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Segmental relationship between somites and vertebral column in zebrafish

Elizabeth M. Morin-Kensicki*, Ellie Melancon and Judith S. Eisen{dagger}

Institute of Neuroscience, 1254 University of Oregon, Eugene, OR 97403-1254, USA
* Present address: Department of Genetics, Campus Box 7264, University of North Carolina, Chapel Hill, NC, 27599-7264, USA



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  		Fig. 1. Models of resegmentation during formation of vertebrae from somites. (A) Primary elements of the axial skeleton include the centrum (c) or vertebral body, which develops around the notochord (n); neural arches (na), paired dorsal elements that together with the dorsal margin of the centrum enclose regions of the neural tube (nt); hemal arches (ha), paired ventral elements that extend from the centrum adjacent to midline blood vessels (bv); and ribs (r), which extend ventrolaterally from the centrum or hemal arch. Viewed from anterolateral with anterior towards the left and dorsal towards the top. Resegmentation models (B,C) describe development of vertebrae from somites. Views of a generic vertebrate from the dorsal aspect. (B) Compartmental resegmentation model proposes that somites (S) are comprised of anterior (A) and posterior (P) halves. Each vertebra (V) develops by recombination of posterior half-sclerotomes from one somite pair with anterior half-sclerotomes from the next posterior somite pair (yellow arrows). In this strict model, half-sclerotome derivatives are restricted to one-half of the vertebra as well. This model resembles the contribution of parasegmental anterior and posterior compartments to segments in fly integument, in which patterning is maintained by lineage restriction (reviewed by Dahmann and Basler, 1999Go). (C) Leaky resegmentation model in which sclerotome cells from one somite (S) contribute to (yellow arrows) two adjacent vertebrae (V) in a manner that is not strictly dependent upon the A or P domain of origin in the somite.

 


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  		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.

 


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  		Fig. 3. Mesenchymal cells derived from ventral somite contribute to vertebrae. Cells of the ventromedial cell cluster of each somite migrate dorsally and spread along the AP axis (Morin-Kensicki and Eisen, 1997Go). (A) Shown is a single cell (arrow) labeled at 18 h and the distribution of its progeny (arrows) at 24 h (B) and 48 h (C). Arrowheads mark myotomal boundaries. (D,E) Serving as an early marker for vertebral column components, {alpha}-coll2a1 is localized to anterior neural arches (arrows) by 10 d in the side view in D (compare with Fig. 2C) and in transverse section (E). (F,G) Progeny derived from a single, labeled ventromedial cluster cell of an anterior trunk somite contributed to vertebral components when visualized at 10 d. (F) Cells visualized with an antibody to fluorescein;. The arrow indicates a cell that is incorporated into a neural arch and the arrowheads indicate cells in other regions of the developing vertebra. (G) The same section as in F labeled with Xylenol Orange to reveal developing bone; the cells indicated in F are also positive for this bone marker. Asterisk in E marks pigment cells. n, notochord; nt, neural tube. Scale bar: 10 µm in A; 15 µm in B,D,F,G; 20 µm in C; 35 µm in E.

 


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  		Fig. 4. Segmental patterns of somites and vertebral column are out of register. A region of the trunk of a 15 d zebrafish stained to reveal vertebral (arrows) and myotomal (broken line) boundaries. Viewed from the side with anterior towards the left, dorsal towards the top. Myotomes retain the original segmental pattern of the somites. Adjacent to the axis, a single myotome spans the posterior three-quarters of one vertebral centrum and the anterior quarter of the next posterior centrum. Scale bar: 45 µm.

 


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  		Fig. 5. Relationship of somites or myotomes and vertebral column in zebrafish. To determine the alignment of the somite series with the vertebral column, future myotome cells in somite 5 (S5) were labeled during the first day of development and then visualized in conjunction with Alizarin Red staining of the vertebral column at 12 d as shown schematically in A. Cells originally labeled in S5 were found in the fifth myotome (M5) at 12 d (arrows in B). M5 was adjacent to vertebra 2 (V2) and vertebra 3 (V3), as shown schematically in A and in a representative fish in B. The resulting relationship between somite and vertebral alignment is shown in a schematized dorsal view in C.

 


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  		Fig. 6. Schematic representation of relationships between somites, anterior expression boundaries of some Hox genes and vertebrae. Anterior expression boundaries of zebrafish Hox genes [names according to Amores et al. (Amores et al., 1998Go)] are primarily as in 1van der Hoeven et al. (van der Hoeven et al., 1996Go) 2Prince et al. (Prince et al., 1998aGo) and 3Sordino et al. (Sordino et al., 1996Go). Differing anterior boundaries indicated by stripes have been reported (Sordino et al., 1996Go; Prince et al., 1998aGo) for hoxa10b. Zebrafish hoxc6 homologs, with anterior expression boundaries at somite 5, align with the transition between cervical and thoracic (rib-bearing) vertebrae as in chick and mouse (Burke et al., 1995Go). By contrast, the anterior expression boundaries of zebrafish Hox paralog group 9 genes fall within the anterior thoracic domain, as apposed to Hox paralog group 9 alignment with the thoracic/lumbar transition zone in chick and mouse (Burke et al., 1995Go). The hoxd12a anterior expression boundary is the posterior-most described and yet also falls within the region that will contribute to rib-bearing vertebrae. Posterior somites are indicated by stripes because the resolution of total somite and vertebral numbers remains unclear.

 

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© The Company of Biologists Ltd 2002