Teratogenicity of high vitamin A intake

Susceptibility in utero and upon neonatal exposure

Important determinants or principles in developmental toxicology are: (1) genotype; (2) developmental stage when an insult is hitting; (3) mechanisms of action; (4) pharmacokinetics of the drug in the mother, conceptus and the neonate; (5) the manifestations of embryo/foeto- and neonatal toxicity such as death, malformations, growth inhibition and functional disturbances; and (6) dose-effect and dose-response relationships. The present paper will give a broad review of some important developmental events and sensitivity periods, such as the preimplantation period, the period of gastrulation, organogenesis and placental formation, the foetal and neonatal period during which xenobiotics can cause perturbation in the normal development. Mostly pharmaceuticals are used as examples due to their often well documented effects and sometimes known sensitivity periods. For the postnatal period, some neurotoxic pesticides and environmental pollutants, known to affect adult behaviour in experimental animals after perinatal exposure, are given as examples.

Randomised trial of effects of vitamin supplements on pregnancy outcomes and T cell counts in HIV-1-infected women in Tanzania

BACKGROUND

In HIV-1-infected women, poor micronutrient status has been associated with faster progression of HIV-1 disease and adverse birth outcomes. We assessed the effects of vitamin A and multivitamins on birth outcomes in such women.

METHODS

In Tanzania, 1075 HIV-1-infected pregnant women at between 12 and 27 weeks' gestation received placebo (n=267), vitamin A (n=269), multivitamins excluding vitamin A (n=269), or multivitamins including vitamin A (n=270) in a randomised, double-blind, placebo-controlled trial with a 2x2 factorial design. We measured the effects of multivitamins and vitamin A on birth outcomes and counts of T lymphocyte subsets. We did analyses by intention to treat.

RESULTS

30 fetal deaths occurred among women assigned multivitamins compared with 49 among those not on multivitamins (relative risk 0.61 [95% CI 0.39-0.94] p=0.02). Multivitamin supplementation decreased the risk of low birthweight (<2500 g) by 44% (0.56 [0.38-0.82] p=0.003), severe preterm birth (<34 weeks of gestation) by 39% (0.61 [0.38-0.96] p=0.03), and small size for gestational age at birth by 43% (0.57 [0.39-0.82] p=0.002). Vitamin A supplementation had no significant effect on these variables. Multivitamins, but not vitamin A, resulted in a significant increase in CD4, CD8, and CD3 counts.

INTERPRETATION

Multivitamin supplementation is a low-cost way of substantially decreasing adverse pregnancy outcomes and increasing T-cell counts in HIV-1-infected women. The clinical relevance of our findings for vertical transmission and clinical progression of HIV-1 disease is yet to be ascertained.

Food plant toxicants and safety Risk assessment and regulation of inherent toxicants in plant foods

The ADI as a tool for risk management and regulation of food additives and pesticide residues is not readily applicable to inherent food plant toxicants: The margin between actual intake and potentially toxic levels is often small; application of the default uncertainty factors used to derive ADI values, particularly when extrapolating from animal data, would prohibit the utilisation of the food, which may have an overall beneficial health effect. Levels of inherent toxicants are difficult to control; their complete removal is not always wanted, due to their function for the plant or for human health. The health impact of the inherent toxicant is often modified by factors in the food, e.g. the bioavailability from the matrix and interaction with other inherent constituents. Risk-benefit analysis should be made for different consumption scenarios, without the use of uncertainty factors. Crucial in this approach is analysis of the toxicity of the whole foodstuff. The relationship between the whole foodstuff and the pure toxicant is expressed in the `product correction factor' (PCF). Investigations in humans are essential so that biomarkers of exposure and for effect can be used to analyse the difference between animals and humans and between the food and the pure toxicant. A grid of the variables characterising toxicity is proposed, showing their inter-relationships. A flow diagram for risk estimate is provided, using both toxicological and epidemiological studies.

Birth weight, body length, and cranial circumference in newborns with cleft lip or palate

OBJECTIVE

This paper reports on birth weight, body length, body mass index, and cranial circumference at birth of infants with cleft lip and/or palate born between 1973 and 1992.

METHODS

Data were obtained from two nationwide Swedish health registries. Infants with syndromes, twins, and infants with immigrant parentage were excluded from the study. Comparisons were made with all singleton births with the same exclusion criteria (n = 2,031,140).

RESULTS

The body dimensions of infants with isolated cleft lip (n = 865) were found not to differ from those of control subjects, but infants with isolated cleft palate (n = 811) or cleft lip and palate (n = 1139) were found to be lighter and shorter than control subjects. Also, infants with the Pierre Robin sequence (n = 121) had a tendency to be lighter and shorter than control subjects, but these differences did not reach statistical significance despite the large study population.

Basic fibroblast growth factor is a morphogenic modulator in kidney vessel development

During kidney organogenesis the development of renal vessels must be synchronized with the maturation of nephrons and the collecting duct system. Several reports showed that hormones and mitogenic peptides as basic fibroblast growth factor (bFGF) or vascular endothelial growth factor (VEGF) are involved in this regulatory process. It is a known fact that bFGF receptors are expressed by differentiating tubular epithelium and mesenchyme, but little information is available about the function of bFGF in kidney organogenesis. The role of bFGF during kidney development was investigated using an organotypic culture system and immunohistological techniques. Renal cortex explants were prepared from the kidneys of neonatal rabbits with a microsurgical method, retaining the natural tissue composition. The explants were cultured serum free under continuous medium perfusion. Our results indicate a new and unexpected role of bFGF during the differentiation process. When bFGF alone was applied, vessels could no longer be detected. The inhibitory influence of bFGF could be overcome by addition of VEGF or hormones such as retinoic acid and aldosterone/vitamin D3. The combination of these factors with bFGF resulted in the expression of small vessel-like structures. We conclude that bFGF has a morphogenic rather than a mitogenic function during kidney vessel development.

Nutrient intakes from foods and dietary supplements in women at risk for breast cancer recurrence. The Women's Healthy Eating and Living Study Group

Dietary supplements have been suggested to have a role in cancer prevention and treatment. The purpose of this study was to describe the nutrient intakes from foods and dietary supplements in women at the time of enrollment into a clinical trial to prevent breast cancer recurrence. Subjects were within four years of diagnosis with Stage I, II, or IIIA breast cancer and had completed medical treatment (n = 435). Intakes were assessed with four 24-hour recalls over two weeks. Dietary nutrient intakes in supplement users were compared with intakes in nonusers, and supplement nutrient intakes in participants consuming diets providing < 75% were compared with those in participants consuming > or = 75% of recommended levels. Intakes of participants with diets meeting general guidelines for disease prevention were compared with intakes of those whose diets did not meet these guidelines. Dietary supplement use was reported by 352 (80.9%) of the participants, but frequency of excess intakes did not exceed 5% for all micronutrients examined. Women whose diets provided higher levels of most vitamins and minerals were more likely to obtain additional amounts of these micronutrients from dietary supplements. Participants reporting use of any supplement consumed diets providing more dietary fiber (p < 0.04) and less dietary fat (p < 0.001) than nonusers of any supplement. These results illustrate the importance of monitoring dietary supplement use in clinical trials with a focus on preventing cancer recurrence, because supplements can contribute substantially to nutrient intakes in the population under study.

Periconceptional vitamin A use: how much is teratogenic?

The objective of the review is to determine whether preformed vitamin A (retinol and retinyl esters) is teratogenic at dosages commonly used by women living in industrialized countries. Published human and animal data and research developed by the authors are reviewed. It is well known that vitamin A is essential for normal reproduction and development. Although doses of 10,000 IU/d or less of preformed vitamin A (retinyl esters and retinol) are considered safe, doses > 10,000 IU/d as supplements have been reported to cause malformations in a single epidemiologic study. Nonhuman primate data show no teratogenicity at doses of 30,000 IU/d. Daily periconceptional exposures greater than 25,000 IU/d of preformed vitamin A have not been sufficiently studied to establish specific risk. Because no study reports adverse effects of 10,000 IU/d preformed vitamin A supplements and this dose is more than the Recommended Dietary Allowance for pregnant women (2670 IU or 800 RE/d), we recommend that women living in industrialized countries or who otherwise have nutritionally adequate diets may not need to ingest more than the Recommended Dietary Allowance of preformed vitamin A as supplements. If periconceptional vitamin A exposures to levels up to 30,000 IU/d (9,000 micrograms RE/d) do occur unintentionally, multiple animal studies do support only very low risk. Human epidemiologic studies do not establish at what level vitamin A becomes teratogenic; however, pharmacokinetic data presented in this paper indicate that blood levels of retinoids from women taking 30,000 IU/d of preformed vitamin A are not greater than retinoid blood levels in pregnant women during the first trimester who delivered healthy babies. Interestingly, neither teratogenicity nor vitamin A toxicity has been observed in multiple species exposed to high doses of beta-carotene.

Development of anorectal malformations using etretinate

PURPOSE

To investigate the pathogenesis of anorectal malformations (ARM), the authors studied cell proliferation and programmed cell death (apoptosis) patterns in murine embryos that develop ARM as a result of administering an overdose of etretinate, a long-acting vitamin A analogue (retinoid).

METHODS

Pregnant mice were fed 60 mg/kg of etretinate on the ninth gestational day (E9). Embryos were obtained between E9.5 and E13, and prepared for histological study. Cell proliferation was examined using proliferative cell-specific nuclear antigen (PCNA) expression. Apoptosis was identified by detecting in situ DNA fragmentation using the TdT-mediated dUTP-digoxigenin nick end-labeling (TUNEL) method.

RESULTS

Over 95% of etretinate-treated embryos had ARM including rectoprostatic urethral or rectocloacal fistula. In the histological study, ARM embryos showed defective cell proliferation in the cloacal membrane and excessive apoptosis in the dorsocaudal region on E11, which resulted in a lack of apoptosis in the anal orifice and a short tail on E12, respectively. Cells forming the urorectal septum showed the same pattern of cell proliferation and apoptosis both in ARM embryos and the controls. These results suggest that impairments of embryonal cellular dynamics in the cloacal membrane and dorsocaudal region induce some types of ARM.

A survey of vitamin A concentrations in the liver of food-producing animals

High oral doses of vitamin A have been shown to be teratogenic and accordingly the World Health Organization has recommended that the daily intake for pregnant women should not exceed 3.3 mg. Liver contains high concentrations of this vitamin and consequently it has become necessary to assess its level in the livers of food animals. A survey of fresh, frozen, imported and home-produced retail liver samples from calf (42), ox (121), lamb (228), pig (133) and chicken (125) was carried out between August 1992 and April 1993 using a modified version of the method of Ashoor and Knox (1987) to determine retinol and retinyl esters. Interlaboratory comparison showed no significant difference in results for this method and a method which employed hydrolysis of retinyl esters, and there were significant advantages in specificity, simplicity, cost and quality control. The mean +/- sd total vitamin A concentration calculated as retinol equivalents, for all species surveyed was 139 +/- 96 with a range of 3-1267 mg/kg liver. Mean values for individual species were: calf 188 +/- 125, ox 142 +/- 110, lamb 173 +/- 104, pig 174 +/- 118 and chicken 97 +/- 44 mg/kg liver. No change in the vitamin A concentrations in liver were observed compared with previous surveys in the UK, which suggests that the intake of vitamin A from dietary and supplementary sources by different species has not changed. The data from this survey confirm that liver cannot be recommended as a food for pregnant women.

Mother-child HIV-1 transmission: Timing and determinants

Prevention of mother-to-child transmission of HIV is a significant public health priority. A regimen of zidovudine administered during pregnancy, intrapartum, and to the newborn significantly reduces transmission, and incorporation of this regimen into clinical practice has been associated with significant decreases in perinatal transmission in industrialized countries. This regimen, however, is not applicable in the developing world (where most perinatal transmission occurs), and simpler, shorter, less costly regimens are urgently needed. An understanding of the pathogenesis of perinatal transmission is crucial for the design of new preventive and therapeutic regimens, and current knowledge is reviewed in this article, with an emphasis on relevance to prevention.

[Teratogenic effects of vitamin A and its derivates]

Retinoids, the synthetic derivates of vitamin A, have a key role on cellular differentiation and developmental tissue specificity. Their effects are mediated by nuclear receptors which transactivate homeobox genes. They are teratogenic to animals and they all induce similar malformations dependent on the dose and the duration of exposure. This is a review of the teratogenic effects of vitamin A and its synthetic derivates--isotretinoin, acitretine and topical retinoids--in humans. High dose vitamin A have a potent teratogenic effect and are therefore contra-indicated during pregnancy. Isotretinoin is responsible for a syndrome including malformations of the central nervous system, heart and thymus, together with craniofacial defects. The incidence rate is high and comparable to thalidomide (ie, 25%). This high teratogenic potency justifies a strict limitation of such a prescription in women susceptible to become pregnant. Acitretine, which replaces etretinate because of its long half life of 120 days, might also be teratogenic in humans. In addition, it may be back transformed into etretinate, thus contraindicating pregnancy for 2 years after withdrawal. Finally, despite a low percutaneous resorption, available data on the use of retinoids as topicals are limited and their use during pregnancy is therefore not recommended. Although they are efficient in skin diseases, the use of retinoids in women of the child bearing age is very limited because of their potent teratogenic effect.

The epidemiology of preaxial limb malformations

The epidemiology of preaxial limb malformations was studied in an effort to find similarities and differences between different types that could help to define suitable groups of malformations for etiologic studies. Material from three large congenital malformation registers was used, with a total of 1646 infants with such malformations among over 5 million births. We compared different types of preaxial malformations with respect to bilaterality, maternal age and parity, racial differences, sex distribution, twinning, and associated malformations. From many epidemiologic aspects, different types of preaxial malformations showed similarities, but there were some distinct differences between upper and lower limb anomalies. We conclude that in searches for etiologic factors, it may be useful to group all preaxial malformations together but to differentiate between upper and lower limb defects.

Vitamin A and birth defects

OBJECTIVE

Our objective was to determine whether moderate doses of vitamin A are teratogenic.

STUDY DESIGN

This was a geographically based case-control study. Women whose pregnancies produced offspring with neural tube defects (n = 548) or major malformations other than neural tube defects (n = 387) and normal control subjects (n = 573) were interviewed to determine periconceptional vitamin A supplement exposure levels.

RESULTS

The proportion of women consuming doses of vitamin A between 8000 and 25,000 IU was no greater in the major malformations group or the group with neural tube defects than in the normal control group. For exposure from supplements and fortified cereals combined, women consuming >8000 and >10,000 IU daily had odds ratios for major malformations of 0.79 (95% confidence interval 0.40 to 1.53) and 0.73 (95% confidence interval 0.27 to 1.96), respectively, compared with women consuming <5000 IU. The results for neural tube defects were similar. For cranial neural crest defects the odds ratios were 0.76 (0.22 to 2.56) and 1.09 (0.24 to 4.98) for exposure to >8000 and >10,000 IU, respectively, versus exposure to <5000 IU.

CONCLUSIONS

This study found no association between periconceptional vitamin A exposure at doses >8000 IU or >10,000 IU per day and malformations in general, cranial neural crest defects, or neural tube defects. If vitamin A is a teratogen, the minimum teratogenic dose appears to be well above the level consumed by most women during organogenesis.

The future of prenatal HIV testing

The use of zidovudine for the prevention of mother-to-child transmission of human immunodeficiency virus, as well as continuing advances in our understanding of the determinants of that transmission, and of potential new avenues of intervention augur ever more efficient perinatal prevention strategies in the near future. In this paper the evolving role of prenatal testing in an era of ever more successful interventions will be discussed. At least four factors will be critical in determining what that role will be: clinical, legal, ethical and economic. Each of these are parochial, i.e. they will feature in eradication strategies in different ways in different geographical, economic and cultural settings. An understanding of these factors will help to suggest what course testing policy should take and to determine the role clinicians must play if any testing policy is to succeed in contributing to the end of pediatric HIV disease.

Promoting healthy eating and ensuring adequate weight gain in pregnant adolescents: issues and strategies

The seven specific strategies discussed for promoting the nutritional health of pregnant adolescents can be categorized into three broad areas: (1) improve nutrition knowledge and skills of adolescents, as well as health care providers; (2) improve programs, including access to prenatal care, and developing effective nutrition interventions; and (3) direct research efforts to better understand barriers to behavior change and conduct scientifically rigorous program evaluations of efficacy of nutrition interventions. Nutritional health should be viewed within the context of improving overall health and lifestyles. Simple solutions focused solely at the individual level are not effective. We need to advocate for and support policy efforts to address the economic, familial, and social issues that impact health and adolescent pregnancy.

Worldwide prevention of vertical human immunodeficiency virus (HIV) transmission

Most human immunodeficiency virus (HIV) infected children acquire the infection by perinatal transmission. Strategies for the prevention of perinatal HIV transmission mainly focus on conditions that prevail in industrialized countries, although most of these children are born in areas with less privileged circumstances. This review takes these circumstances into consideration when addressing six potential strategies to reduce perinatal HIV transmission.

Maternal serum vitamin A levels are not associated with mother-to-child transmission of HIV-1 in the United States

HIV-1 transmission from mother to child has been associated with maternal vitamin A status in studies of women living in Africa. This finding has raised the question of whether vitamin A supplementation might help reduce transmission in the United States as well as worldwide. In industrialized nations, however, both the vitamin A nutritional status of HIV-1-infected pregnant women and the association of vitamin A levels with vertical transmission were unknown. Furthermore, vitamin A is teratogenic, and supplements during pregnancy have caused birth defects. To investigate whether maternal serum levels of vitamin A (retinol) and three other micronutrients correlate with vertical transmission of HIV-1 in the United State, we studied 95 HIV-1-infected pregnant women and followed their infants to determine whether transmission occurred. Sera were obtained during the third trimester of pregnancy from 95 HIV-1-infected women living in the New York and Los Angeles metropolitan areas. The two cohorts were established to study vertical transmission of HIV-1 and to reflect the racial, ethnic, and socioeconomic status of HIV-1-infected in women in the United States. We measured serum levels of vitamin A (retinol) and three other micronutrients, vitamin E (alpha-tocopherol), beta-carotene, and lycopene, in the mothers using reverse-phase high-performance liquid chromatography and determined the HIV-1 infection status of their infants using virus cultivation and polymerase chain reaction. Sixteen of the 95 women transmitted HIV-1 to their infants. Statistical analysis of the data indicated that low maternal serum retinol levels during the third trimester of pregnancy were not associated with mother-to-child transmission of HIV-1. None of the women had retinol levels so low as to have clinical symptoms of vitamin A deficiency. The serum levels of alpha-tocopherol, beta-carotene, and lycopene, three micronutrients that act as antioxidants and enhance immune function, were also measured. Statistical analysis of the data revealed no association of the levels of these three micronutrients with vertical transmission of HIV-1. Analysis of the data obtained from 95 women in the United States indicates that vitamin A deficiency is rare, and serum retinol levels are not associated with risk of vertical HIV-1 transmission. In view of the teratogenic effects of vitamin A when taken as a supplement during pregnancy, pregnant HIV-1-infected women living in nations where vitamin A deficiency is not a public health problem should not be advised to take extra vitamin A supplements.

Vitamin A and its congeners

Vitamin A (retinol) is a fat-soluble vitamin that is necessary for cell growth and differentiation. Excess vitamin A has been associated with teratogenic effects in animals and humans. Because vitamin A deficiency is very uncommon in the industrialized world, the current recommendation is that routine vitamin A supplementation is not necessary. If vitamin A supplements are used, they should be limited to less than 5,000 IU per day. Systemic administration of the naturally occurring retinoid tretinoin has been associated with birth defects, fetal resorption, and stillbirths in animals; however, topical use is not associated with increased birth defects and is classified as a category B drug during pregnancy. The synthetic retinoids isotretinoin, etretinate, and etretin are strictly contraindicated during pregnancy (category X) as they have been associated with teratogenic syndromes in humans. In addition, owing to the prolonged elimination half-life of aromatic retinoids, effective contraception should be used for at least 2 years following discontinuation of treatment with these drugs.

Retinoic acid, neoplasia, differentiation and development

Retinoic acid has pronounced effects on cultures of neoplastic cells. These have attracted the attention of pathologists, but it is important to note that much of the critical data about retinoic acid has been obtained from the current extension of our knowledge in the field of development. Some of these changes are reviewed here.

Preconception genetic counseling

The purpose of this chapter has not been to be all-inclusive, but to raise awareness of the benefits of preconception consultation in the prevention of birth defects. If there is to be a significant decrease in the incidence of congenital malformations, it will come only by intervention that occurs prior to organogenesis, and that opportunity is lost by the first prenatal visit. For the practitioner interested in a more in-depth analysis of the subject of preconception counseling, the two books listed as supplementary reading provide a wealth of information essential to the care of the patient planning a pregnancy.

Rat metanephric organ culture in terato-embryology

The development of the permanent mammalian kidney, or metanephros, depends on mesenchymal-epithelial interactions, leading to branching morphogenesis of the ureteric bud that forms the collecting ducts and to conversion of the metanephric mesenchyme into epithelium that forms the nephrons. Rat metanephric organ culture in which these interactions are maintained is a valuable in vitro model system for investigating normal and abnormal renal organogenesis. Methods were designed to evaluate either the capacity of the ureteric bud to branch or that of the mesenchyme to form nephrons. Both are based on specific staining of the ureteric bud and the glomeruli with lectins. Using this approach, we have shown that retinoids are potent stimulating factors of nephrogenesis, acting through an increase in the branching capacity of the ureteric bud. On the other hand, several drugs such as gentamicin and cyclosporin A were found to reduce the number of nephrons formed in vitro. While gentamicin affects the early branching pattern of the ureteric bud, cyclosporin may affect the capacity of the mesenchyme to convert into epithelium. This methodology therefore appears a potentially useful tool for toxicological studies of new drugs.

Oral retinoids and pregnancy

The oral retinoids isotretinoin and etretinate are uniquely effective in the treatment of severe cystic acne and keratinisation disorders. Because of their known teratogenicity, there are strict prescription guidelines, but exposure during pregnancy still occurs. A dedicated effort by women and their clinicians is required, involving patient selection, education and informed consent, detailed contraceptive counselling, and careful monitoring and management, including pregnancy testing before commencement of therapy.

Identification of 9-cis-retinoic acid, 9,13-di-cis-retinoic acid, and 14-hydroxy-4,14-retro-retinol in human plasma after liver consumption

Vitamin A is a well-established teratogen in several animal species. Case reports as well as a recent epidemiological study suggest that vitamin A intake in excess of 25,000 or 10,000 IU respectively, can result in retinoid-specific defects in the offspring. A single meal of liver contains, on the average, a 10- to 20-fold higher amount of vitamin A than what is already suspected to be teratogenic. To evaluate the risk of liver consumption during pregnancy, we have studied levels of vitamin A and a number of potentially active retinoid metabolites in plasma of ten healthy male volunteers following consumption of fried turkey liver (2 g raw weight/kg body weight). HPLC, UV spectroscopy and mass spectrometry were used for identification and quantitation of retinoids in plasma. As shown previously, vitamin A intake via liver consumption resulted in greatly increased plasma levels of 13-cis-retinoic acid (13-cis-RA) and 13-cis-4-oxo-RA, and low levels of all-trans-RA and all-trans-4-oxo-RA. In our present investigation 9-cis-RA, 9,13-di-cis-RA, and 14-hydroxy-4,14-retro-retinol (14-HRR) were identified for the first time in humans as physiological metabolites of vitamin A. 9-cis-RA is a potent teratogen as well as a high affinity ligand of retinoid receptors, and 14-HRR was previously shown to promote lymphocyte activation in vitro. The present study bears on the issue of a possible teratogenic risk of liver consumption, as active retinoids were identified in human plasma, and their levels could be related to previous human studies as well as to experimental studies in sensitive animal species.

Metanephros organogenesis is highly stimulated by vitamin A derivatives in organ culture

Vitamin A and its metabolic derivatives are known to be key signalling molecules in regulating morphogenetic events in vertebrate development. Here we investigated their possible involvement during mammalian kidney development using paired rat metanephros organ culture. Metanephroi were explanted from 14-day-old embryos and cultured for six days in a chemically defined medium containing a retinoid at a dose of 10(-11) to 10(-4) M. Retinol, all-trans and 9-cis retinoic acid were able to promote metanephros growth and differentiation in vitro. A significant increase in the number of nephrons was observed from 10(-8) M of retinol and 10(-10) M of all-trans retinoic acid, before any change in growth parameters. A threefold increase in the number of nephrons was obtained at a dose of 10(-6) M. At low retinoid concentrations, there was a modulating effect of triiodothyronine on retinoid-stimulated nephrogenesis since the absence of triiodothyronine in the medium enhanced the nephrogenic stimulation. Exposure of metanephroi from 13-day-old embryos to all-trans retinoic acid (10(-7) M) led to a sixfold increase of nephron formation. Finally, we analyzed the branching pattern of the ureteric bud and showed that within 48 hours of culture, it was significantly more developed upon retinoid exposure. In conclusion, this study demonstrates that retinoic acid is a key regulator of renal organogenesis in controlling nephrogenic induction processes and ureteric bud patterning, and that the younger the metanephros, the greater the effect.