The induction of supernumerary ribs in rodents: Role of the maternal stress

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Abstract
Increased incidences of supernumerary ribs (SNR) are a relatively common finding in standard teratology bioassays, and previous studies have indicated a possible correlation between their occurrence and general maternal stress. The present study describes the effects of immobilization stress on the induction of supernumerary ribs. To isolate the sensitive period of SNR induction, Sprague-Dawley rats and CD-1 mice were treated with 300 and 1,500 mg/kg, respectively, of sodium salicylate on single days 7–11 of gestation. In the rat, day 10 was found to be the sensitive period of lumbar rib induction (32% SNR vs 10% on other days) while day 9 was critical in the mouse (71% SNR vs <29% on other days). In a second set of experiments, maternal stress was accomplished by restraining two groups of gravid females in the supine position for 12 hours on the predetermined sensitive day. One group was immobilized from 9 am to 9 pm, while the second group was restrained from 9 pm to 9 am. Concurrent controls were food and water deprived for similar periods. Additional untreated controls were also included. An increase in supernumerary ribs was noted in stressed mice but not in rats. The 9 am to 9 pm mouse group exhibited the highest increase in supernumerary ribs (41%) as well as significant incidences of fused ribs and exencephaly. A significant linear relationship between maternal weight loss during treatment and increases in supernumerary ribs was also noted. Results suggest that for some compounds, supernumerary ribs may be the indirect result of agent-induced, generalized maternal stress in the CD-1 mouse.