural arch on the last vertebra. Centra were also sometimes decorated with dorsoposterior projections (black arrowheads in G,H) and ventroposterior projections (H, white arrowhead) with a characteristic distribution along the AP axis. Scale bars: 15 µm in A-C; 50 µm in D-F,I-J; 400 µm in G,H.