A Practical Approach, Second Edition=Ronald D. Ho.pdf

648 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONI. INTRODUCTIONCurrent applications of in vitro developmental systems can be broadly divided into two areas: (1)prescreening for developmental toxicants and (2) testing for elucidation of mechanisms of normaland abnormal embryogenesis. Manifestations of abnormal development are, herein, described asdysmorphogenesis or developmental toxicity to identify the functional or morphological abnormalitiesproduced by chemical or environmental insults in vitro. The relative merits of these modelsfor their respective purposes have been discussed previously at great length. 1–9 It is generally agreedthat in vitro models, such as the rodent whole embryo culture system, may require additionalvalidation before they can be used universally as predictive teratogen prescreens. Nevertheless,clear advantages support the continued development of the various methods as in vitro tools tostudy mechanisms of developmental toxicity. This is especially true for developmental investigationsin viviparous species. Major obstacles encountered in understanding mechanisms of abnormaldevelopment and embryotoxicity include experimental inaccessibility, a paucity of material, andthe numerous confounding maternal, nutritional and physiological influences present in utero. Mostof these limitations can now be overcome through systematic study of the whole conceptus or itsdissociated component parts in vitro at various stages of gestation. A complete understanding of anymechanism, however, is unlikely to emerge from analyses using only a single approach. Ultimately,several in vivo and in vitro methods, utilizing different levels of biological organization and complexity,must be combined in a systematic manner to elucidate mechanisms of developmental toxicity. Inaddition, the likelihood that a single universal mechanism will be found to explain embryotoxicmanifestations caused by environmental extremes and chemical exposure is highly improbable.Various manifestations of embryotoxicity are not expressed uniformly throughout the complex,dynamic and interactive continuum of development and are exhibited during the species’ periodof sensitivity for that specific tissue. Identical maternal exposures to a given chemical agent atdifferent times in gestation may, for example, produce a spectrum of deleterious effects characteristiconly of disturbances in specific developmental processes particularly vulnerable at the timeof exposure. Lesions produced by chemical agents and environmental extremes will, therefore, bemanifest both temporally and spatially at all levels of biological organization. These range fromeffects on the entire conceptus to highly selective alterations at the cellular and molecular levels.In the discussion to follow, emphasis is placed on the use of mammalian cells, tissues, organs, andembryos, although nonmammalian species are highlighted as deemed appropriate.The study of mechanisms of developmental toxicity bears the close and expected resemblanceto similar in vitro methods used for investigating a broad spectrum of toxicological effects in matureorganisms. Systems employing decreasing levels of biological complexity are commonly used toisolate specific processes and probe for specific biochemical and molecular mechanisms that maybe involved in developmental toxicology. Endpoints for analysis can be developed and customized,based on the needs of a particular study. The range of possibilities for analytical endpoints mayinclude (1) an evaluation of gross malformations using visual examination, light microscopy,scanning and transmission electron microscopy, scanning laser confocal microscopy, and computerassisted morphometric analysis; (2) functional and physiological alterations in organs or tissues,including studies of transport, metabolism, biosynthesis, turnover, electrical and metabolic coupling,and maintenance of cellular redox status, and (3) molecular events, including studies of time andtissue-specific gene expression, differentiation, pattern formation, induction, receptor binding andsignal transduction, programmed cell death, and imprinting. Experimental models with higher levelsof biological organization and complexity are useful in the evaluation of mechanisms involvingorgan-organ interactions, pharmacokinetics, and systemic metabolic regulation. Recent advancesin molecular and developmental biology have created new possibilities for evaluation of the dynamicmolecular and biochemical processes of selected cell populations within an intact embryo. Thetechniques involved include the use of specific fluorescent antibody probes and ribonucleotidehybridization markers.© 2006 by Taylor & Francis Group, LLC