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 x enobiotics and/or general maternal stress. The significance of this de fect in cross-species extrapolations has been problematic and recent s tudies, including this one, have shown that this anomaly is more compl ex than previously thought. The SNR in mice have a bimodal distributio n composed of 'rudimentary ribs' (RR) with a mode of 0.3-0.4 mm and 'e xtra ribs' (ER) with a mode of 0.9-1.1 mm. The studies reported here e xamine the relationship between the presence of SNR and the 13th rib l ength and the gross morphological development of the anomaly. Supernum erary ribs were induced in CD-1 mice by surgical stress (subcutaneous micropump implanted on gestational day (GD), restraint stress (GD8), f ood and water deprivation (GD8) or maternal administration of the pest icide dinoseb (50 mg kg(-1) on GD7 and GD8). Fetuses from untreated li tters 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 1 3th and 14th ribs were measured and other anomalies were recorded. Fem ur length was used as an indicator of fetal size. The SNR frequency wa s higher in all treatment groups compared to controls. We found that E R and RR were morphologically distinct. The ER were Bat ended and dist ally 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 S NR, whether treated or untreated. This relationship was present in all fetal ages examined and with both ER and RR groups. These results sug gest that SNR are indicative of basic alterations in the development o f the axial skeleton.