Role of myometrial constriction in the induction of wavy ribs in rat fetuses

Mechanism of Developmental Effects in Rats Caused by an N-Phenylimide Herbicide: Transient Fetal Anemia and Sequelae during Mid-to-Late Gestation

BACKGROUND

Rat developmental toxicity including embryolethality and teratogenicity (mainly ventricular septal defects [VSDs] and wavy ribs) was produced by an N-phenylimide herbicide that inhibits protoporphyrinogen oxidase (PPO) common to chlorophyll and heme biosynthesis. Major characteristics of the developmental toxicity included species difference between rats and rabbits, compound-specific difference among structurally similar herbicides, and sensitive period. Protoporphyrin accumulation in treated fetuses closely correlated with the major characteristics. Iron deposits in erythroblastic mitochondria and degeneration of erythroblasts were observed in treated rat fetuses. In this study we investigated fetal anemia and subsequent developmental effects in rats, and inhibition of PPO in rats, rabbits, and humans by the herbicides in vitro.

METHODS

Fetuses were treated on gestational day (GD) 12 and removed on GDs 13 through 20. All litters were examined externally. One half of litters were examined for blood and skeletal development, and the other half for interventricular foramen closure. Effects on PPO were determined in mitochondria from embryos and adult livers.

RESULTS

Fetal anemia in rats was evident on GDs 13 through 16. Subsequently, enlarged heart, delayed closure of the foramen, reduced serum protein, and retarded rib ossification were observed. In vitro PPO inhibition exhibited species- and compound-specific differences corresponding to the developmental toxicity.

CONCLUSION

We propose that developmental toxicity results from PPO inhibition in primitive erythroblasts, causing transient fetal anemia followed by death. Compensatory enlargement of the fetal heart results in failure of interventricular foramen closure and VSD. Reduced serum protein leads to delayed ossification and wavy ribs.

Relationship between bent long bones, bent scapulae, and wavy ribs: malformations or variations?

BACKGROUND

Shortened and bent long bones and bent scapulae are sometimes reported in fetuses with wavy ribs (Carney and Kimmel, ). Wavy ribs are typically seen in the presence of maternal and developmental toxicity, are transient and reversible postnatally, and are considered to be variations rather than malformations.

METHODS

We further assessed the literature cited in Kimmel and Carney () as well as papers published since then to determine under what conditions bent long bones in the absence of gross limb defects and bent scapulae were reported and whether information was available on the transient or permanent nature of these effects.

RESULTS

Long bone and/or scapular changes almost always occurred at a lower incidence than wavy ribs. In every case, maternal and fetal toxicity occurred at the same dose levels. In a few studies, pups were followed sequentially after birth and bent long bones and scapulae were transient in nature and appeared normal by the time of weaning. Rabbits were much less likely to show wavy ribs or long bone and scapular changes at birth, even in the presence of severe maternal and fetal toxicity. This species difference may be due in part to the great increase in bone mass and remodeling that occurs during the first few postnatal weeks in rodents, but which takes place during the longer fetal period in rabbits.

CONCLUSION

Our conclusion from this review is that bent long bones and scapulae, like wavy ribs, appear to be secondary to maternal and developmental toxicity, are transient, and like wavy ribs should be considered variations rather than malformations.

"Wavy ribs". A reversible pathologic finding in rat fetuses

A literature of 74 scientific papers addressing the occurrence of "wavy ribs" in fetuses of small rodents has been collected with the support of a computer-assisted search on MEDLINE, TOXLINE, BIOSIS and EXCERPTA MEDICA systems (1982-1993) and has been critically examined. Numerous compounds of a large variation in chemical structure and biological activity, if given to pregnant rats during the later period of organogenesis, are known to cause "wavy ribs" in their fetuses, a reversible pathologic finding which is rarely observed in untreated controls. According to literature, causative factors, like delayed development of alkaline phosphatase, decreased fetal protein, dietary contents, maternal toxicity, renal loop diuretics, myometrial constriction, endocrine disturbances and beta-adrenoceptor stimulation or blocking have been observed and discussed, but, a general explanation for the lesions is still missing. Therefore, the following hypothesis may be added: the high doses used in reproductive toxicology may induce maternal and placental circulatory disturbances leading to reductions in fetal serum proteins, with delayed development of osteoblast progenitors and low fetal serum alkaline phosphatase in the day 17-20 fetus. Normalization of the neonate's homeostasis (serum protein) leads to enhanced ossification and concomitant bone repair until weaning.

Effects of sodium bicarbonate and ammonium chloride on the incidence of furosemide-induced fetal skeletal anomaly, wavy rib, in rats

Furosemide produces fetal wavy ribs when administered to pregnant rats during late gestation. The compound is also known to produce metabolic alkalosis in laboratory animals and man. In order to evaluate the effect of furosemide on maternal blood pH, Crj:CD(SD) female rats received an oral administration of 150 or 200 mg/kg of furosemide by gavage on day 16 of gestation and were bled at 4 hr post-dose. Compared to an average pH of 7.39 in control females, there was a significant elevation in blood pH in these furosemide-treated females (average pH of 7.44 to 7.48). When 2% sodium bicarbonate was provided in the drinking water for females treated with 150 mg/kg of furosemide, there was a further rise in maternal blood pH (7.52) compared to females treated with furosemide alone. Associated with this elevation in maternal blood pH was a marked increase in the incidence of fetal wavy ribs (87.6% compared to 27.6%). When females treated with 200 mg/kg of furosemide were provided with 0.5% ammonium chloride, furosemide-induced maternal alkalosis was corrected (pH decreased to 7.35) and there was a reduction in the incidence of fetal wavy ribs (7.0% compared to 37.2%). In addition, maternal blood pH among individual females was positively correlated with the incidence of fetal wavy ribs (r = 0.714). These results suggest that maternal metabolic alkalosis is involved in the pathogenesis of furosemide-induced wavy ribs.

Role of maternal toxicity in assessing developmental toxicity in animals: a discussion

The belief that any drug or chemical, when administered at a high enough dose, can be expected to produce fetal malformations is not consistent with the facts. However, the stress associated with maternally toxic doses can be expected to result in associated, often transient, fetal abnormalities that may not be the result of deviant organogenesis. Sometimes the toxicity toward the pregnant animal, including her embryos/fetuses since they are hardly in a sanctuary, is severe enough to result in resorption of the embryo or abortion of the fetus. Thus, it is possible that the embryolethality and other indications of developmental toxicity, produced by some drugs and chemicals, may be the result of a mechanism(s) other than selective toxicity toward the embryo. Also, some test materials have been shown to affect maternal homeostasis, thereby disrupting support to the embryo, without causing significant overt toxicity to the embryo or dam; e.g., the endocrine system of the dam is altered. Routine testing has thus far revealed a relatively limited number of true teratogens, although a large number of drugs and chemicals have resulted in fetal effects such as developmental variations when administered at doses that approach lethal levels. Such effects on the fetus should be expected when the maternal animals are stressed by the high dosages usually employed. A better understanding of the etiology and biological relevance of the embryo/fetal deviations often seen in developmental toxicology studies might help to avoid the sometimes unjustified withholding of potentially useful drugs and chemicals from the marketplace.

Developmental toxicity of isomalt in rats

The sugar replacer isomalt, a 1:1 mixture of the disaccharides glucopyranosylsorbitol and glucopyranosylmannitol, was incorporated in the diet of rats. Female Bay FB:30 rats were adapted to isomalt by feeding them a diet containing a gradually increasing amount of isomalt for several days, prior to mating. Subsequently, they were mated. Isomalt was fed continuously in concentrations of 2.5, 5 and 10% up to day 20 of pregnancy. In addition, one group of female Wistar rats was mated and fed 10% isomalt incorporated in the diet from day 0 up to day 20 of pregnancy, without previous adaptation to isomalt. Finally, one group of untreated female Wistar rats served as controls. Half of the number of females in each group was selected for caesarian section on day 20 of pregnancy. The other half was allowed to litter and rear their pups for 2 weeks (Wistar rats) or 3 weeks (Bay FB:30 rats). In the females of the Bay FB:30 rats, a decreased body-weight gain and food consumption were observed in the 5 and 10% isomalt group. Minor retardation in the development of the foetuses was observed in the 10% isomalt group only with the Bay FB:30 rats and was therefore considered to be related to maternal toxicity. In addition, a dose-related increase in the incidence of wavy ribs occurred in foetuses of the Bay FB:30 rats. However, none of the observed effects were persistent in neonates. Isomalt appeared to have slight toxic effects in the dams of the Bay FB:30 strain but no toxicity in the offspring. In Wistar rats no toxicity and no effects on maternal performance or on embryonic, foetal or neonatal development were seen. Isomalt, when fed at dietary levels up to 10%, did not induce structural or functional teratogenic effects in rats of either the Wistar or the Bay FB:30 strain.