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First published online May 12, 2005
doi: 10.1242/10.1242/dev.01812

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Structure and function of the notochord: an essential organ for chordate development

Vertebrate Development and Genetics, Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SA, UK

View larger version (119K): [in a new window]   Fig. 1. A comparison of notochords. (A) At 24 hours post fertilisation (hpf), the zebrafish notochord (red) occupies a considerable volume of the embryo. (B) This is especially apparent in cross-section, where the area of the notochord is nearly equal to that of the neural tube. (C) By contrast, in an embryonic day (E) 9 mouse embryo, the notochord is proportionally smaller, (D) taking up a cross-sectional area of only a fraction of the neural tube. (A) Lateral view of live 24 hpf zebrafish embryo. (B) Trunk-level cross section of a 24 hpf zebrafish embryo. (C) Three dimensional reconstruction of an E9 mouse. White line indicates level of section shown in D, with notochord highlighted in red. (C,D) Adapted, with permission, from the Edinburgh Mouse Atlas Project (Baldock et al., 2001; Brune et al., 1999).

View larger version (48K): [in a new window]   Fig. 2. Separable organiser activities. (A,B) The zebrafish dorsal organiser is the embryonic shield (between the arrowheads), and is shown in (A) lateral and (B) animal-pole views. (C) Deep organiser tissue expresses gooscoid (gsc), while (D) superficial tissue expresses floating head (flh) mRNA. (E) By separating deep (grey) and superficial (red) organiser tissues and grafting the individual pieces, superficial tissue was found to induce a secondary axis that possessed only trunk and tail (F), while deep tissue induced a secondary axis that possessed only a head (G). (F,G) Embryos are stained for sonic hedgehog expression by in situ hybridisation. Arrows indicate (F) anterior limit of secondary axis and (G) posterior limit of secondary axis. Adapted, with permission, from Saúde et al. (Saúde et al., 2000).

View larger version (41K): [in a new window]   Fig. 3. Stages of notochord development affected by mutations. The transition from dorsal organiser to chordamesoderm is affected by mutations in several genes. At the top of the dorsal specification cascade are ichabod and ß-catenin proteins (Kelly et al., 2000). In their absence, ventral types of mesoderm can form but not chordamesoderm. The bozozok and floating head loci each encode homeodomain-containing proteins that are necessary for chordamesoderm specification. In the complete absence of oep, as in the maternal-zygotic (Mz) mutant, mesendoderm is not specified, including the chordamesoderm. The laminins (bashful, grumpy and sleepy), the coatomers (sneezy, happy and dopey), brachyury (no tail) and one unknown locus, doc, are all required for proper notochord differentiation. Arrowheads indicate (A) chordamesoderm progenitor region of the shield, (B) the chordamesoderm and (C) the differentiated notochord.

View larger version (66K): [in a new window]   Fig. 4. Structural aspects of the notochord. (A) A lateral view of a living zebrafish tail at 24 hpf, showing the main features of the notochord. Dorsal to the notochord is the floor plate, in the ventral-most part of the forming spinal cord. Ventral to the notochord is the hypochord. (B) A schematic diagram of lateral and cross-sections of the notochord, showing the floor plate and hypochord acting as cables running along the top and bottom of the notochord. (C) As well as the notochord, the floor plate and hypochord express type II collagen (Yan et al., 1995). cc, central canal; fp, floor plate; hy, hypochord; no, notochord; nt, neural tube.

View larger version (145K): [in a new window]   Fig. 5. Peri-notochordal basement membrane. (A) The tri-laminar structure of the peri-notochordal basement membrane (between the arrowheads) shown using electron microscopy, from the sheath of a wild-type notochord cell. i, inner layer; m, medial layer; o, outer layer. (B) This structure is disrupted in the sheath of a dopey mutant notochord cell, where the outer and medial layers are missing. The inner layer is likely to consist of laminin as (C) wild-type levels of laminin immunoreactivity are clearly seen in (D) sneezy mutant embryos, but are abolished in (E) sleepy mutants.