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

670 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONC. Nonmammalian Whole Embryo Cultures1. Caenorhabditis elegans CulturesThere are numerous advantages to the use of the relatively ubiquitous nematode, Caenorhabditiselegans (C. elegans). Because C. elegans can be hermaphroditic, self-fertilization is possible.Cultures are relatively easy to grow and maintain, with a generation time of only 3 days. The wormsare also transparent throughout their entire developmental period, making observations of internaldevelopment possible with the use of a light microscope. The C. elegans genome is very small (97megabases) and has been completely mapped. Another advantage is that the mature C. elegans hasrelatively few somatic cells, approximately 1000 in adults, and embryonic cell lineages have beencarefully defined, 168,169 so each mature cell can be traced back to its embryonic precursor. Thisadvantage was exploited in laser ablation experiments, where specific cells were killed and developmentwas followed to adulthood. This approach allows for understanding cell-cell interactionsduring development, an approach that is very difficult, if not impossible, in other animal modelswithout further genetic manipulations.While many developmental pathways that may be relevant to developmental toxicology can bestudied in C. elegans, this model has primarily been utilized in developmental biology. Pathwaysstudied in C. elegans have concentrated on receptor tyrosine kinases, 170,171 transforming-growthfactor β, 172–174 Notch and Delta, 175,176 Wnt, 177 and apoptosis 178,180 pathways. The use of C. elegansin developmental toxicology has predominantly been limited to embryo toxicity screens. Theorganism has not been widely used as a model for the evaluation of mechanisms of mammaliandevelopmental toxicity, despite its obvious advantages.2. Drosophila CulturesDrosophila larvae have been used extensively in studies of the control and regulation of geneexpression and pattern formation during embryogenesis. The virtues of mechanistic toxicity studiesin Drosophila mirror those of C. elegans toxicity testing. Drosophila are inexpensive and generallyeasy to maintain and grow, but the greatest advantage to their use is the wealth of informationabout them that has accrued since the 1920s. Drosophila genetics were first elucidated by T.H.Morgan at the University of Columbia, and in the 1970s Drosophila studies led to the discoveryof early egg organization and body patterning. Among the genes that were discovered during theseinitial experiments, the homeobox genes were found to be highly conserved in species rangingfrom flies to mammals. Later, it was found that many other Drosphila genes had mammalianhomologues, providing an easily manipulable nonmammalian model for developmental biology.Such knowledge and relatively well-understood developmental genes and specific pathways shouldrender Drosophila valuable for furthering an awareness of developmental toxicology mechanisms.However, much like C. elegans, Drosophila has not been used to elucidate developmental toxicologymechanisms but rather has been primarily employed as a potential developmental toxicity screeningtest organism.3. Amphibian Embryo CultureSome of the earliest studies of mechanisms of development were conducted in amphibians. Amphibiansare likely have somewhat more relevance to humans than C. elegans or Drosophila becausethey are vertebrates and thus develop more like humans. These simple organisms have been usedto study neural crest cells, osteogenesis, vasculogenesis, cardiogenesis, and development of gutderivatives (e.g., liver and pancreas), all of which have much in common with human developmentalprocesses.© 2006 by Taylor & Francis Group, LLC