1 2 3 4 5 6 7 8


In the early development of every member of the chordate phylum, somites arise from the dorsal mesoderm. These segments increase in number as the organism grows and are a useful determination of the embryo's age. Somite segmentation has been shown to be dependent on specific cell-cell and cell-matrix interactions during embryogenesis, and these interactions are thought to be regulated by the expression of pattern-forming genes (Barnes et. al., 1996). The formation and patterning of the axial vertebral skeleton is the target of a number of teratogenic agents during embryonic development.

The Pax-1 gene has been shown to be an important regulator of axial skeletal patterning and is specifically implicated in the formation and maturation of the distinct somite sclerotome that gives rise to the axial skeleton as vertebrae, intervertebral discs, ribs, and neural arches (Smith & Tuan, 1995). Researchers determined that Pax-1 gene expression is critical to the normal development of somites in chicks, and has been found to play a similar role in the somites of rats and mice (Vorhees, 1987). The Pax-1 gene is expressed in newly formed epithelial somites as well as sclerotomal cells and remains a target of teratogens that generate anomalies of the axial skeleton.

Valproic acid is an anticonvulsant drug used to treat epilepsy. Since its release for human use in 1967, it has been found to cause a number of congenital birth defects in the children of women who use the drug. Such defects include facial malformations and skeletal abnormalities (Lammer et. al., 1987). Since nearly 1 out of 200 pregnant women is epileptic and a candidate for valproic acid treatment, researchers have aimed to determine which gene was affected by the presence of valproic acid. Recent studies have concluded that the mechanism by which valproic acid causes these birth defects is by down-regulating the expression of the Pax-1 gene (Lammer et. al., 1987). The molecular pathway of valproic acid, as in a large number of pharmaceutical teratogens, currently remains unknown. However one theory suggests that valproic acid acts via a deficiency in the vitamin folic acid, as folic acid deficiency is linked to an increased incidence of neural tube defects in humans (Barnes, et. al., 1996). Specifically, when tested on chicken embryos, valproic acid affected Pax-1 expression, causing the disorganization or fusion of somites (Barnes et. al., 1996). Other studies showed that rats treated with valproic acid demonstrated similar results of fused ribs, missing vertebrae, and kinky tails (Vorhees, 1987). This research demonstrates that the Pax-1 gene is crucial to the normal development of somites in chickens, mice, and rats, and the down-regulation of Pax-1 due to valproic acid will result in somite malformations in all of these organisms (Vorhees, 1987).

Figure 1. Danio rerio embryo at 28 hours with normal development of somites.


© Cebra-Thomas, 2001

Last Modified: 31 May 2001

[Lab Protocols | Students | Cebra-Thomas | Course | Links ]