- Supernumerary ribs in developmental toxicity bioassays and in human populations: incidence and biological significance. [Review]J Toxicol Environ Health B Crit Rev 2004 Nov-Dec; 7(6):437-49JT
- Supernumerary or accessory ribs (SNR), either lumbar (LR) or cervical (CR), are a common finding in standard developmental toxicology bioassays. The biological significance of these anomalies within the regulatory arena has been problematic and the subject of some debate. In rodents, the spontaneous incidence of SNR is species and strain related and ranges from <1% to >30%. Compound-induced LR ar…
Supernumerary or accessory ribs (SNR), either lumbar (LR) or cervical (CR), are a common finding in standard developmental toxicology bioassays. The biological significance of these anomalies within the regulatory arena has been problematic and the subject of some debate. In rodents, the spontaneous incidence of SNR is species and strain related and ranges from <1% to >30%. Compound-induced LR are induced by a wide variety of chemical and physical agents when pregnant animals are exposed during specific gestational periods. A significant portion of the agent-induced LR may be due to maternal factors, as it has been shown that stress alone will induce LR in rodents. SNR are not isolated phenomena and signify basic alterations in the architecture of the axial skeleton. LR are associated with longer ribs, increased numbers of vertebrosternal ribs, and the presence of extra presacral vertebrae ("anteriorization"). CR are associated with reduced numbers of vertebrosternal ribs (posteriorization). It is evident that SNR are not a single anomaly, but consist of two unrelated structures: an extra rib that has a cartilaginous segment at the distal end, and an ossification site that lacks cartilage. These have a bimodal size distribution, with the population of extra ribs being significantly longer than the ossification sites, and 0.6 mm can be used as an approximate length for distinguishing the two populations in mice. Extra ribs are permanent structures in contrast to ossification sites that disappear postnatally, probably becoming part of the lateral transverse vertebral processes. SNR are also found in humans although, in contrast to laboratory species, CR are more commonly noted. SNR are associated with adverse heath effects, and CR with inducing thoracic outlet disease characterized by diminished blood flow and altered position of the ganglia and nerve roots in the area of the C7-T1 vertebrae. LR are associated with lower back pain and L4-5 degeneration. The incidence of CR is greatly reduced in adult humans as compared to fetuses, and it has been hypothesized that fetal "SNR" may be largely composed of ossification sites that disappear postnatally. The mechanisms involved in the formation of extra ribs are not understood at this time, although the fact that the early sensitive periods for their initiation during embryogenesis is coupled with the associated changes in the axial skeleton argues for their induction being due to fundamental changes in gene expression. The sum of the experimental evidence supports the idea of SNR being composed of two different structures: extra ribs that are permanent dysmorphological structures that may be induced by xenobiotics and/or maternal stress, and ossification sites that may be transient variations in the formation of the lateral processes of the vertebrae.
- Axial skeletal and Hox expression domain alterations induced by retinoic acid, valproic acid, and bromoxynil during murine development. [Journal Article]J Biochem Mol Toxicol 2003; 17(6):346-56JB
- Retinoic acid (RA) alters the developmental fate of the axial skeletal anlagen. "Anteriorizations" or "posteriorizations," the assumption of characteristics of embryonic areas normally anterior or posterior to the affected tissues, are correlated with altered embryonal expression domains of Hox genes after in utero RA treatment. These "homeotic" changes have been hypothesized to result from alter…
Retinoic acid (RA) alters the developmental fate of the axial skeletal anlagen. "Anteriorizations" or "posteriorizations," the assumption of characteristics of embryonic areas normally anterior or posterior to the affected tissues, are correlated with altered embryonal expression domains of Hox genes after in utero RA treatment. These "homeotic" changes have been hypothesized to result from alterations of a "Hox cod" which imparts positional identity in the axial skeleton. To investigate whether such developmental alterations were specific to RA, or were a more general response to xenobiotic exposure, CD-1 pregnant mice were exposed to RA, valproic acid (VA), or bromoxynil (Br) during organogenesis. Additionally, the expression domains of two Hox genes, Hoxa7 and Hoxa10, were examined in gestation day (GD) 12.5 embryos obtained from control, RA, VA, or Br, treated gravid dams exposed on GD 6, 7, or 8. The anterior expression boundary of Hoxa7 is at the level of the C7/T1 vertebrae and that of Hoxa10 is at L6/S1. Compound-induced changes in the incidence of skeletal variants were observed. These included supernumerary cervical ribs (CSNR) lateral to C7, 8 vertebrosternal ribs, supernumerary lumbar ribs (LSNR) lateral to L1, extra presacral vertebrae, and the induction of vertebral and/or rib malformations. RA and VA administration on GD 6 caused posteriorization in the cervico-thoracic region (CSNR) while GD 8 exposure to any of the three compounds resulted in anteriorizations in the thoraco-lumbar area (LSNR and an increase in the number of presacral vertebrae). These effects occurred across regions of the axial skeleton. Analysis of gene expression demonstrated changes in the anterior boundaries of Hoxa7 expression domains in embryos treated on GD 6 and 8 with RA. VA and Br did not induce any statistically significant alterations in Hoxa7 and none of the compounds caused alterations in Hoxa10 expression domains. The studies indicate that RA GD 6 treatment-induced Hoxa7 shifts were rostral (posteriorization) while the RA-induced GD 8 anterior expression boundary shift was caudal (anteriorization), correlating with the axial skeletal changes noted. These data suggest that xenobiotic compounds such as VA and Br may induce similar axial skeletal changes by affecting different components of the developmental processes involved in the patterning of the axial skeleton.