Supernumerary lumbar rib: manifestation of basic alteration in embryonic development of ribs.

Supernumerary ribs (SNR) are a common variant in some strains of mice used in standard teratology bioassays. We have previously demonstrated that increased incidence of SNR may be induced by a wide variety of xenobiotics and/or general maternal stress. The significance of this defect in cross-species extrapolations has been problematic and recent studies, including this one, have shown that this anomaly is more complex than previously thought. The SNR in mice have a bimodal distribution composed of ‘rudimentary ribs’ (RR) with a mode of 0.3–0.4 mm and ‘extra ribs’ (ER) with a mode of 0.9–1.1 mm. The studies reported here examine the relationship between the presence of SNR and the 13th rib length and the gross morphological development of the anomaly. Supernumerary ribs were induced in CD-1 mice by surgical stress (subcutaneous micropump implanted on gestational day (GD), restraint stress (GD8), food and water deprivation (GD8) or maternal administration of the pesticide dinoseb (50 mg kg−1 on GD7 and GD8). Fetuses from untreated litters were also examined. Dinoseb-treated mice were killed on GD14, 15, 16 or 17. All other groups were killed on GD17. The lengths of the 13th and 14th ribs were measured and other anomalies were recorded. Femur length was used as an indicator of fetal size. The SNR frequency was higher in all treatment groups compared to controls. We found that ER and RR were morphologically distinct. The ER were flat ended and distally joined by a cartilaginous portion, while RR were usually rounded distally and were without cartilaginous extensions. The 13th ribs were significantly longer in fetuses having SNR than in those not having SNR, whether treated or untreated. This relationship was present in all fetal ages examined and with both ER and RR groups. These results suggest that SNR are indicative of basic alterations in the development of the axial skeleton.