All-trans retinoic acid and 13-cis-retinoic acid in the rat whole-embryo culture: abnormal development due to the all-trans isomer

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.

The use of whole embryo culture to elucidate teratogenic mechanisms

In the overview presented, only selected examples for the applicability of whole embryo culture, and a few examples for its use and possible misuse, are mentioned. In general, the system has proved suitable for elucidating mechanisms involved in abnormal development at this embryonic stage. However, it should be remembered that only a small window of prenatal development can be judged with this technique, and that the system is not well suited for elucidating many special effects and abnormalities induced at later stages of development. For this reason, no single in vitro system is suitable to study all kinds of problems in prenatal biology or toxicology. As emphasized before, the real significance of the whole embryo culture technique rests on the variety of methodological possibilities that can be modified by any creative scientist. In connection with in vivo and appropriate pharmacokinetic studies, this system will play an important role in the field of developmental biology and toxicology in the future.

Postulated pathogenic pathway in triazole fungicide induced dysmorphogenic effects

Triazole fungicides are used in medicine as well as in agricultural treatment of mycoses. The pharmacological mechanism is related to the inhibition of CYP enzymes involved in the formation of the fungal walls. A similar inhibition of human CYP enzymes has been suggested as the cause of triazole side effects in humans. An important role of some CYP isoforms (CYP26 isoforms) expressed during mammalian development is the catabolism of retinoic acid, a known morphogen in vertebrates and invertebrates. The adverse effects on morphogenesis, observed after exposure of mammalian, amphibian and ascidiacea, are compared to the reported effects of triazole in humans. The possible pathogenic pathway in triazole-related teratogenesis is discussed on the basis of different experimental approaches.

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.

Study on the common teratogenic pathway elicited by the fungicides triazole-derivatives

Triazole-derivatives alter the pharyngeal apparatus morphogenesis of rodent embryos cultured in vitro. The hindbrain segmentation and the rhombencephalic neural crest cell (NCCs) migration are altered by Fluconazole exposure in vitro. The aim of the present work is to identify if a common pathogenic pathway is detectable also for other molecules of this class of compounds. 9.5 days post coitum (d.p.c.) old rat embryos were exposed in vitro to the teratogenic concentrations of Flusilazole, Triadimefon and Triadimenol and cultured for 24, 48 or 60 h. The expression and localisation of Hox-b1 and Krox-20 proteins (used as markers for hindbrain segmentation) were evaluated after 24 h of culture. The localisation and distribution of NCC was evaluated after 24, 30 and 48 h of culture. The morphology of the embryos was analysed after 48 h, while the branchial nerve structures were evaluated after 60 h of culture. Hindbrain segmentation and NCC migration alteration as well as pharyngeal arch and cranial nerve abnormalities were detected after exposure of the tested molecules. A common severe teratogenic intrinsic property for the tested molecules of this chemical class has been found, acting through alteration of the normal hindbrain developmental pattern.

Craniofacial and axial skeletal defects induced by the fungicide triadimefon in the mouse

BACKGROUND

Triadimefon is an antifungal derived from triazole. In in vitro whole-rodent embryo cultures, triazole-derivatives showed specific teratogenic effects at the branchial apparatus. The aim of the present work was to test in vivo triadimefon (FON), in order to verify a relationship between triazole exposure, embryonic abnormalities, and/or fetal malformations.

METHODS

Pregnant CD-1 mice were treated with 0-300 mg/kg FON by gavage on day 8 post coitum (p.c.) at 10:00 AM, and sacrificed on day 8 p.c. at 1:00 PM, on day 9 p.c. at 10:00 AM, on day 10 p.c. at 10:00 AM, and at term of gestation (day 18 p.c.). At midgestation, the embryos were processed for specific immunostainings to visualize the hindbrain segmentation (day 8 p.c.) and the neural crest cell migration (days 8 and 9 p.c.). Fetuses explanted at term were all processed for skeletal examination after double-staining of osseous and cartilaginous tissues.

RESULTS

At midgestation, the immunostaining of rhombomeres 3 and 5 showed a light scattering of the immunostained areas; the neural crest cell migration was unaffected, but their localization at the branchial arch level was abnormal. At term, several severe malformations were observed at the craniofacial and at the axial skeletal level. Ectopic cartilage was observed at the upper jaw.

CONCLUSIONS

Triadimefon is teratogenic. The observed craniofacial malformations could be explained by an alteration of the rhombomeric organization and neural crest migration to the branchial arches; the axial abnormalities could be explained by the abnormal segmental identity specification.

A study of vehicles for dosing rodent whole embryo culture with non aqueous soluble compounds

In rodent whole embryo culture (WEC), finding vehicles for non-aqueous-soluble compounds has been problematic due to developmental toxicity associated with many solvents. The purpose of this study was to identify alternative vehicles for insoluble compounds. In WEC, we evaluated carrier solutions containing bovine serum albumin (BSA) and glycerol as well as the solvents, formamide, dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and ethanol, for relative teratogenicity and delivery of the insoluble teratogen, all-trans retinoic acid (RA). At a concentration of <or=0.04%, formamide and DMF exhibited no significant toxicity to cultured rat embryos and were effective at delivering RA to the embryo. The BSA and glycerol carrier solutions were not teratogenic, although both inhibited robust formation of yolk sac vasculature. Both solutions delivered RA to the cultured rat embryos at higher doses. In summary, all four solvents/solutions may have utility as vehicles dependent upon the chemical properties of the compound to be solubilized.

Relationship between hindbrain segmentation, neural crest cell migration and branchial arch abnormalities in rat embryos exposed to fluconazole and retinoic acid in vitro

Fluconazole (FLUCO) and retinoic acid (RA) can perturb morphogenesis of the branchial apparatus in rodent embryos exposed in vitro. The aim of the present study was to compare the effects induced by in vitro exposure to FLUCO or to RA on rhombomere organisation, neural crest cell (NCC) migration and cranial nerve differentiation using specific antibodies. For this purpose 9.5 d.p.c. rat embryos were exposed to teratogenic concentrations of FLUCO or RA; another group was exposed to no-effect concentrations of both agents. Expression of Hox-b1 and Krox20 (markers of specific rhombomeres) was altered after FLUCO and RA exposure. Furthermore, FLUCO and RA showed a synergistic effect. These results suggest that the observed branchial abnormalities are due to anomalous NCC migration related to incorrect organisation of specific rhombomeres.

Optimal nutrition: vitamin A and the carotenoids

There are two major dietary sources of vitamin A: easily absorbed retinyl palmitate in foods of animal origin, and poorly bioavailable carotenoids from plant foods. Plasma retinol is tightly controlled, probably by regulation of retinol-binding protein (RBP) formation in the liver, and only hormonal factors (e.g. oral contraceptives) and infection will alter the homeostasis. Delivery of retinol to the tissues is facilitated by the RBP-retinol complex; however, there is evidence that this mechanism can be bypassed when very high doses of vitamin A are given. Some retinyl ester may be released to tissues from chylomicrons when the latter bind to tissue lipoprotein receptors during their passage from the gut to the liver following a meal. High-dose vitamin A therapy is a means of rapidly improving vitamin A status in persons with sub-optimal vitamin A nutrition but there are dangers of toxic symptoms (e.g. teratogenicity) from excess vitamin A usage. Evidence is presented to suggest that the plasma retinol: RBP may be a guide to optimal vitamin A status, since values less than one frequently occur in less-developed countries and during infection. In contrast to plasma retinol, plasma carotenoids reflect the dietary intake of plant foods. However, absorption is limited by poor bioavailability and a saturable uptake mechanism in competition with other phytochemicals. Recent work on bioavailability suggests that the calculation of plant food vitamin A activity should be re-examined. Illness has little influence on plasma levels except by suppressing appetite. Carotenoids are generally regarded as non-toxic yet intervention studies with beta-carotene in smokers have been associated with increased lung cancer and heart disease. Some carotenoids are important as vitamin A precursors, but the physiological importance of their antioxidant properties is not known and consequently the amount needed for optimal nutrition is uncertain.

From gastrulation to neurulation: transition in retinoic acid sensitivity identifies distinct stages of neural patterning in the rat

Early neural development is a multistep process with morphologically distinct stages; however, the molecular events that underlie morphologic development are poorly understood. Retinoic acid (RA) was chosen as a teratogen to perturb development because this endogenous molecule is thought to play an integral role in normal neuraxis formation in many vertebrate species. We have examined the effects of RA on early neural patterning in the rat at three morphologically distinct stages: late streak, foregut pocket, and early somite. In this model exogenous RA exposure during mid-gastrulation (late streak stage) leads to severe disruption of anterior neural development as determined by morphologic and molecular (Engrailed [En] gene expression) markers. This disruption in anterior neural development is associated with excessive cell death in the hindbrain posterior to the En expression domain. In contrast, at the time the neural folds begin to elevate (foregut pocket stage) there is a dramatic reduction in the sensitivity of anterior neural development to exogenous RA as reflected by En expression and cell death patterns. These results suggest that we have identified a major transition in the development of the anterior neuraxis that is reflected in a transition in sensitivity to RA. This transition in sensitivity demonstrates that the fundamental patterning mechanisms that separate fore- and midbrain from hindbrain occurs very early in neurogenesis.

The teratogenic metabolites of vitamin A in women following supplements and liver

Ten healthy female volunteers were given 5 doses of retinol as the palmitate; 50 and 150 mg retinol as an oral supplement, 50 and 150 mg as fried calf liver (50 and 150 g) and 3, 9 or 30 mg by intra-muscular injection. Plasma concentrations of retinyl palmitate were higher after 50 mg retinol given as an oral supplement compared with 50 mg as liver; there was no significant difference between the 150 mg doses. Plasma concentrations of retinol showed only small increases. The peak plasma concentrations (Cmax) of all-transretinoic acid, the principal teratogenic metabolite of retinol, and the area under the concentration-time curve (AUC) were up to 20-times higher after supplements compared to the same dose as liver. Plasma concentrations of all-trans-4-oxo-retinoic acid, 13-cis-retinoic acid and 13-cis-4-oxo-retinoic acid showed smaller differences between supplements and liver. Intra-muscular administration of 30 mg retinol gave retinyl palmitate concentrations similar to those found after the oral doses but did not increase circulating concentrations of the acid metabolites. Based on the formation of all-trans-retinoic acid, liver and supplements are not of equivalent teratogenic potential. Advice to pregnant women on the consumption of liver based on the reported teratogenicity of vitamin A supplements should be reconsidered.

Comparative teratogenicity and metabolism of all-trans retinoic acid, all-trans retinoyl beta-glucose, and all-trans retinoyl beta-glucuronide in pregnant Sprague-Dawley rats

When single large equimolar doses (0.38-0.41 mmol/kg BW) of all-trans retinoic acid (RA), all-trans retinoyl beta-glucose (RBGL), and all-trans retinoyl beta-glucuronide (RBG) are administered orally in oil on day 8.5 of pregnancy to Sprague-Dawley rats, RA and RBGL proved highly teratogenic, whereas RBG was not. Indeed, fetuses from RBG-treated dams were 16% heavier (P < 0.01) than control fetuses. After dosing with RA and RBGL, RA appeared in large amounts within 0.5 h in the maternal plasma and within 1.0 h in the embryo. In contrast, orally administered RBG seemed to be absorbed much more slowly, to be converted very slowly to RA, and not to accumulate either as RBG or as RA in the embryo. When incubated in vitro with embryos and attached membranes, however, both all-trans RBG and all-trans RA were partially converted to 13-cis RA. The nonteratogenicity of RBG, in contrast to RA, seems to be due to a much slower rate of GI absorption, a slow rate of hydrolysis to RA, a limited passage from the maternal circulation into the embryo, and a lower inherent toxicity.

Early heart development in the chick embryo: effects of isotretinoin on cell proliferation, alpha-actin synthesis, and development of contractions

Isotretinoin is a potent retinoic acid used in the treatment of skin disorders. Though very effective, it is teratogenic if administered during pregnancy, and its teratogenic effect may be related to the normal activity of retinoids as signalling molecules in the embryo. Although its exact mechanism of action is unknown, it has been suggested that it causes its characteristic pattern of defects that includes heart defects, by inhibiting the migration of neural crest cells. However, other effects on cells are known. We studied early cardiac cell proliferation using incorporation of bromodeoxyuridine (BrdU) and detection with a monoclonal anti-BrdU. Proliferation in heart tissue of whole embryo cultures was inhibited in medium with 10(-6) M isotretinoin to 62% of the control level in myocardium. We studied its effects in culture on precardiac explant development in the absence of the neural crests. Culture of precardiac mesodermal-endodermal explants revealed that development of heart vesicles from the mesoderm was little affected, but the development of heartbeat was inhibited depending on dose in the 10(-5) to 10(-7) M range. The effect on development of contractions was augmented in the presence of serum; it could be duplicated by all-trans-retinoic acid, and it was reversible. Synthesis of the alpha-actin isotype, analyzed by isoelectric focusing, was found to be inhibited or delayed. The results suggest multiple effects of retinoids on growth, morphogenesis, and differentiation of early cardiac tissue, and are discussed in relation to the potential role of retinoids in early embryogenesis.

Teratogenicity of arotinoids (retinoids) in the rat whole embryo culture

Structural modifications of the arotinoid molecule RO 13-7410 led to a difference in the teratogenic potencies of more than five orders of magnitude in mice in vivo and in micromass cultures of rat embryonic limb bud cells (Kistler et al. 1990). Five of these retinoids were selected and tested in rat whole embryo culture to determine the suitability of this in vitro test system for the identification of potentially non-teratogenic derivatives among this class of chemicals. The highest concentrations of the compounds with no effects (NOAEL) on general conceptus growth, on differentiation and on the frequency of dysmorphogenic embryos in vitro were compared with the lowest effective teratogenic doses in vivo (LOAEL) or with the concentrations leading to 50% inhibition of limb bud cell differentiation (IC50) in vitro. NOAEL's for the parameters of conceptus development ranged from 10(-5) micrograms/ml (0.03 nM) to 10 micrograms/ml (28.7 microM) for the compounds tested. These correlated very well with LOAEL and IC50 (R greater than 0.95). The types of dysmorphogenesis in vitro were those typical for retinoids, and for the most part resembled the malformations found in vivo. We conclude that the whole embryo culture system is a useful tool for the preliminary testing of retinoids.

Parameters determining isotretinoin teratogenicity in rat embryo culture

At the in vitro threshold serum concentration of 500 ng/ml, isotretinoin induces defects of visceral arch development in 9.5-day rat embryos grown in culture for 48 h. Experiments were performed to determine the minimum period of exposure necessary to induce these arch defects and whether an increase in concentration of isotretinoin could compensate for reduced exposure time. The results showed that a minimum 6-h exposure to 500 ng/ml immediately prior to cranial neural crest migration was necessary to induce severe defects of the second visceral arch in a majority of embryos. Maximal increase in isotretinoin concentration to 16,000 ng/ml did not compensate for shorter exposure periods. These results suggest that to cause malformations of the visceral arches, the embryo must be exposed to isotretinoin for a minimum period of time regardless of the concentration of isotretinoin above the threshold.