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. 2011 Jun 28;2:373.
doi: 10.1038/ncomms1355.

Identification of Vertebra-Like Elements and Their Possible Differentiation From Sclerotomes in the Hagfish

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Free PMC article

Identification of Vertebra-Like Elements and Their Possible Differentiation From Sclerotomes in the Hagfish

Kinya G Ota et al. Nat Commun. .
Free PMC article

Abstract

The hagfish, a group of extant jawless fish, are known to lack true vertebrae and, for this reason, have often been excluded from the group Vertebrata. However, it has yet to be conclusively shown whether hagfish lack all vertebra-like structures, and whether their somites follow developmental processes and patterning distinct from those in lampreys and gnathostomes. Here we report the presence of vertebra-like cartilages in the in-shore hagfish, Eptatretus burgeri. These elements arise as small nodules occupying anatomical positions comparable to those of gnathostome vertebrae. Examination of hagfish embryos suggests that the ventromedial portion of a somite transforms into mesenchymal cells that express cognates of Pax1/9 and Twist, strikingly similar to the pattern of sclerotome development in gnathostomes. We conclude that the vertebra-like elements in the hagfish are homologous to gnathostome vertebrae, implying that this animal underwent secondary reduction of vertebrae in most of the trunk.

Figures

Figure 1
Figure 1. The putative vertebral elements of E. burgeri.
(a) Lateral views of whole-mount Alcian blue-stained adult hagfish. Anterior is to the left. (b) This high-magnification view of the caudal fin area shows small cartilaginous nodules (the putative vertebral elements) located on the ventral side of the notochord at the post-cloacal level (arrowheads). More posteriorly, dorsal and ventral fin rays are connected with median cartilaginous plates, previously called 'median dorsal and ventral bars'. (ce) Transverse views of paraffin sections of E. burgeri stained with haematoxylin and eosin and Alcian blue. (c) The putative vertebral elements (arrowheads) are located ventral to the notochord surrounding the dorsal aorta at the post-cloacal region. (d) Higher magnification of the box shown in c. (e) The median dorsal bar is attached to a pair of cartilaginous arches on both sides of the notochord at the caudalmost level (arrow). ao, dorsal aorta; cl, cloaca; fr, fin ray; mdb, median dorsal bar; mu, mucus gland; mvb, median ventral bar; n, notochord; nsh, notochordal sheath; nt, neural tube; ve, vein. Scale bars, 1 cm (a); 1 mm (b); 100 μm (ce).
Figure 2
Figure 2. Comparison of axial skeletal elements.
(af) Transverse and horizontal sections of a whole-mount hagfish (E. burgeri), a lamprey (Lethenteron japonicum) and a catshark (Galeus nipponesis). (a) A transverse view of E. burgeri. (b) Ventral view of a horizontally sectioned E. burgeri specimen. (c) Transverse view of L. japonicum. (d) Ventral view of a horizontally sectioned L. japonicum specimen. (e) Transverse view of G. nipponesis. (f) Ventral view of a horizontally sectioned G. nipponesis specimen. The levels of the horizontal sections in each specimen are indicated by arrows in a, c, and e. The ventral view of the horizontally sectioned E. burgeri (b) shows an asymmetric and non-metameric distribution of cartilaginous nodules along the anterior–posterior axis. Myosepta are indicated by broken lines. The dorsal view of the sectioned lamprey (d) shows dorsal vertebral elements segmentally arranged in register with myomeres. The ventral view of the horizontal section of the shark (f) shows a clear metameric distribution of the hemal arches. These horizontal sections of the lamprey and the shark show that two cartilaginous nodules are located between two myosepta along the anterior–posterior axis (d, f). ao, dorsal aorta; ce, centrum; fr, fin ray; hem, hemal arch; n, notochord; nsh, notochordal sheath; nt, neural tube; ve, vein. Scale bars, 1 mm.
Figure 3
Figure 3. Three stages of E. burgeri embryos.
(ac) Whole-mount cleared embryos of E. burgeri: (a) dorsal view of a 121 dpd embryo (early pharyngula); (b) dorsal view of a 145 dpd embryo (middle pharyngula); (c) lateral view of a 150 dpd embryo (late pharyngula) with tentacles and laterally flattened tail. (df) High-magnification view of the caudal regions: (d) early pharyngula embryo showing an epithelial somite; (e) middle pharyngula embryo; (f) late pharyngula embryo. Dotted lines indicate the levels of sections shown in Figure 3a–j. mu, mucus gland; nt, neural tube; som, somite; tent, tentacles. Scale bars, 1 mm (ac); 100 μm (df).
Figure 4
Figure 4. Somite derivatives of E. burgeri.
(af) Early pharyngula. (a) Epithelial somite in the caudalmost region of the trunk. (b) Twist is strongly expressed in the ventral somite. (c) Pax1/9 is weakly expressed in the median ventral somite. (d) Sagittal section showing rostrocaudally migrating putative sclerotomal cells at the anterior level of the trunk. (e, f) Transverse sections at the anterior level of the trunk showing migrating putative sclerotomal cells expressing Twist (e) and Pax1/9 (f) (arrowheads). (gj) Middle pharyngula. (g) Mesenchymal cells located at the lateral side of the notochord. (h) Pax1/9 expression by mesenchymal cells. (i) Pax3/7 expression on the lateral side of a somite. (j) MyoD is strongly expressed at the medial side of a somite. (k, l) Late pharyngula. (k) Mesenchymal cells located at the dorsal aorta and the lateral side of the notochord. (l) Clear expression of Pax1/9 is evident in the late pharyngula. (m, n) Schematic summary of the development of putative vertebral elements. (m) Medial ventral somites differentiate into mesenchymal cells (blue) in the early pharyngula. (n) The mesenchymal cells differentiate into the cartilaginous nodules in the adult. ao, dorsal aorta; mu, mucus gland; n, notochord; nt, neural tube; sc, sclerotome; som, somite. Arrows indicate the migrating sclerotomes. Scale bars, 100 μm.
Figure 5
Figure 5. Hypothetical scenario of vertebral evolution.
This phylogenetic tree is based mainly on molecular data and hagfishes are clustered with lampreys as members of monophyletic cyclostomes. The origins of the cartilaginous vertebrae, consisting of dorsal and ventral elements, as well as associated developmental mechanisms are assumed to have been obtained before the divergence of gnathostomes, lampreys, and hagfish. The vertebral elements of the extant vertebrates are coloured blue, and those of the common ancestor of entire extant vertebrates are indicated by dotted lines. bd, basi-dorsal; bv, basi-ventral; id, inter-dorsal; iv, inter-dorsal; n, notochord; nt, neural tube.

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