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Biologically based dose-response (BBDR) models represent an emerging approach to improving the current practice of human health-risk assessment. The concept of BBDR modeling is to incorporate mechanistic information about a chemical that is relevant to the expression of its toxicity into descriptive mathematical terms, thereby providing a quantitative model that will enhance the ability for low-dose and cross-species extrapolation. Construction of a BBDR model for developmental toxicity is particularly complicated by the multitude of possible mechanisms. Thus, a few model assumptions were made. The current study illustrates the processes involved in selecting the relevant information for BBDR modeling, using an established developmental toxicant, 5-fluorouracil (5-FU), as a prototypic example. The primary BBDR model for 5-FU is based on inhibition of thymidylate synthetase (TS) and resultant changes in nucleotide pools, DNA synthesis, cell-cycle progression, and somatic growth. A single subcutaneous injection of 5-FU at doses ranging from 1 to 40 mg/kg was given to pregnant Sprague-Dawley rats at gestational day 14; controls received saline. 5-FU was absorbed rapidly into the maternal circulation, and AUC estimates were linear with administered doses. We found metabolites of 5-FU directly incorporated into embryonic nucleic acids, although the levels of incorporation were low and lacked correlation with administered doses. On the other hand, 5-FU produced dose-dependent inhibition of thymidylate synthetase in the whole embryo, and recovery from enzyme inhibition was also related to the administered dose. As a consequence of TS inhibition, embryonic dTTP and dGTP were markedly reduced, while dCTP was profoundly elevated, perhaps due to feedback regulation of intracellular nucleotide pools. The total contents of embryonic macromolecules (DNA and protein) were also reduced, most notably at the high doses. Correspondingly, dose-related reductions of fetal weight were seen as early as GD 15, and these deficits persisted for the remainder of gestation. These detailed dose-response parameters involved in the expression of 5-FU developmental toxicity were incorporated into mathematical terms for BBDR modeling. Such quantitative models should be instrumental to the improvement of high-to-low dose and cross-species extrapolation in health-risk assessment.

Estrogenic activity of certain xenobiotics is an established mechanism of toxicity that can impair reproductive function in adults of either sex, lead to irreversible abnormalities when administered during development, or cause cancer. The concern has been raised that exposure to ambient levels of estrogenic xenobiotics may be having widespread adverse effects on reproductive health of humans and wildlife. The purpose of this review is to evaluate (a) the nature of the evidence supporting this concern, and (b) the adequacy of toxicity screening to detect, and risk assessment procedures to establish safe levels for, agents acting by this mechanism. Observations such as adverse developmental effects after maternal exposure to therapeutic levels of the potent estrogen diethylstilbestrol or male fertility problems after exposure to high levels of the weak estrogen chlordecone clearly demonstrate that estrogenicity is active as a toxic mechanism in humans. High level exposures to estrogenic compounds have also been shown to affect specific wildlife populations. However, there is little direct evidence to indicate that exposures to ambient levels of estrogenic xenobiotics are affecting reproductive health. Reports of historical trends showing decreasing reproductive capacity (e.g., decreased sperm production over the last 50 years) are either inconsistent with other data or have significant methodologic inadequacies that hinder interpretation. More reliable historical trend data show an increase in breast cancer rate, but the most comprehensive epidemiology study to data failed to show an association between exposure to persistent, estrogenic organochlorine compounds and breast cancer. Clearly, more work needs to be done to characterize historical trends in humans and background incidence of abnormalities in wildlife populations, and to test hypotheses about ambient exposure to environmental contaminants and toxic effects, before conclusions can be reached about the extent or possible causes of adverse effects. It is unlikely that current lab animal testing protocols are failing to detect agents with estrogenic activity, as a wide array of estrogen-responsive endpoints are measured in standard testing batteries. Routine testing for aquatic and wildlife toxicity is more limited in this respect, and work should be done to assess the validity of applying mammalian toxicology data for submammalian hazard identification. Current risk assessment methods appear to be valid for estrogenic agents, although the database for evaluating this is limited. In conclusion, estrogenicity is an important mechanism of reproductive and developmental toxicity; however, there is little evidence at this point that low level exposures constitute a human or ecologic health risk. Given the potential consequences of an undetected risk, more research is needed to investigate associations between exposures and effects, both in people and animals, and a number of research questions are identified herein. The lack of evidence demonstrating widespread xenobiotic-induced estrogenic risk suggests that far-reaching policy decisions can await these research findings.

For topographical analysis of developing embryos, investigators typically rely on scanning electron microscopy (SEM) to provide the surface detail not attainable with light microscopy. SEM is an expensive and time-consuming technique, however, and the preparation procedure may alter morphology and leave the specimen friable. We report that by using a high-resolution compound epifluorescence microscope with inexpensive low-power objectives and the fluorochrome acridine orange, we were able to obtain surface images of fixed or fresh whole rat embryos and fetal palates of considerably greater topographical detail than those obtained using routine light microscopy. Indeed the resulting high-resolution images afford not only superior qualitative documentation of morphological observations, but the capability for detailed morphometry via digitization and computer-assisted image analysis.

We sought to determine whether flow cytometric analysis of circulating fetal blood cells could be used to rapidly detect perturbations of fetal erythropoiesis. In addition, we wanted to determine whether this approach would allow sample collection by exsanguination instead of fetal cardiac puncture, a difficult technique used to prevent contamination of samples with maternal erythrocytes. To monitor fetal erythropoiesis from gestational day (GD) 14-20, we analyzed the cell size, RNA content, and percentage of circulating liver-derived reticulocytes relative to yolk-sac-derived erythroblasts. As a model toxicant, we chose 5-fluorouracil (5-FU), since we previously observed that maternal administration at 20-40 mg/kg on gestational day (GD) 14 produced fetal anemia on GD 16-17, as evidenced by dose-dependent decreases in the cell counts, hematocrit, and hemoglobin content of fetal blood obtained by cardiac puncture. We report herein that 48 hr after maternal 5-FU administration, both cardiac and peripheral blood samples exhibited a dose-dependent decrease in the relative percentage of reticulocytes, indicating a reduced rate of reticulocyte release from the fetal liver. Moreover, at 30 and 40 mg/kg, reticulocytes exhibited increased size and reduced RNA content on GD 16, but elevated RNA content (indicative of premature release) by GD 18. These data suggest that 5-FU inhibits both erythroid cell proliferation and RNA synthesis reversibly, resulting in an anemia that triggers compensatory release of immature reticulocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanistically based dose-response models for developmental toxicity require elucidation of critical biological events that intervene between maternal exposure and adverse developmental outcome. We have examined some of the major events in the rat embryo/fetus following a subcutaneous injection of 5-fluorouracil (5-FU; 0-40 mg/kg) to the dam on Day 14 of gestation. This treatment resulted in reduced fetal weight that was significant at doses of 20 mg/kg and higher, generalized reduced ossification at doses above 25 mg/kg, and wavy ribs at doses of 30 mg/kg and higher. Numerous malformations including cleft palate and hindlimb defects were substantially increased at doses of 35 and 40 mg/kg. 5-FU inhibits thymidylate synthetase (TS), resulting in inhibited growth of rapidly proliferating tissues. To identify early events in the pathogenesis of hindlimb defects, we examined the effects of 5-FU on TS activity, cell cycle, growth, and morphology in the developing hindlimb as a function of dose and time. The rate of decline of TS activity following 5-FU exposure was dose related, although maximal inhibition and recovery were similar at doses within (20 and 40 mg/kg) and below (10 mg/kg) the range of detectable developmental toxicity. Flow cytometric analysis of nuclei from embryonic hindlimbs revealed a transient increase in the percentage of cells in S phase and decrease in G0/G1 phase 8 hr after maternal injection of 20-40 mg 5-FU/kg, but not at lower doses. Reduction in growth and morphometric changes of hindlimbs were observed only after maternal exposure to 40 mg/kg. The tissue specificity of these effects was examined by comparing the hindlimb with other embryonic tissues. There was also a dose-related decline of TS activity in the embryonic liver. However, the pattern of recovery of TS activity and cell cycle alterations were different in the liver than in the hindlimb, probably reflecting the higher cell proliferative rate in the liver at this stage. We have derived a quantitative, empirical model for induction of hindlimb defects based on TS inhibition and subsequent cellular events following 5-FU exposure. The model predicted a dose response similar to that of the observed data although the predicted curve was shifted toward lower doses. These results suggest that while this model may not capture all of the critical events involved in the induction of hindlimb defects following maternal exposure to 5-FU, it does reflect a central mechanism of its developmental toxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Previous studies from our laboratory have shown that maternal 5-fluorouracil (5-FU) exposure on day 14 of gestation (GD14) in the rat results in dose-dependent retardation of both cell cycle progression and growth of embryonic liver. At this developmental stage, hepatic erythropoiesis is the primary source of new circulating fetal erythrocytes. This study examined dose-dependent hematological changes in the fetus after maternal 5-FU exposure (0, 20, 30, 40 mg/kg on GD14) to assess 1) hematopoiesis as a potential target for 5-FU developmental toxicity and 2) the role of the observed 5-FU-induced fetal anemia in adverse developmental outcome. Standard clinical hematological parameters, including hematocrit, hemoglobin content, and erythrocyte counts, were measured in fetal blood drawn by cardiac puncture. Dose-related deficits were observed in all of these parameters within 48 hr of 5-FU administration. Calculation of various red cell indices revealed a concomitant increase in mean cell volume and mean cell hemoglobin. These changes were preceded by depletion of hepatic precursor populations which was evident by 24 hr after maternal exposure to 30 or 40 mg/kg. At doses of 20 and 30 mg/kg there was full and moderate recovery, respectively, in these endpoints by 72 hr after dosing, but persistent deficits were observed at 40 mg/kg. Fluorescence microscopy of Höechst-stained fetal blood smears revealed that at both 48 and 72 hr after dosing, the proportion of nucleated yolk sac-derived erythrocytes was increased relative to control.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell-cycle analysis of nuclei obtained from the circulating erythroblasts (gestational day [GD] 11-16), livers (GD 14-19), and whole embryos (GD 10-13) or remaining (extrahepatic) tissues (GD 14-16) of rat embryos/fetuses revealed age- and tissue-dependent variations in the relative percentages of cells in the G0/G1, S, and G2/M phases of the cell cycle. With development, the rate of cell proliferation declined resulting in decreases in the relative percentage of S-phase cells and increases in the G0/G1 percentage, while the percentage of G2/M-phase cells remained relatively constant. Comparing tissue cell-cycle profiles during development, erythroblasts exhibited the most rapid age-dependent decline in S-phase percentage (from 75% at GD 11 to 8% by GD 14), embryos/extrahepatic tissues exhibited a more gradual reduction (from 55% at GD 10 to 14% by GD 15), while the hepatic isolates exhibited a relatively constant S-phase percentage of approximately 40% from GD 14 to GD 18 before decreasing to 23% at GD 19. These age-dependent variations suggest that cell-cycle distribution may be useful in staging embryogenesis and in detecting abnormal development. To determine how these developmental and organ-specific cell-cycle variations affect toxic response, we sampled GD 11-13 embryos 6 hr after maternal administration of a teratogenic dose of 5-fluorouracil (5-FU), a thymidylate synthetase inhibitor that induces S-phase accumulation. The results indicate that, on a relative basis, the amount of induced S-phase accumulation in erythroblasts and whole embryos 6 hr postdosing increased with development.(ABSTRACT TRUNCATED AT 250 WORDS)

Murine erythroleukemic cells (MELC) exposed to 2'-deoxy-5-azacytidine (D-AZA) or to the active cyclophosphamide (CP) metabolites phosphoramide mustard (PAM) and 4-hydroxycyclophosphamide (OHCP) exhibit cell-cycle perturbations similar to those seen in limb bud nuclei of gestational day (GD) 10 CD-1 mouse embryos exposed in utero to D-AZA or CP, respectively. The similarities in response suggest MELC may be a useful model for determining mechanisms of action of DNA-active developmental toxicants. As such, we used the MELC model to investigate the mechanism of action of 5-fluorouracil (5-FU), an antimetabolite that induced in GD 14 rat fetuses an apparent S-phase accumulation in limb cells 8 hr after in utero exposure, but S-phase depletion in liver cells 24 hr postexposure. MELC timed-recovery and synchronization studies suggest that in proliferative tissues, 5-FU induces an early S-phase accumulation, followed by a synchronous, concentration-dependent delay in progression through the cell cycle. Consequently, it is the tissue-specific rate of delay, rather than different mechanisms of action, that results in apparent tissue-specific perturbations. Moreover, growth and cell-cycle data suggest that cells entering S phase (when TS activity is greatest) are the most sensitive to 5-FU toxicity. Assays of the TS activity of recovering MELC reveal that although the initial extent of TS inhibition does not appear to be concentration-dependent, the time to recovery is, suggesting that the rate of S-phase progression is closely associated with TS activity. Together, the induction of similar cell-cycle perturbations in embryonic/fetal tissues and MELC following exposure to CP (or CP metabolites), D-AZA, or 5-FU, as well as the adaptability of MELC to a variety of kinetic assays suggests that, for those developmental toxicants suspected of inducing cell-cycle perturbations in embryonic/fetal tissues, MELC may prove useful for elucidating mechanisms of action.

The neurotransmitter serotonin (5-HT) was localized in the ectoplacental cone (EPC) and placenta of the day 9-12 (E9-12) mouse embryo in vivo and in whole embryo cultures, using immunocytochemistry with a specific 5-HT antiserum. In uncultured conceptuses, 5-HT immunoreactivity (5-HT IR) was most intense in the EPC at E9 (2-7 somites), particularly in giant cells around the periphery. Nuclear staining was observed in lightly staining giant cells and in small cells in the core of the cone. By E10 (18-24 somites) 5-HT IR in the placenta was less intense and almost exclusively limited to giant cells, where it was localized to chromatin-like material in nuclei. The same pattern and level of 5-HT IR persisted through E12. In the placenta, 5-HT IR appeared to be most intense in giant cells located near aggregations of platelets in decidual blood vessels. 5-HT IR was enhanced in cultured conceptuses, and further increased when exogenous 5-HT was added to the culture medium. Immunoreactivity was greatly reduced by adding the 5-HT uptake inhibitor fluoxetine to the culture medium, or culturing conceptuses in medium containing 5-HT depleted rat serum. Thus, 5-HT was apparently taken up from the culture medium. In conceptuses exposed to exogenous 5-HT, immunoreactivity in the placenta appeared as a gradient from the giant cells to the inner layers, suggesting that these cells may transport 5-HT toward the embryo. No evidence of 5-HT synthesis by the EPC/placenta was found. These results suggest that 5-HT present in the EPC/placenta is due to uptake, not synthesis. Possible sources and functions of 5-HT in the developing placenta are discussed.

During craniofacial development in the mouse embryo (days 9-12 of gestation; plug day = day 1), transient expression of serotonin (5-HT) uptake in epithelial structures of this region correlates with critical morphogenetic events (Lauder et al., '88; Shuey, '91; Shuey et al., '89, '92). The purpose of the present investigation was to assess the possible functional significance of these uptake sites by examination of patterns of dysmorphology following exposure of embryos to selective 5-HT uptake inhibitors. Exposure of mouse embryos in whole embryo culture to sertraline, at a concentration (10 microM) which produced no evidence of general embryotoxicity, caused craniofacial malformations consistent with direct action at 5-HT uptake sites. Two other 5-HT uptake inhibitors, fluoxetine and amitriptyline, produced similar defects. The critical period of sertraline exposure occurred on days 10-11. The observed craniofacial defects were associated with decreased proliferation and extensive cell death in mesenchyme located 5-6 cell layers deep from the overlying epithelium. In contrast, the subepithelial mesenchymal layers showed normal or elevated levels of proliferation. From these results it appears that inhibition of 5-HT uptake into craniofacial epithelia may produce developmental defects by interference with serotonergic regulation of epithelial-mesenchymal interactions important for normal craniofacial morphogenesis.

Colchicine, a neurotoxin that preferentially destroys dentate gyrus granule cells and mossy fibers, was injected into the hippocampus of adult rats. Three weeks later, the rats were tested for colchicine-induced hypermotility after which they received fetal hippocampal explants. Locomotor activity was retested three weeks later, after which the rats were trained over a period of four weeks on a food-reinforced, spatial, working memory task in an 8-arm radial maze. Fetal hippocampal explants were found to attenuate significantly the colchicine-induced hypermotility and spatial learning deficits. Histological observations showed the presence of surviving hippocampal explants in both the lesioned and the control rat brains, suggesting that the presence of viable implants facilitates the recovery of behavioral function in rats with spatial memory deficits.