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WIREs Dev Biol
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Scoliosis and segmentation defects of the vertebrae

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The vertebral column derives from somites, which are transient paired segments of mesoderm that surround the neural tube in the early embryo. Somites are formed by a genetic mechanism that is regulated by cyclical expression of genes in the Notch, Wnt, and fibroblast growth factor (FGF) signaling pathways. These oscillators together with signaling gradients within the presomitic mesoderm help to set somitic boundaries and rostral–caudal polarity that are essential for the precise patterning of the vertebral column. Disruption of this mechanism has been identified as the cause of severe segmentation defects of the vertebrae in humans. These segmentation defects are part of a spectrum of spinal disorders affecting the skeletal elements and musculature of the spine, resulting in curvatures such as scoliosis, kyphosis, and lordosis. While the etiology of most disorders with spinal curvatures is still unknown, genetic and developmental studies of somitogenesis and patterning of the axial skeleton and musculature are yielding insights into the causes of these diseases. WIREs Dev Biol 2012, 1:401–423. doi: 10.1002/wdev.34

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Figure 1.

Skeletal elements of the vertebral column. (a) Lateral view of the vertebral column with cervical, thoracic, lumbar, sacral, and coccygeal vertebrae indicated. (b) Morphological features of a typical human vertebra from superior (top) and left lateral (bottom) views. The zygapophyseal joint is a synovial joint formed between the superior and inferior articular processes of adjacent vertebrae. (c) Lateral view of major ligaments of the spine including the ligamentum flava, supraspinous, interspinous, and anterior and posterior longitudinal ligaments. (Reprinted with permission from Ref 221. Copyright 2010 Springer)

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Figure 2.

Major muscles of the back. (a) The superficial splenius muscles (left) and the erector spinae muscles, including iliocostalis, longissimus, and spinalis (right), are illustrated. (b) The transversospinalis muscles, including semispinalis, multifidus, and rotatores (left), and the levatores costarum, intertransversarii, and interspinales muscles (right) are illustrated. Drawings by Brent Adrian. Reproduced with permission of the Arizona Board of Regents.

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Figure 3.

Developmental stages in somitogenesis in a mouse embryo at E8.5, equivalent to a 22‐day human embryo. Mesp2 expression is visualized by whole‐mount in situ hybridization (left) and boundaries of the presomitic mesoderm and somites are illustrated on the right. Major steps in somitogenesis are indicated on the right. By convention, the forming somite is labeled ‘0’ and the most recently formed somite is +I. The region of unsegmented paraxial mesoderm that will next form somite 0 is labelled −I.

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Figure 4.

An example of phases of cycling gene expression in the presomitic mesoderm (PSM) of a mouse embryo. Phases I–III are defined as described previously.28 The cycling gene expression of the Nrarp gene is shown at E10.5,29 equivalent to 30 days in human development. The diagrams below summarize key features of Nrarp gene expression as it displays oscillatory expression within the PSM. Embryos are oriented with rostral toward the top.

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Figure 5.

Gradients of fibroblast growth factor (FGF) and Wnt signaling (blue) from the caudal presomitic mesoderm together with a retinoic acid gradient (green) from the somites coordinate the positioning of the determination front.

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Figure 6.

Diagram of the mesenchymal to epithelial transition (MET) associated with somite formation from a lateral view.

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Figure 7.

Diagram illustrating the maturation and compartmentalization of somites in birds and mammals (a) compared to teleosts and anurans (b). D, dorsal; V, ventral; M, medial; L, lateral; yellow, dermomyotome; green, myotome; blue, sclerotome; red, syndeotome.

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Figure 8.

Segmentation defects of the vertebrae, including vertebral fusions (a and b), hemivertebra (c), and midline defects (d). Yellow arrowheads point to malformations.

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Figure 9.

Segmentation defects of the vertebrae, including multiple defects observed in congenital scoliosis (a and b), single vertebral fusion (c), and multiple fusions (d) in Klippel–Feil anomaly. Yellow arrowheads indicate sites of malformations.

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Vertebrate Organogenesis > Musculoskeletal and Vascular
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Early Embryonic Development > Development to the Basic Body Plan