Nervous system development related gene expression regulation in the zebrafish embryo after exposure to valproic acid and retinoic acid: A genome wide approach.

The evaluation of chemical and pharmaceutical safety for humans is moving from animal studies to New Approach Methodologies (NAM), reducing animal use and focusing on mechanism of action, whilst enhancing human relevance. In developmental toxicology, the mechanistic approach is facilitated by the assessment of predictive biomarkers, which allow mechanistic pathways perturbation monitoring at the basis of human hazard assessment. In our search for biomarkers of maldevelopment, we focused on chemically-induced perturbation of the retinoic acid signaling pathway (RA-SP), a major pathway implicated in a plethora of developmental processes. A genome-wide expression screening was performed on zebrafish embryos treated with two teratogens, all-trans retinoic acid (ATRA) and valproic acid (VPA), and a non-teratogen reference compound, folic acid (FA). Each compound was found to have a specific mRNA expression profile with 248 genes commonly dysregulated by both teratogenic compounds but not by FA. These genes were implicated in several developmental processes (e.g., the circulatory and nervous system). Given the prominent response of neurodevelopmental gene sets, and the crucial need to better understand developmental neurotoxicity, our study then focused on nervous system development. We found 62 genes that are potential early neurodevelopmental toxicity biomarker candidates. These results advance NAM-based safety assessment evaluation by highlighting the usefulness of the RA-SP in providing early toxicity biomarker candidates.

Retinoic acid signaling pathway perturbation impacts mesodermal-tissue development in the zebrafish embryo: Biomarker candidate identification using transcriptomics.

The zebrafish embryo (ZE) model provides a developmental model well conserved throughout vertebrate embryogenesis, with relevance for early human embryo development. It was employed to search for gene expression biomarkers of compound-induced disruption of mesodermal development. We were particularly interested in the expression of genes related to the retinoic acid signaling pathway (RA-SP), as a major morphogenetic regulating mechanism. We exposed ZE to teratogenic concentrations of valproic acid (VPA) and all-trans retinoic acid (ATRA), using folic acid (FA) as a non-teratogenic control compound shortly after fertilization for 4 h, and performed gene expression analysis by RNA sequencing. We identified 248 genes specifically regulated by both teratogens but not by FA. Further analysis of this gene set revealed 54 GO-terms related to the development of mesodermal tissues, distributed along the paraxial, intermediate, and lateral plate sections of the mesoderm. Gene expression regulation was specific to tissues and was observed for somites, striated muscle, bone, kidney, circulatory system, and blood. Stitch analysis revealed 47 regulated genes related to the RA-SP, which were differentially expressed in the various mesodermal tissues. These genes provide potential molecular biomarkers of mesodermal tissue and organ (mal)formation in the early vertebrate embryo.

Dynamic regulation of gene expression and morphogenesis in the zebrafish embryo test after exposure to all-trans retinoic acid.

The zebrafish embryotoxicity test (ZET) is widely used in developmental toxicology. The analysis of gene expression regulation in ZET after chemical exposure provides mechanistic information about the effects of chemicals on morphogenesis in the test. The gene expression response magnitude has been shown to change with exposure duration. The objective of this work is to study the effect of the exposure duration on the magnitude of gene expression changes in the all-trans retinoic acid (ATRA) signaling pathway in the ZET. Retinoic acid regulation is a key driver of morphogenesis and is therefore employed here as an indicator for the regulation of developmental genes. A teratogenic concentration of 7.5 nM of ATRA was given at 3 hrs post fertilization (hpf) for a range of exposure durations until 120 hrs of development. The expression of a selection of genes related to ATRA signaling and downstream developmental genes was determined. The highest magnitudes of gene expression regulation were observed after 2-24 hrs exposure with an optimal response after 4 hrs. Longer exposures showed a decrease in the gene expression response, although continued exposure to 120 hpf caused malformations and lethality. This study shows that assessment of gene expression regulation at early time points after the onset of exposure in the ZET may be optimal for the prediction of developmental toxicity. We believe these results could help optimize sensitivity in future studies with ZET.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny-part III: how is substance-mediated thyroid hormone imbalance in pregnant/lactating rats or their progeny related to neurodevelopmental effects?

This review investigated which patterns of thyroid- and brain-related effects are seen in rats upon gestational/lactational exposure to 14 substances causing thyroid hormone imbalance by four different modes-of-action (inhibition of thyroid peroxidase, sodium-iodide symporter and deiodinase activities, enhancement of thyroid hormone clearance) or to dietary iodine deficiency. Brain-related parameters included motor activity, cognitive function, acoustic startle response, hearing function, periventricular heterotopia, electrophysiology and brain gene expression. Specific modes-of-action were not related to specific patterns of brain-related effects. Based upon the rat data reviewed, maternal serum thyroid hormone levels do not show a causal relationship with statistically significant neurodevelopmental effects. Offspring serum thyroxine together with offspring serum triiodothyronine and thyroid stimulating hormone appear relevant to predict the likelihood for neurodevelopmental effects. Based upon the collated database, thresholds of ≥60%/≥50% offspring serum thyroxine reduction and ≥20% and statistically significant offspring serum triiodothyronine reduction indicate an increased likelihood for statistically significant neurodevelopmental effects; accuracies: 83% and 67% when excluding electrophysiology (and gene expression). Measurements of brain thyroid hormone levels are likely relevant, too. The extent of substance-mediated thyroid hormone imbalance appears more important than substance mode-of-action to predict neurodevelopmental impairment in rats. Pertinent research needs were identified, e.g. to determine whether the phenomenological offspring thyroid hormone thresholds are relevant for regulatory toxicity testing. The insight from this review shall be used to suggest a tiered testing strategy to determine whether gestational/lactational substance exposure may elicit thyroid hormone imbalance and potentially also neurodevelopmental effects.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - part II: how can key events of relevant adverse outcome pathways be addressed in toxicological assessments?

The current understanding of thyroid-related adverse outcome pathways (AOPs) with adverse neurodevelopmental outcomes in mammals has been reviewed. This served to establish if standard rodent toxicity test methods and in vitro assays allow identifying thyroid-related modes-of-action potentially leading to adverse neurodevelopmental outcomes, and the human relevance of effects - in line with the European Commission's Endocrine Disruptor Criteria. The underlying hypothesis is that an understanding of the key events of relevant AOPs provides insight into differences in incidence, magnitude, or species sensitivity of adverse outcomes. The rodent studies include measurements of serum thyroid hormones, thyroid gland pathology and neurodevelopmental assessments, but do not directly inform on specific modes-of-action. Opportunities to address additional non-routine parameters reflecting critical events of AOPs in toxicological assessments are presented. These parameters appear relevant to support the identification of specific thyroid-related modes-of-action, provided that prevailing technical limitations are overcome. Current understanding of quantitative key event relationships is often weak, but would be needed to determine if the triggering of a molecular initiating event will ultimately result in an adverse outcome. Also, significant species differences in all processes related to thyroid hormone signalling are evident, but the biological implications thereof (including human relevance) are often unknown. In conclusion, careful consideration of the measurement (e.g. timing, method) and interpretation of additional non-routine parameters is warranted. These findings will be used in a subsequent paper to propose a testing strategy to identify if a substance may elicit maternal thyroid hormone imbalance and potentially also neurodevelopmental effects in the progeny.

Toward a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - part I: which parameters from human studies are most relevant for toxicological assessments?

The 2018 European Food Safety Authority/European Chemicals Agency Guidance on the Identification of Endocrine Disruptors lacks clarity on how the presence or absence of substance-induced maternal thyroid hormone imbalance, or the potential for subsequent deleterious consequences in child neurodevelopment, should be established by toxicological assessments. To address these uncertainties, this narrative review evaluates human evidence on how altered maternal thyroid function may be associated with child neurodevelopmental outcomes; and seeks to identify parameters in human studies that appear most relevant for toxicological assessments. Serum levels of free thyroxine (fT4) and thyroid stimulating hormone (TSH) are most frequently measured when assessing thyroid function in pregnant women, whereas a broad spectrum of neurodevelopmental parameters is used to evaluate child neurodevelopment. The human data confirms an association between altered maternal serum fT4 and/or TSH and increased risk for child neurodevelopmental impairment. Quantitative boundaries of effects indicative of increased risks need to be established. Moreover, it is unknown if altered serum levels of total T4, free or total triiodothyronine, or parameters unrelated to serum thyroid hormones might be more relevant indicators of such effects. None of the human studies established a link between substance-mediated liver enzyme induction and increased serum thyroid hormone clearance, let alone further to child neurodevelopmental impairment. This review identifies research needs to contribute to the development of toxicity testing strategies, to reliably predict whether substances have the potential to impair child neurodevelopment via maternal thyroid hormone imbalance.

Assessing the population relevance of endocrine-disrupting effects for nontarget vertebrates exposed to plant protection products.

The European Commission intends to protect vertebrate wildlife populations by regulating plant protection product (PPP) active substances that have endocrine-disrupting properties with a hazard-based approach. In this paper we consider how the Commission's hazard-based regulation and accompanying guidance can be operationalized to ensure that a technically robust process is used to distinguish between substances with adverse population-level effects and those for which it can be demonstrated that adverse effects observed (typically in the laboratory) do not translate into adverse effects at the population level. Our approach is to use population models within the adverse outcome pathway framework to link the nonlinear relationship between adverse effects at the individual and population levels in the following way: (1) use specific protection goals for focal wildlife populations within an ecosystem services framework; (2) model the effects of changes in population-related inputs on focal species populations with individual-based population models to determine thresholds between negligible and nonnegligible (i.e., adverse) population-level effects; (3) compare these thresholds with the relevant endpoints from laboratory toxicity tests to determine whether they are likely to be exceeded at hazard-based limits or the maximum tolerated dose/concentration from the experimental studies. If the population threshold is not exceeded, then the substance should not be classified as an endocrine disruptor with population-relevant adversity unless there are other lines of evidence within a weight-of-evidence approach to challenge this. We believe this approach is scientifically robust and still addresses the political and legal requirement for a hazard-based assessment. Integr Environ Assess Manag 2019;15:278-291. © 2018 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

The urgency for optimization and harmonization of thyroid hormone analyses and their interpretation in developmental and reproductive toxicology studies.

In recent years several OECD test guidelines have been updated and some will be updated shortly with the requirement to measure thyroid hormone levels in the blood of mammalian laboratory species. There is, however, an imperative need for clarification and guidance regarding the collection, assessment, and interpretation of thyroid hormone data for regulatory toxicology and risk assessment. Clarification and guidance is needed for 1) timing and methods of blood collection, 2) standardization and validation of the analytical methods, 3) triggers for additional measurements, 4) the need for T4 measurements in postnatal day (PND) 4 pups, and 5) the interpretation of changes in thyroid hormone levels regarding adversity. Discussions on these topics have already been initiated, and involve expert scientists from a number of international multisector organizations. This paper provides an overview of existing issues, current activities and recommendations for moving forward.

Best practices for developmental toxicity assessment for classification and labeling.

Many chemicals are going through a hazard-based classification and labeling process in Europe. Because of the significant public health implications, the best science must be applied in assessing developmental toxicity data. The European Teratology Society and Health and Environmental Sciences Institute co-organized a workshop to consider best practices, including data quality and consistency, interpretation of developmental effects in the presence of maternal toxicity, human relevance of animal data, and limits of chemical classes. Recommendations included larger historical control databases, more pharmacokinetic studies in pregnant animals for dose setting and study interpretation, generation of mechanistic data to resolve questions about whether maternal toxicity is causative of developmental toxicity, and more rigorous specifications for what constitutes a chemical class. It is our hope that these recommendations will form the basis for subsequent consensus workshops and other scientific activities designed to improve the scientific robustness of data interpretation for classification and labeling.

Guidance on classification for reproductive toxicity under the globally harmonized system of classification and labelling of chemicals (GHS).

The Globally Harmonised System of Classification (GHS) is a framework within which the intrinsic hazards of substances may be determined and communicated. It is not a legislative instrument per se, but is enacted into national legislation with the appropriate legislative instruments. GHS covers many aspects of effects upon health and the environment, including adverse effects upon sexual function and fertility or on development. Classification for these effects is based upon observations in humans or from properly designed experiments in animals, although only the latter is covered herein. The decision to classify a substance based upon experimental data, and the category of classification ascribed, is determined by the level of evidence that is available for an adverse effect on sexual function and fertility or on development that does not arise as a secondary non-specific consequence of other toxic effect. This document offers guidance on the determination of level of concern as a measure of adversity, and the level of evidence to ascribe classification based on data from tests in laboratory animals.

Identification of transcription factors and coactivators affected by dibutylphthalate interactions in fetal rat testes.

Previous analysis of in utero dibutylphthalate (DBP)-exposed fetal rat testes indicated that DBP's antiandrogenic effects were mediated, in part, by indirect inhibition of steroidogenic factor 1 (SF1), suggesting that peroxisome proliferator-activated receptor alpha (PPARα) might be involved through coactivator (CREB-binding protein [CBP]) sequestration. To test this hypothesis, we have performed chromatin immunoprecipitation (ChIP) microarray analysis to assess the DNA binding of PPARα, SF1, CBP, and RNA polymerase II in DBP-induced testicular maldevelopment target genes. Pathway analysis of expression array data in fetal rat testes examined at gestational day (GD) 15, 17, or 19 indicated that lipid metabolism genes regulated by SF1 and PPARα, respectively, were overrepresented, and the time dependency of changes to PPARα-regulated lipid metabolism genes correlated with DBP-mediated repression of SF1-regulated steroidogenesis genes. ChIP microarrays were used to investigate whether DBP-mediated repression of SF1-regulated genes was associated with changes in SF1 binding to genes involved in DBP-induced testicular maldevelopment. DBP treatment caused reductions in SF1 binding in CYP11a, StAR, and CYP17a. Follicle-stimulating hormone receptor (FSHR), regulated by SF1 but unaffected by DBP-treatment, also contained SF1-binding peaks, but DBP did not change this compared with control. GD15 and GD19 fetal testes contained PPARα protein-binding peaks in CYP11a, StAR, and CYP17a regulatory regions. In contrast to its repressive effect on SF1, DBP treatment caused increases in these peaks compared with control. PPARα-binding peaks in the FSHR promoter were not detected in GD15 samples. Hence, the repressive effect of DBP on SF1-regulated steroidogenic genes correlates with inhibition of SF1-DNA binding and increased PPARα-DNA binding. The data indicate that PPARα may act as an indirect transrepressor of SF1 on steroidogenic genes in fetal rat testes in response to DBP treatment.

Science based guidance for the assessment of endocrine disrupting properties of chemicals.

The European legislation on plant protection products (Regulation (EC) No. 1107/2009) and biocides (Directive 98/8/EC), as well as the regulation concerning chemicals (Regulation (EC) No. 1907/2006 'REACH') only support the marketing and use of chemical products on the basis that they do not induce endocrine disruption in humans or non-target species. However, there is currently no agreed guidance on how to identify and evaluate endocrine activity and disruption. Consequently, an ECETOC task force was formed to provide scientific criteria that may be used within the context of these three legislative documents. Specific scientific criteria for the determination of endocrine disrupting properties that integrate information from both regulatory (eco)toxicity studies and mechanistic/screening studies are proposed. These criteria combine the nature of the adverse effects detected in studies which give concern for endocrine toxicity with an understanding of the mode of action of toxicity so that adverse effects can be explained scientifically. The criteria developed are presented in the form of flow charts for assessing relevant effects for both humans and wildlife species. In addition, since not all chemicals with endocrine disrupting properties are of equal hazard, assessment of potency is also proposed to discriminate chemicals of high concern from those of lower concern. The guidance presented in this paper includes refinements made to an initial proposal following discussion of the criteria at a workshop of invited regulatory, academic and industry scientists.

Effects of monobutyl and di(n-butyl) phthalate in vitro on steroidogenesis and Leydig cell aggregation in fetal testis explants from the rat: comparison with effects in vivo in the fetal rat and neonatal marmoset and in vitro in the human.

BACKGROUND: Certain phthalates can impair Leydig cell distribution and steroidogenesis in the fetal rat in utero, but it is unknown whether similar effects might occur in the human. OBJECTIVES: Our aim in this study was to investigate the effects of di(n-butyl) phthalate (DBP), or its metabolite monobutyl phthalate (MBP), on testosterone production and Leydig cell aggregation (LCA) in fetal testis explants from the rat and human, and to compare the results with in vivo findings for DBP-exposed rats. We also wanted to determine if DBP/MBP affects testosterone production in vivo in the neonatal male marmoset. METHODS: Fetal testis explants obtained from the rat [gestation day (GD)19.5] and from the human (15-19 weeks of gestation) were cultured for 24-48 hr with or without human chorionic gonadotropin (hCG) or 22R-hydroxycholesterol (22R-OH), and with or without DBP/MBP. Pregnant rats and neonatal male marmosets were dosed with 500 mg/kg/day DBP or MBP. RESULTS: Exposure of rats in utero to DBP (500 mg/kg/day) for 48 hr before GD21.5 induced major suppression of intratesticular testosterone levels and cytochrome P450 side chain cleavage enzyme (P450scc) expression; this short-term treatment induced LCA, but was less marked than longer term (GD13.5-20.5) DBP treatment. In vitro, MBP (10(-3) M) did not affect basal or 22R-OH-stimulated testosterone production by fetal rat testis explants but slightly attenuated hCG-stimulated steroidogenesis; MBP induced minor LCA in vitro. None of these parameters were affected in human fetal testis explants cultured with 10(-3) M MBP for up to 48 hr. Because the in vivo effects of DBP/MBP were not reproduced in vitro in the rat, the absence of MBP effects in vitro on fetal human testes is inconclusive. In newborn (Day 2-7) marmosets, administration of a single dose of 500 mg/kg MBP significantly (p = 0.019) suppressed blood testosterone levels 5 hr later. Similar treatment of newborn co-twin male marmosets for 14 days resulted in increased Leydig cell volume per testis (p = 0.011), compared with co-twin controls; this is consistent with MBP-induced inhibition of steroidogenesis followed by compensatory Leydig cell hyperplasia/hypertrophy. CONCLUSIONS: These findings suggest that MBP/DBP suppresses steroidogenesis by fetal-type Leydig cells in primates as in rodents, but this cannot be studied in vitro.

Time-dependent and compartment-specific effects of in utero exposure to Di(n-butyl) phthalate on gene/protein expression in the fetal rat testis as revealed by transcription profiling and laser capture microdissection.

We undertook transcription profiling of fetal testis RNA on gestational days e15.5, 17.5, and 19.5 in offspring from dams treated daily from e12.5 with 500 mg/kg di(n-butyl) phthalate (DBP). At e17.5-19.5, reduced expression of genes involved in cholesterol uptake/metabolism and steroidogenesis was identified in DBP-exposed animals, including scavenger receptor B1 (SCARB1), HMGCoA synthase, steroidogenic acute regulatory protein, Cyp11a, and Cyp17. Genes encoding inhibin-alpha, phosphatidylethanolamine-binding protein (PEBP), and cellular retinoic acid-binding protein 2 (CRABP2) were also downregulated. Most of the aforementioned genes are regulated by steroidogenic factor 1 (SF1) but no consistent change in SF1 mRNA or protein expression was detected. Expression of the aforementioned genes was unaffected at e15.5, but expression of other genes was significantly altered (mostly upregulated). To gain further insight, RNA from interstitial (INT) and seminiferous cord (CORD) tissue obtained by laser capture microdissection (e19.5) was used for transcription profiling. This confirmed most gene expression changes identified for whole testes, but some were remarkably compartment specific. Inhibin-alpha, PEBP, and CRABP2 gene expression were all downregulated in INT but not in CORD, as confirmed by immunohistochemistry; similarly, SCARB1 was downregulated 4.6-fold in INT but only 2.3-fold in CORD. DBP-induced gene expression changes specific to CORD involved small magnitude (less than twofold) reductions or upregulation. These results extend earlier findings and point to the Leydig cells as a primary target of DBP-induced dysfunction. The observed gene expression changes, and their compartmentalization, suggest a possible role for peroxisome proliferator-mediated alteration of cofactor availability as a mechanism underlying DBP-induced Leydig cell dysfunction.

Role of androgens in fetal testis development and dysgenesis.

This study sought to establish whether reduced androgen levels/action in the fetal rat testis induced by di(n-butyl) phthalate (DBP) contributes to dysgenetic features, namely reduced Sertoli cell number, occurrence of multinucleated gonocytes (MNG), and Leydig cell aggregation. Pregnant rats were administered treatments or cotreatments designed to manipulate testosterone levels [DBP, testosterone propionate (TP)] or action [flutamide, 7,12-dimethyl-benz[a]anthracene (DMBA)]. The aforementioned end points were analyzed and related to intratesticular testosterone (ITT) levels and peripheral androgen action (anogenital distance). Dysgenetic features were also evaluated in mice with inactivation of the androgen receptor (testicular feminized or ARKO mice). Exposure to DBP alone, or combined with flutamide, DMBA, or TP, resulted in reduced Sertoli cell number and ITT levels, as did exposure to TP alone; coadministration of DBP + TP caused the most severe reduction in both parameters. A positive correlation between ITT levels and Sertoli cell number was found (r = 0.791; P = 0.019). Similarly, exposure to DBP alone, or as a cotreatment, significantly increased occurrence of MNG and Leydig cell aggregation, and these were negatively correlated with ITT levels. Exposure to flutamide or DMBA alone had no significant effect on these dysgenetic end points. These findings suggest that reduced ITT decreases fetal Sertoli cell numbers and might be involved in Leydig cell aggregation and MNG. However, of these three end points, only Sertoli cell number was affected significantly in ARKO/testicular feminized mice with absent androgen action. Therefore, induction of MNG and Leydig cell aggregation might result from DBP-induced effects other than suppression of ITT levels.

In utero exposure to di(n-butyl) phthalate and testicular dysgenesis: comparison of fetal and adult end points and their dose sensitivity.

BACKGROUND: Fetal exposure of male rats to di(n-butyl) phthalate (DBP) induces reproductive disorders similar to those in human testicular dysgenesis syndrome (TDS), including infertility, cryptorchidism, focal "dysgenetic areas," and Sertoli cell-only tubules in the adult testis. Humans are widely exposed to DBP, but at much lower levels than those causing adverse effects in rats. OBJECTIVES: The objective of this study was to evaluate end points affected by DBP action in rats in fetal and adult life that are relevant to human TDS, and to compare their dose sensitivity. METHODS: Pregnant rats were gavaged daily with corn oil (control) or with 4, 20, 100, or 500 mg/kg DBP. We examined adult end points of TDS (infertility, cryptorchidism) and indicators within the fetal testis of dysgenesis [abnormal Leydig cell (LC) aggregation, multinucleated gonocytes (MNGs)], as well as conditions that may result from these indicators in adulthood (occurrence of focal dysgenetic areas). Fetal testis weight and testicular testosterone levels were also evaluated. RESULTS: The fetal end points analyzed (testicular testosterone levels, abnormal LC aggregation, occurrence of MNGs) were most sensitive to disruption by DBP, as all were significantly affected at a dose of 100 mg/kg/day DBP, with a trend toward effects occurring at 20 mg/kg/day DBP; adult end points were affected consistently only by 500 mg/kg/day DBP. CONCLUSIONS: The fetal end points we evaluated can be objectively quantified and may prove helpful in evaluating the health risk of exposure to DBP and other phthalates, as well as identifying DBP-sensitive fetal events that have adult consequences/end points that are identifiable in human TDS.

Acute and long-term effects of in utero exposure of rats to di(n-butyl) phthalate on testicular germ cell development and proliferation.

This study investigated effects of in utero exposure [embryonic day (e)13.5-e21.5] to di(n-butyl) phthalate (DBP) on fetal gonocytes and postnatal germ cell (GC) development in rats and focused on changes (delayed development) relevant to the postulated origins of human carcinoma-in situ cells. DBP treatment resulted in both early (e15.5-e17.5) and late (e19.5-e21.5) effects on gonocytes. The former involved delayed entry of proliferating gonocytes into quiescence, as indicated by prolongation/overexpression of octamer-binding transcription factor 3/4 and retinoblastoma protein phosphorylated at Ser 807/811 and Ki67 plus a 2- to 4-fold increase in gonocyte apoptosis. The late effect of DBP was to induce a greater than 10-fold increase in occurrence of multinucleated gonocytes. GC numbers in DBP-exposed males were reduced (P < 0.01) by 37, 53, 79, and 80% at e21.5 and postnatal d (d) 4, 8, and 15, respectively, with recovery to normal in scrotal testes between postnatal d 25 and 90. The DBP-induced decrease in GC numbers at d 4-8 was associated with delayed exit from quiescence, as indicated by retinoblastoma protein expression and a 28% reduction (P < 0.001) in GC proliferation index at d 6, although the latter was increased by 84% at d 25. The postnatal GC changes were associated with the early, but not late, effects of DBP on gonocytes as short-term DBP treatment from e19.5 to e20.5, induced multinucleated gonocytes as effectively as did treatment from e13.5 to e20.5, but did not reduce GC numbers on d 4. In conclusion, fetal DBP exposure delays normal GC development in both fetal (as early as e15.5) and postnatal life with the possibility of consequences for fertility.

Cellular origins of testicular dysgenesis in rats exposed in utero to di(n-butyl) phthalate.

Foetal exposure of male rats to di(n-butyl) phthalate (DBP) induces testicular changes similar to testicular dysgenesis syndrome in humans, including the formation of focal 'dysgenetic areas' within post-natal testes, surrounded by otherwise normal tubules exhibiting complete spermatogenesis. We hypothesize that these dysgenetic areas form when Sertoli (and other) cells are 'trapped' during the abnormal formation of large Leydig cell (LC) clusters in foetal life and by post-natal day (d) 4 these groups of intermingled cells attempt to form seminiferous tubules. It is likely that the malformed tubules resulting correspond to the dysgenetic areas evident in later life. This also provides a plausible explanation for the occurrence of LCs within seminiferous cords/tubules in or bordering the dysgenetic areas. In our previous studies intratubular LCs (ITLCs) were identified by immunostaining for 3beta-hydroxysteroid dehydrogenase (3beta-HSD), the definitive LC cytoplasmic marker. However, the possibility remained that the 'presumptive' ITLCs were in fact Sertoli cells that had aberrantly gained the ability to express 3beta-HSD. Therefore, the aim of the present study was to fully characterize the ITLCs induced by in utero DBP exposure in d25 rats using a number of LC- (3beta-HSD, P450 side-chain cleavage enzyme, insulin-like factor 3, oestrogen receptor alpha) and Sertoli cell- (vimentin, Wilm's tumour-1) specific markers. Our results show that ITLCs express all four LC-specific markers but do not express either of the Sertoli cell markers. It is therefore concluded that the ITLCs are bona fide LCs that are abnormally located within the seminiferous tubules of DBP-exposed rats in post-natal life.