Detection of teratogens by differentiating embryonic neural crest cells in culture: evaluation as a screening system

Alternative Tests for Teratogenicity

Summary

The major features of the tests surveyed are shown in Table I. In a tier system of tests for teratogenicity, the Chernoff test is at a different level than the other assays described here. It is not appropriate for screening large numbers of chemicals, but may be useful for studies of smaller groups of agents, for example to confirm data from a prescreen. Although the test is certainly easier, cheaper and uses less than half the animals of a Segment II test, it is still much more expensive and time-consuming than most alternative tests.

Of the remaining alternatives, whole embryos or organs in culture encompass the widest range of mammalian developmental events and are invaluable in the study of teratogenic mechanisms. They are, however, also inappropriate for screening large numbers of chemicals. The methods are technically demanding, relatively expensive and use reasonably large numbers of pregnant mammals. To screen a group of, say, 20 chemicals involves a considerable investment of time and, in fact, no study of this size has been reported. In certain specific circumstances, they may be a useful adjunt to testing; for example, if treated human serum samples are freely available, if a drug has a unique action on rodent dams which confounds evaluation of the standard in vivo tests, or if human metabolism is important and can be mimicked in vitro.

Sub-mammalian and sub-vertebrate species offer considerable advantages; reduced cost, relative rapidity and no requirement for laboratory animals. FETAX provides some indication of teratogenicity in relation to embryotoxicity, while CHEST and the planarian and Drosophila assays measure only teratogenic potential, or more strictly speaking, embryotoxic potential, although it should be possible to derive some assessment of hazard with each of the latter systems. The Hydra system is cheap, quick and easy and is commercially available. It is the only assay specifically designed to estimate teratogenic hazard and may offer considerable advantages as an alternative screen.

The metabolic cooperation assay has not generated sufficient data to enable evaluation. The neural crest cell assay is not well developed as a routine screen, and objective endpoints which are not measures of general cytotoxicity must be devised. The viral morphogenesis and Drosophila embryo cell assays have both produced encouraging validation data. With further assessment, the viral system may be shown to be useful, but it is a relatively complex assay and its relevance to teratogenesis is obscure. The Drosophila system is easier, has been used with more chemicals and is developmentally relevant. However, it has not produced dose-response data to evaluate potency or hazard, and must be improved so that it can more clearly distinguish cytotoxicity. The measurement of endpoints in the neuroblastoma cell line assay requires further refinement, and contributions of growth inhibition or stimulation to effects on differentiation must be examined. In combination, tumour cell attachment and HEPM may prove valuable. Alone, HEPM appears to be an assay for cellular toxicity, not teratogenicity, and the attachment assay suffers from a high rate of false negatives because it measures only one cell phenomenon. Although micromass cultures use mammalian tissue, are not the cheapest assays and require some skill for full evaluation of the results obtained, they show considerable promise. Validation data are encouraging, the assay includes several developmental processes and the use of multiple endpoints permits specific developmental toxicities to be evaluated.

Implications for the Predictivity of Cell-Based Developmental Toxicity Assays Developed Two Decades Apart

Many in vitro developmental toxicity assays have been proposed over several decades. Since the late 1980s, we have made intermittent attempts to introduce in vitro assays as screening tests for developmental toxicity of in-house candidate products. Two cell-based assays which were developed two decades apart were intensively studied. One was an assay of inhibitory effects on mouse ascites tumor cell attachment to a concanavalin A-coated plastic sheet surface (MOT assay), which we studied in the early days of assay development. The other was an assay of inhibitory effects on the differentiation of mouse embryonic stem cell to beating heart cells (EST assay), which we assessed more recently. We evaluated the suitability of the assays for screening in-house candidates. The concordance rates with in vivo developmental toxicity were at the 60% level. The EST assay classified chemicals that inhibited cell proliferation as embryo-toxic. Both assays had a significant false positive rate. The assays were generally considered unsuitable for screening the developmental toxicity of our candidate compounds. Recent test systems adopt advanced technologies. Despite such evolution of materials and methods, the concordance rates of the EST and MOT systems were similar. This may suggest that the fundamental predictivity of in vitro developmental toxicity assays has remained basically unchanged for decades. To improve their predictivity, in vitro developmental toxicity assays should be strictly based on elucidated pathogenetic mechanisms of developmental toxicity.

Connecting teratogen-induced congenital heart defects to neural crest cells and their effect on cardiac function

Neural crest cells play many key roles in embryonic development, as demonstrated by the abnormalities that result from their specific absence or dysfunction. Unfortunately, these key cells are particularly sensitive to abnormalities in various intrinsic and extrinsic factors, such as genetic deletions or ethanol-exposure that lead to morbidity and mortality for organisms. This review discusses the role identified for a segment of neural crest in regulating the morphogenesis of the heart and associated great vessels. The paradox is that their derivatives constitute a small proportion of cells to the cardiovascular system. Findings supporting that these cells impact early cardiac function raises the interesting possibility that they indirectly control cardiovascular development at least partially through regulating function. Making connections between insults to the neural crest, cardiac function, and morphogenesis is more approachable with technological advances. Expanding our understanding of early functional consequences could be useful in improving diagnosis and testing therapies.

Discriminative power of an assay for automated in vitro screening of teratogens

Screening for potential teratogenicity of 20 test compounds was performed using a computerised microscope workstation for determination of cytotoxicity, proliferation and morphology of fibroblastoid murine L929-cells. The test compounds, which were divided into four classes according to teratogenicity were: 5-bromo-2(')-deoxyuridine, 6-aminonicotinamide, acrylamide, boric acid, D-(+)-camphor, dimethadione, dimethyl phthalate, diphenhydramine, hydroxyurea, isobutyl-ethyl-valproic acid, lithium chloride, methyl mercury chloride, methotrexate, methoxyacetic acid, penicillin G, all-trans-retinoic acid, pentyl-4-yn-valproic acid, saccharin, salicylic acid and valproic acid. All compounds, with the exception of dimethadione inhibited proliferation in a linear dose-dependent manner, and there were statistically significant compound class-dependent differences between the IC(50)-values for the compounds (p<0.0374), the strongest teratogens being the most potent. Furthermore, the average efficacies (maximum relative change) for 10 parameters describing cell morphology exhibited statistically significant compound class-dependent differences (p<0.0001), the class I and II compounds exhibiting significantly lower efficacies than the class III and IV compounds (p<0.01). Thus, test compounds affected both the proliferation and morphology of L-cells in manner demonstrating a general relationship with the teratogenic potency of the compounds. However, the moderate teratogens dimethadione and lithium chloride only had minor effects on the morphology and proliferation of the cells whereas the non-teratogen diphenhydramine had effects on both proliferation and morphology comparable to the strong teratogens.

Bendectin: review of the medical literature of a comprehensively studied human nonteratogen and the most prevalent tortogen-litigen

OBJECTIVE

to review the extensive literature pertaining to the reproductive and teratogenic effects of Bendectin and the opinions of the scientific experts for the defense and plaintiff. These data were evaluated with regard to the reproductive risks of Bendectin providing a scientific framework for evaluating the views of the experts in the Bendectin litigation.

DESIGN

the Bendectin literature was primarily obtained from articles cited in Research Alert of the Institute for Science Information. Other articles were obtained from Medline, review articles, and colleagues. An attempt was made to be all-inclusive, citing and reviewing all articles related to each subject being discussed. The literature includes epidemiologic studies, animal studies, in vitro studies, and basic science articles related to the principles of teratology and reproductive toxicology. Review articles, meta analyses, editorials, commentaries, articles in the press, and case reports were also included.

METHODOLOGY

the methodology utilized for the evaluation of Bendectin teratogenicity was presented. It consists of a five-part analysis of epidemiologic studies, secular trend analysis, animal studies, dose-response relationships, and biologic plausibility.

CONCLUSION

the five-part analysis of Bendectin reproductive effects indicates that therapeutic use of Bendectin has no measurable teratogenic effects. Presentations by many of the plantiff's experts failed to meet the scientific standards that should be expected of knowledgeable scientists and contributed to the persistence of Bendectin litigation.

Lung organoid culture

An organoid culture system for lung cells is described in which morphogenesis of lung histotypic structures and differentiation of both pneumocytes type II and mesenchyme occur. The principle of this technique is the culture of mouse fetal lung cells at high density on a membrane filter at the medium/air interface. In the course of cultivation, cell sorting-out, epithelial cell aggregation, formation of an alveolar-like lumen in the organoids and formation of a basal lamina occur. Epithelial differentiation culminates in the production of lamellar bodies, and the mesenchyme develops into mature connective tissue. Morphogenesis and differentiation depend on the stage of fetal development from which the lung cells were derived but appear independent of the formation of a basal lamina. Various drugs have been tested for their effects on morphogenesis and differentiation in this lung organoid culture: some of them inhibit differentiation or damage the mesenchyme, others stimulate surfactant production. Due to the quite complex morphogenetic and cellular events occurring in lung organoid culture, it may be an applicable tool for alternative in vitro screening methods.

Anomalous neural differentiation induced by 5-bromo-2'-deoxyuridine during organogenesis in the rat

The influence of 5-bromo-2'-deoxyuridine (BrdU) on rat embryo development and neurogenesis was investigated using a rat conceptus culture system during organogenesis (pregnancy days 10-13). The embryos and visceral yolk sacs of conceptuses cultured with BrdU were examined for overall growth, morphological anomalies, incorporation of radiolabeled BrdU into DNA, and neurotransmitter enzyme activities in embryos. In addition, neural tubes from cultured whole embryos were isolated and mechanically dissociated into fragments and cultured again to assess neural cell differentiation into neuron-like cells. BrdU was found to incorporate differentially into embryonic and visceral yolk sac DNA with simultaneous stage-specific retardation and anomalous organogenesis in proportion to the increasing concentrations used. Neural tube differentiation of cultured embryos was markedly altered, and there were morphologically distinct neural anomalies. The neurite outgrowth from neuroblast cells (type 1) of explanted spinal neural tube fragments from BrdU-treated embryos was markedly reduced in length and number compared to those from similar areas of embryos grown without BrdU. In contrast, BrdU at the same doses did not affect differentiation of a number of neural tissue-related enzymes. These results indicate that BrdU incorporation into DNA of primordial embryonic cells significantly affects neurogenesis and differentiation of neurites from neuroblasts, which is a specific neural cytodifferentiation characteristic of neuronal cells.

Application of the preliminary developmental toxicity screen for chemical hazard identification under the Toxic Substances Control Act

The Office of Toxic Substances (OTS) within the U.S. Environmental Protection Agency (EPA) is authorized to carry forth the mandates of the Toxic Substances Control Act (TSCA). Included among the provisions of TSCA are the development of requirements for testing of "new" and "existing" chemicals that may present an unreasonable risk of injury to health or the environment. There are over 63,000 "existing" chemicals on the TSCA inventory, and EPA in recent years has been receiving an average of over 1,300 submissions for "new" chemicals a year. Since it is illogical and unrealistic to expect that all of these chemicals should be subjected to detailed testing for all potential adverse health and environmental effects, OTS views screening assays as highly useful tools to assign priorities to chemicals for further testing according to standard methodologies. The Chernoff/Kavlock assay (preliminary developmental toxicity screen) was specifically developed to address the need for a developmental toxicity assay to prioritize for further testing the large number of "new" and "existing" chemicals. OTS has been involved in seeking the development of data through the preliminary developmental toxicity screen for purposes of validating the screen and to obtain critical data necessary for evaluating chemicals. OTS believes that the screen has a role in the risk assessment process and has developed a testing protocol, which is included along with other OTS test guidelines; has provided internal guidance on when the screen may be recommended; and has discussed how the data may be applied in prioritizing chemicals for further study.

Xenobiotic influences on embryonic differentiation, growth and morphology in vitro

Direct in vitro exposure of post-implantation rat embryos to 18 known teratogens induced typical malformations in all cases. Of 21 non-teratogens in vivo, 20 induced, in vitro either no malformations at all, even at high concentrations, or abnormal development could only be observed at concentrations which affected growth and differentiation significantly. Responses of chemically induced exposed embryos in vitro occurred within wide concentration ranges. Actinomycin D, for example, affected embryonic development at a concentration as low as 3 X 10(-4) micrograms/ml, whereas other substances had no effect at concentrations up to 9 X 10(2) micrograms/ml.

Analysis of the activity of DNA, RNA, and protein synthesis inhibitors on Xenopus embryo development

The teratogenic and growth-inhibiting potential of DNA, RNA, and protein synthesis inhibitors was explored using the Frog Embryo Teratogenesis Assay: Xenopus (FETAX). Endpoints measured in 96-h static tests were survival, malformation, ability to swim, skin pigmentation, stage of development, and growth. The DNA synthesis inhibitors hydroxyurea, cytosine arabinoside, and ethidium bromide proved to be teratogenic by the severity of malformations induced. Hydroxyurea gave an LC50 of 1.82 mg/ml, an EC50 (malformation) of 0.43 mg/ml, while the values for cytosine arabinsode were 5.41 and 0.76, respectively. The values for ethidium bromide were 0.05 and 0.035. The RNA synthesis inhibitor actinomycin D and the protein synthesis inhibitor cycloheximide were more embryolethal than teratogenic but significantly inhibited growth as determined by head-tail length measurements. Actinomycin D caused severe malformations, while cycloheximide caused relatively minor abnormalities. The LC50 for actinomycin D was 1.89 mg/ml, while the EC50 (malformation) was 2.17 mg/ml. For cycloheximide, the values were 1.59 and 1.19, respectively. FETAX advantages include rapid data collection, the ability to measure stage-dependent effects, and the ability to use a large number of embryos to obtain excellent dose-response curves with narrow confidence limits. Disadvantages include lack of a metabolic activation system, absence of a placental relationship, and the inability to detect specific abnormalities such as limb defects in 96 h.

Effects of anticonvulsant drugs on brain cultures from chick embryos: a comparison with cultures from embryos treated in ovo

The effects of anticonvulsant drugs phenytoin, phenobarbitone, and sodium valproate on neurons and glia from embryonic chicken brain have been tested. These effects have been compared with those produced in neuronal and glial cultures established from embryos that were injected with the drugs in ovo. Choline acetyl transferase activity and accumulation of gamma-aminobutyric acid were measured in neuronal cultures, and carbonic anhydrase and 2,'3'-cyclic nucleotide 3' phosphohydrolase activities were measured in glial cultures. Similarities have been observed in the morphological and biochemical changes brought about by the in vitro and in vivo treatment with the drugs. The use of in vitro cell culture systems for screening drugs for potential behavioural teratogenic effects is discussed.

Toxic and teratogenic effects of chemical class fractions of a coal-gasification electrostatic precipitator tar

Dimethyl sulfoxide slurries of a coal gasifier electrostatic precipitator tar and its chemical class fractions were assayed for their toxicity and teratogenicity using early embryos of the frog Xenopus laevis. Of the 5 tar fractions the ether-soluble base and polyaromatic were found to be the most teratogenic and the ether-soluble acid and ether-soluble base were the most toxic. The teratogenic effects of the raw tar suggest synergism. The toxic effects to newly metamorphosed froglets is 1-2 orders of magnitude less than those observed for embryos. Chemical analysis shows dihydroxybenzenes and organonitrogen compounds to be the major components of the acid and base fractions, respectively. The neutral fractions contain mainly alkyl-substituted two-ring hydrocarbons.