Utility of fluorescence microscopy in embryonic/fetal topographical analysis

Effect of nano-encapsulation of β-carotene on Xenopus laevis embryos development (FETAX)

Vitamin A plays a vital role during embryo development as most precursor of embryonic retinoic acid, a key morphogen during embryogenesis. Carotenoids, including β-carotene, are important vegetal source of Vitamin A and in contrast to teratogenic potential of animal-derived retinoids, β-carotene is usually considered freed from embryotoxic effects and supplements in pregnancy with β-carotene are suggested. The aim of the present work is to evaluate the effect of bulk and nano-encapsulated β-carotene on embryo development, by using the animal model Frog Embryo Teratogenesis Assay: Xenopus- FETAX. Xenopus laevis embryos were exposed from late gastrula till pharyngula (the phylotypic stage for vertebrates) to the concentrations of BULK β-carotene 150-3000 ng/mL and NANO β-carotene 0.75-30 ng/mL. At pharyngula stage, some embryos were processed for whole mount neural crest cell immunostaining, while others embryos were allowed to develop till tadpole for morphological and histological evaluation of neural crest cells-derived structures. In this model, the nano-encapsulated β-carotene induced specific malformations at craniofacial and eye level, while the bulk formulation only induced developmental delays. Finally, the applied alternative animal model resulted a rapid and sensitive screening method able to re-evaluate the teratogenic profile of nano-encapsulated micronutrients.

Short-term exposures to dihaloacetic acids produce dysmorphogenesis in mouse conceptuses in vitro

The haloacetic acids (HAAs) are a family of xenobiotics found in tap water as a result of drinking water disinfection. Administration of HAAs to rats produces a variety of adverse effects, including developmental toxicity. The dysmorphogenic potencies of all nine bromo/chloro-acetic acids have been determined in rodent whole embryo culture using standard 26-h exposure. Since the half-lives of the HAAs in vivo are typically <8 h, the developmental effects of short-term exposures to dihaloacetates were evaluated. Gestation day 8 (3-6 somite pairs) CD-1 mouse conceptuses were exposed to 11,000 microM dichloroacetic acid (DCA), 300 microM dibromoacetic acid (DBA) or 300 microM bromochloroacetic acid (BCA) for culture periods of 1, 3, 6 or 26 h. Following 1, 3 or 6 h of exposure to HAAs, conceptuses were transferred to control medium to complete a 26-h culture period. The amounts of HAAs present in embryos after 1, 3 and 6h of exposure were determined. Increased incidences of dysmorphic embryos were produced by 6 or 26-h exposures to DCA; a 26-h exposure to DBA; or 3, 6 or 26-h exposures to BCA. The dysmorphology produced was dependent upon the length of exposure and chemical. The embryonic concentration of each HAA (104.5, 2.5 and 2.6 pmol/microg protein for DCA, DBA and BCA, respectively) was reached by 1h of exposure and did not change at the subsequent time points examined. The current studies demonstrate that BCA is more potent than DBA or DCA at disrupting embryogenesis since shorter exposures alter morphogenesis. Since the embryonic HAA concentrations were the same at the three time points measured, the time-dependence in dysmorphogenesis does not appear to be a simple function of increasing embryonic concentration of these chemicals. These studies demonstrate that for these dihaloacetic acids relatively high concentrations and long exposures are needed to alter rodent development in vitro.

Dysmorphogenic effects of some fungicides derived from the imidazole on rat embryos cultured in vitro

Like triazole-derivatives, imidazole-derivatives exert their antifungal and toxicological properties by inhibiting P450 enzymes (Cyps). At the embryonic level, Cyp enzymes are involved also in the catabolism of the retinoic acid. Specific effects of triazole-derivatives have been reported on developing rodent embryos, and were correlated to an imbalance of the retinoid homeostasis. The aim of this work was to investigate if imidazole-derivatives are able to induce specific malformations similar to those observed after triazole-derivative exposure. Post implantation rat embryos were exposed in vitro to 1,000 microM Imidazole and to 5-100 microM of the imidazole-derivatives Ketoconazole and Enilconazole. After 48 h in culture, the embryos exposed to the imidazole-derivatives showed specific malformations, quite similar to those observed after triazole-derivative exposure. The common dysmorphogenic effects of the azole-derivatives of the two classes could be due to the inhibition of retinoid catabolism. From this point of view, the contemporaneous exposure to these substances or their therapeutic use could be considered as potentially dangerous for human conceptuses.

Methanol exposure during gastrulation causes holoprosencephaly, facial dysgenesis, and cervical vertebral malformations in C57BL/6J mice

BACKGROUND

Exposure of pregnant outbred CD-1 mice to methanol during the period of gastrulation results in exencephaly, cleft palate, and cervical vertebra malformations [Rogers and Mole, Teratology 55: 364, 1997], while inbred C57BL/6J mice are sensitive to the teratogenicity of ethanol. C57BL/6J fetuses exhibit the holoprosencephaly spectrum of malformations after maternal exposure to ethanol during gastrulation, but the sensitivity of C57BL/6J mice to methanol-induced teratogenesis has not been previously described.

METHODS

Pregnant C57BL/6J mice were administered two i.p. injections totaling 3.4 or 4.9 g/kg methanol or distilled water four hrs apart on gestation day 'GD' 7. On GD 17, litters were examined for numbers of live, dead and resorbed conceptuses, fetuses were weighed as a litter and examined externally, and all fetuses were double stained for skeletal analysis.

RESULTS

No maternal intoxication was apparent, but the high dosage level caused a transient deficit in maternal weight gain. The number of live fetuses per litter was reduced at both dosages of methanol, and fetal weight was lower in the high dosage group. Craniofacial defects were observed in 55.8% of fetuses in the low dosage group and 91.0% of fetuses in the high dosage group, including micro/anophthalmia, holoprosencephaly, facial clefts and gross facial angenesis. Skeletal malformations, particularly of the cervical vertebrae, were observed at both dosages of methanol, and were similar to those previously reported in the CD-1 mouse following methanol exposure.

CONCLUSIONS

The types of craniofacial malformations induced in the C57BL/6J mouse by methanol indicate that methanol and ethanol have common targets and may have common modes of action.

Retinoic acid-induced developmental defects are mediated by RARbeta/RXR heterodimers in the pharyngeal endoderm

Fusion and hypoplasia of the first two branchial arches, a defect typically observed in retinoic acid (RA) embryopathy, is generated in cultured mouse embryos upon treatment with BMS453, a synthetic compound that exhibits retinoic acid receptor beta (RARbeta) agonistic properties in transfected cells. By contrast, no branchial arch defects are observed following treatment with synthetic retinoids that exhibit RARalpha or RARgamma agonistic properties. The BMS453-induced branchial arch defects are mediated through RAR activation, as they are similar to those generated by a selective pan-RAR agonist, are prevented by a selective pan-RAR antagonist and cannot be mimicked by exposure to a pan-RXR agonist alone. They are enhanced in the presence of a pan-RXR agonist, and cannot be generated in Rarb-null embryos. Furthermore, they are accompanied, in the morphologically altered region, by ectopic expression of Rarb and of several other direct RA target genes. Therefore, craniofacial abnormalities characteristic of the RA embryopathy are mediated through ectopic activation of RARbeta/RXR heterodimers, in which the ligand-dependent activity of RXR is subordinated to that of RARbeta. Endodermal cells lining the first two branchial arches respond to treatment with the RARbeta agonist, in contrast to neural crest cells and ectoderm, which suggests that a faulty endodermal regionalization is directly responsible for RA-induced branchial arch dysmorphologies. Additionally, we provide the first in vivo evidence that the synthetic RARbeta agonist BMS453 exhibits an antagonistic activity on the two other RAR isotypes.

Buthionine sulfoximine embryotoxicity is associated with prolonged AP-1 activation

BACKGROUND

Many teratogens induce oxidative stress, altering redox status and redox signaling; this has led to the suggestion that developmental toxicants act by disturbing redox status. The goal of these studies was to determine the consequences of altering glutathione homeostasis during organogenesis on embryo development, total DNA methylation, and activator protein-1 (AP-1) DNA binding activity and gene expression.

METHODS

Gestational day 10.5 rat embryos were cultured in vitro for up to 44 hour in the presence of L-buthionine-S,R-sulfoximine (BSO), an irreversible inhibitor of gamma-glutamyl-cysteine synthetase, the rate limiting step in glutathione biosynthesis. Effects of BSO on total, oxidized and reduced glutathione, embryo development, DNA methylation, AP-1 DNA binding activity and gene expression were investigated.

RESULTS

Significant depletion of glutathione by BSO was first noted at 6 hr in the embryo and at 3 hr in the yolk sac; total glutathione in the conceptus was depleted to the same extent after treatment with either 0.1 or 1.0 mM BSO. Exposure to 0.1 mM BSO did not cause a significant increase in embryotoxicity, although some impairment of growth and development was observed. In contrast, exposure to 1.0 mM BSO severely inhibited growth and development, significantly increasing the incidence of swollen hindbrains and of blebs in the forebrain, limb and maxillary regions. No significant treatment-related differences in total DNA methylation were observed. Interestingly, AP-1 DNA binding activity was similar in control and 0.1 mM BSO-treated conceptuses; however, exposure to 1.0 mM BSO increased AP-1 DNA binding at 6, 24, and 44 hr. The expression of several AP-1 family genes and of gamma-glutamylcysteine synthetase was induced in embryos cultured with 1.0 mM BSO.

CONCLUSION

Exposure of embryos in vitro to BSO at a concentration that was embryotoxic induced prolonged AP-1 DNA binding activity and altered gene expression. These data suggest that AP-1 induction may serve as a biomarker of embryo stress.

Roles of retinoic acid receptors in early embryonic morphogenesis and hindbrain patterning

Mutants mice carrying targeted inactivations of both retinoic acid receptor (RAR) alpha and RAR gamma (A alpha/A gamma mutants) were analyzed at different embryonic stages, in order to establish the timing of appearance of defects that we previously observed during the fetal period. We show that embryonic day (E)9.5 A alpha/A gamma embryos display severe malformations, similar to those already described in retinaldehyde dehydrogenase 2 null mutants. These malformations reflect early roles of retinoic acid signaling in axial rotation, segmentation and closure of the hindbrain; formation of otocysts, pharyngeal arches and forelimb buds; and in the closure of the primitive gut. The hindbrain of E8.5 A alpha/A gamma embryos shows a posterior expansion of rhombomere 3 and 4 (R3 and R4) markers, but fails to express kreisler, a normal marker of R5 and R6. This abnormal hindbrain phenotype is strikingly different from that of embryos lacking RAR alpha and RAR beta (A alpha/A beta mutants), in which we have previously shown that the territory corresponding to R5 and R6 is markedly enlarged. Administration of a pan-RAR antagonist at E8.0 to wild-type embryos cultured in vitro results in an A alpha/A beta-like hindbrain phenotype, whereas an earlier treatment at E7.0 yields an A alpha/A gamma-like phenotype. Altogether, our data suggest that RAR alpha and/or RAR gamma transduce the RA signal that is required first to specify the prospective R5/R6 territory, whereas RAR beta is subsequently involved in setting up the caudal boundary of this territory.

Abnormal development of the sinuatrial venous valve and posterior hindbrain may contribute to late fetal resorption of vitamin A-deficient rat embryos

BACKGROUND

Normal embryonic development and survival in utero is dependent on an adequate supply of vitamin A. Embryos from vitamin A-deficient (VAD) pregnant rats fed an inadequate amount of all-trans retinoic acid (atRA; 12 microg per g of diet or approximately 230 microg per rat per day) exhibit severe developmental abnormalities of the anterior cardinal vein and hindbrain by embryonic day (E) 12.5 and die shortly thereafter.

METHODS

In the present study, we sought to determine whether supplementation of VAD-RA supported (12 microg per g of diet) pregnant rats with retinol (ROL) at the late-gastrula (presomite or rat E9.5) or early somite stages (E10.5), or provision of higher levels of atRA throughout this period could prevent abnormalities in the developing cardiovascular and nervous systems.

RESULTS

A newly described defect in the sinuatrial venus valve along with enlarged anterior cardinal veins and nervous system abnormalities and the later death of embryos are prevented by supplementing pregnant animals with ROL on the morning of E9.5. If ROL supplementation is delayed by 1 day (E10.5), most embryos are abnormal and die by E18.5. Supplementation of VAD rats with atRA (250 microg per g of diet) between E8.5 and E10.5 also prevents the cardiovascular and nervous system abnormalities and a significant number of these embryos survive to parturition. Thus, high levels of atRA can obviate the need for ROL between E9.5 and E10.5.

CONCLUSIONS

These results support an essential role for retinoid signaling between the late gastrula and early somite stages in the rat embryo for normal morphogenesis of the primitive heart tube and the posterior hindbrain. Further, these results suggest that embryonic death occurring at midgestation in the VAD rat may be linked to the abnormal development of one or both of these embryonic structures.

Effects of excess vitamin A on development of cranial neural crest-derived structures: a neonatal and embryologic study

BACKGROUND

Vitamin A and its metabolites have been shown to be teratogenic in animals and humans producing defects of neural crest derived structures that include abnormalities of the craniofacial skeleton, heart, and thymus. Our prior studies with retinoic acid have established that gestational day (gd) 9 is a sensitive embryonic age in the mouse for inducing craniofacial and thymic defects.

METHODS

We exposed pregnant mice to variable doses of vitamin A (retinyl acetate) on gd 9 and embryos were evaluated for changes in developing pharyngeal arch and pouch morphology, neural crest cell migration and marker gene expression. Additionally, we investigated whether a single organ system was more sensitive to low doses of vitamin A and could potentially be used as an indicator of vitamin A exposure during early gestation.

RESULTS

High (100 mg/kg) and moderate (50 and 25 mg/kg) doses of vitamin A resulted in significant craniofacial, cardiac outflow tract and thymic abnormalities. Low doses of vitamin A (10 mg/kg) produced craniofacial and thymic abnormalities that were mild and of low penetrance. Exposed embryos showed morphologic changes in the 2nd and 3rd pharyngeal arches and pouches, changes in neural crest migration, abnormalities in cranial ganglia, and altered expression of Hoxa3.

CONCLUSIONS

These animal studies, along with recent epidemiologic reports on human teratogenicity with vitamin A, raise concerns about the potential for induction of defects (perhaps subtle) in offspring of women ingesting even moderate to low amounts of supplemental vitamin A during the early gestational period.

Vitamin A deficiency results in the dose-dependent acquisition of anterior character and shortening of the caudal hindbrain of the rat embryo

The developing nervous system is particularly vulnerable to vitamin A deficiency. Retinoid has been proposed to be a posteriorizing factor during hindbrain development, although direct evidence in the mammalian embryo is lacking. In the present study, pregnant vitamin A-deficient (VAD) rats were fed purified diets containing varying levels of all-trans-retinoic acid (atRA; 0, 0.5, 1.5, 6, 12, 25, 50, 125, or 250 microg/g diet) or were supplemented with retinol. Hindbrain development was studied from embryonic day 10 to 12.5 ( approximately 6 to 40 somites). Normal morphogenesis was observed in all embryos from groups fed 250 microg atRA/g diet or retinol. The most caudal region of the hindbrain was the most sensitive to retinoid insufficiency, as evidenced by a loss of the hypoglossal nerve (cranial nerve XII) in embryos from the 125 microg atRA/g diet group. Further reduction of atRA to 50 microg/g diet led to the loss of cranial nerves IX, X, XI, and XII and associated sensory ganglia IX and X in all embryos as well as the loss of hindbrain segmentation caudal to the rhombomere (r) 3/4 border in a subset of embryos. Dysmorphic orthotopic otic vesicles or immature otic-like vesicles in both orthotopic and caudally ectopic locations were also observed. As the level of atRA was reduced, a loss of caudal hindbrain segmentation was observed in all embryos and the incidence of otic vesicle abnormalities increased. Perturbations in hindbrain segmentation, cranial nerve formation, and otic vesicle development were associated with abnormal patterning of the posterior hindbrain. Embryos from VAD dams fed between 0.5 and 50 microg atRA/g diet exhibited Hoxb-1 protein expression along the entire neural tube caudal to the r3/r4 border at a time when it should be restricted to r4. Krox-20 protein expression was expanded in r3 but absent or reduced in presumptive r5. Hoxd-4 mRNA expression was absent in the posterior hindbrain, and the rostral limit of Hoxb-5 protein expression in the neural tube was anteriorized, suggesting that the most posterior hindbrain region (r7/r8) had been deleted and/or improperly patterned. Thus, when limiting amounts of atRA are provided to VAD dams, the caudal portion of the hindbrain is shortened and possesses r4/r5-like characteristics, with this region finally exhibiting r4-like gene expression when retinoid is restricted even more severely. Thus, regions of the anterior hindbrain (i.e., r3 and r4) appear to be greatly expanded, whereas the posterior hindbrain (r5-r8) is reduced or absent. This work shows that retinoid plays a critical role in patterning, segmentation, and neurogenesis of the caudal hindbrain and serves as an essential posteriorizing signal for this region of the central nervous system in the mammal.

In vitro teratogenic potential of two antifungal triazoles: triadimefon and triadimenol

The teratogenic potential of two antifungal triazoles (Triadimefon and Triadimenol) has been investigated in vitro by the rat postimplantation whole embryo culture method. Rat embryos 9.5 d old were cultured for 48 h in rat serum with Triadimefon (12.5-250 microM) or Triadimenol (6.25-125 microM) and then examined. Some embryos exposed to Triadimenol (6.25-125 microM) were cultured for 12 extra hours in control serum to improve their developmental degree and then immunostain cranial nerves and ganglia. The exposure to the highest doses of triazoles only moderately reduced some morphometrical developmental parameters. By contrast, 25-250 microM Triadimefon and 25-125 microM Triadimenol induced specific concentration-related teratogenic effects at the level of first and second branchial arches. After immunostaining, embryos exposed to 12.5-125 microM Triadimenol showed specific cranial nerve and ganglia abnormalities. The possible implication of neural crest cell alterations on triazole-related abnormalities is discussed.

Tissue-specific regulation of glutathione homeostasis and the activator protein-1 (AP-1) response in the rat conceptus

Oxidative stress in the conceptus is characterized by an increased oxidized to reduced glutathione (GSSG:GSH) ratio and the induction of fos and jun mRNAs, transcripts for components of the activator protein-1 (AP-1) transcription factor. We investigated the role of glutathione homeostasis in the rat conceptus in the regulation of: (1) AP-1 expression and activity, and (2) the activities of glutathione-dependent cytoprotective enzymes. Glutathione content was enhanced with the addition of l-2-oxothiazolidine-4-carboxylate (OTC), a precursor of cysteine, a rate-limiting substrate in glutathione biosynthesis. Day 10 rat conceptuses were cultured for 44 hr with 0, 5, 10, or 20 mM OTC. High concentrations (10 and 20 mM) of OTC were embryotoxic. Incubation of the conceptus in 5 mM OTC caused mild (not statistically significant) embryotoxicity, increased significantly the embryonic glutathione content, prevented culture-induced oxidative stress, and inhibited the induction of AP-1 transcripts and DNA binding activity in the embryo. In contrast, in the yolk sac, 5 mM OTC failed to increase glutathione content or to prevent oxidative stress or AP-1 induction. Thus, regulation of glutathione status in the conceptus is tissue-specific. Glutathione S-transferase and glutathione peroxidase activities were increased approximately 50% in cultured embryos and yolk sacs. OTC treatment (5 mM) prevented this induction in the embryo, but not in the yolk sac, suggesting a role for glutathione homeostasis in the regulation of these enzymes. Tissue-specific regulation of glutathione status and of cytoprotective enzymes in the conceptus during organogenesis may impact on the consequences of insult with oxidative stress.

Morphological alterations induced by sodium valproate on somites and spinal nerves in rat embryos

The antiepileptic drug valproic acid is a well-known teratogenic agent; its main target organ is the neural tube, though skeletal malformations have also been described. In our recent work, respecifications of vertebrae were described in rat fetuses after treatment with 400 mg/kg of sodium valproate at specific somitogenic stages. The observed malformations were stage-dependent. Morphological segmental respecification was observed at the level of segments in formation at the moment of exposure and at the level of more posterior segments. Recently, specific alterations in the development of cranial nerves and ganglia were described in mouse embryos after in vitro exposure to VPA. The aim of the present work was to analyze dysmorphogenetic effects of VPA on embryonic metameric structures: somites, spinal and cranial nerves, and ganglia. Sodium valproate (400 mg/kg) was subcutaneously injected at specific gestational times corresponding to embryonic stages: presomitic or at about 2, 6, 10, 14, 18, or 22 somites. Females were sacrificed on the day 12 post coitum, and embryos were examined. Morphological examination of somites was performed by staining with acridine orange. Morphological examination of nerves and ganglia was performed by immunostaining, using monoclonal antibodies to the 160-kD neurofilament protein. No abnormalities were observed in the cranial nerves and ganglia. Specific and stage-dependent alterations were observed both at the level of the somites and at the level of the spinal nerves. The following characteristic malformations were observed: fusions, duplications, and reductions of somites and corresponding spinal nerves and ganglia. Our morphological data suggest a morphogenetic action of VPA at the level of the axial segments, with a possible respecification of the identity of the interested segments and their derivatives.

Retinoic acid-induced thymic abnormalities in the mouse are associated with altered pharyngeal morphology, thymocyte maturation defects, and altered expression of Hoxa3 and Pax1

Exogenous retinoic acid is teratogenic in animals and man, causing a spectrum of abnormalities termed retinoic acid embryopathy. Using a mouse model of retinoic acid embryopathy, our results show that exposure to all-trans retinoic acid (RA) on gestational day (gd) 9 results in thymic ectopia, hypoplasia, and thymocyte maturational defects. Immunohistochemical and flow cytometric analyses showed aberrant expression of stromal and thymocyte markers, and abnormalities in thymocyte development. RNA in situ hybridization for the transcription factors Hoxa3 and Pax1 was used to investigate the basis of this defect. Hoxa3 and Pax1 have been shown to be required for normal thymus development, and are normally expressed in the cells of the third pharyngeal pouch and third and fourth pharyngeal arches, involved in thymus organogenesis RA-exposed embryos showed an increased level of Hoxa3 expression in the neural tube and caudal pharyngeal arches as soon as 6 hr after exposure. The Pax1 expression pattern, in conjunction with analysis of the external pharyngeal morphology, showed that the development and structure of the third pharyngeal pouch and cleft were disrupted, resulting in a reduced third pharyngeal arch and/or fusion of the third and fourth arches. Changes in the expression of cellular retinoic acid binding protein (CRABP) and in the morphology of the cranial ganglia were consistent with altered neural crest cell migration from the caudal hindbrain after RA exposure. Together, our findings suggest that the teratogenic effects of RA on thymus development include changes in both the cranial neural crest and pharyngeal endoderm that contribute to thymus development. Further, the observed defects in thymus development may be mediated by RA-induced alterations in the expression of Hoxa3.

Defects in embryonic hindbrain development and fetal resorption resulting from vitamin A deficiency in the rat are prevented by feeding pharmacological levels of all-trans-retinoic acid

Vitamin A is required for reproduction and normal embryonic development. We have determined that all-trans-retinoic acid (atRA) can support development of the mammalian embryo to parturition in vitamin A-deficient (VAD) rats. At embryonic day (E) 0.5, VAD dams were fed purified diets containing either 12 micrograms of atRA per g of diet (230 micrograms per rat per day) or 250 micrograms of atRA per g of diet (4.5 mg per rat per day) or were fed the purified diet supplemented with a source of retinol (100 units of retinyl palmitate per day). An additional group was fed both 250 micrograms of atRA per g of diet in combination with retinyl palmitate. Embryonic survival to E12.5 was similar for all groups. However, embryonic development in the group fed 12 micrograms of atRA per g of diet was grossly abnormal. The most notable defects were in the region of the hindbrain, which included a loss of posterior cranial nerves (IX, X, XI, and XII) and postotic pharyngeal arches as well as the presence of ectopic otic vesicles and a swollen anterior cardinal vein. All embryonic abnormalities at E12.5 were prevented by feeding pharmacological amounts of atRA (250 micrograms/g diet) or by supplementation with retinyl palmitate. Embryos from VAD dams receiving 12 micrograms of atRA per g of diet were resorbed by E18.5, whereas those in the group fed 250 micrograms of atRA per g of diet survived to parturition but died shortly thereafter. Equivalent results were obtained by using commercial grade atRA or atRA that had been purified to eliminate any potential contamination by neutral retinoids, such as retinol. Thus, 250 micrograms of atRA per g of diet fed to VAD dams (approximately 4.5 mg per rat per day) can prevent the death of embryos at midgestation and prevents the early embryonic abnormalities that arise when VAD dams are fed insufficient amounts of atRA.

Effects of boric acid on axial skeletal development in rats

Prenatal exposure to elevated levels of boric acid (BA) causes reduced incidences of supernumerary ribs and shortening/absence of the 13th rib in multiple laboratory species. To explore this further, Sprague-Dawley rats received 500 mg/kg b.i.d. on gestation days (gd) 5-9, 6-9, 6-10, or on single days between gd 6 and 11 (plug day = gd 0); gd-21 fetuses were stained for skeletal examination. Following multiday exposures, malformations of the axial skeleton involved the head, sternum, ribs, and vertebrae. Shortening/absence of the 13th rib was seen particularly in the gd 5-9 and 6-10 exposure groups. Although most groups exposed on single days were generally unaffected, about 90% of the gd-9 exposed fetuses had only six cervical vertebrae; the deficient region was usually C3-C5. In contrast, gd-10 treatment caused agenesis of a thoracic/lumbar vertebra in over 60% of the fetuses; the deficient region was usually T11. For 13-ribbed fetuses, the length of rib 13 was shortened compared to controls. Postnatal assessment suggested increased mortality for gd-10 exposed pups. Embryos in culture showed reduced development when exposed to BA for 48 h. These findings demonstrate the critical periods for axial development in the rat and provide an experimental model for the study of homeotic shifts.