Minireview: Endocrine Disruptors: Past Lessons and Future Directions.

Within the past few decades, the concept of endocrine-disrupting chemicals (EDCs) has risen from a position of total obscurity to become a focus of dialogue, debate, and concern among scientists, physicians, regulators, and the public. The emergence and development of this field of study has not always followed a smooth path, and researchers continue to wrestle with questions about the low-dose effects and nonmonotonic dose responses seen with EDCs, their biological mechanisms of action, the true pervasiveness of these chemicals in our environment and in our bodies, and the extent of their effects on human and wildlife health. This review chronicles the development of the unique, multidisciplinary field of endocrine disruption, highlighting what we have learned about the threat of EDCs and lessons that could be relevant to other fields. It also offers perspectives on the future of the field and opportunities to better protect human health.

Expression of Dazl and Vasa in turtle embryos and ovaries: evidence for inductive specification of germ cells.

In bilaterian animals, germ cells are specified by the inductive/regulative mode or the predetermined (germ plasm) mode. Among tetrapods, mammals and urodeles use the inductive mode, whereas birds and anurans use the predetermined mode. From histological data it has been predicted that some reptiles including turtles use the inductive mode. Examining turtle oocytes, we find that Dazl RNA, Vasa RNA, and Vasa protein are not localized, suggesting that germ plasm is not present. In turtle embryos at somite stages, primordial germ cells (PGCs) expressing Dazl lie on a path from the lateral posterior extraembryonic endoderm through the gut to the gonad as previously described. In gastrulating embryos, cells expressing Dazl are found in the blastoporal plate and subsequently below the blastoporal plate, indicating that PGCs are generated at the equivalent of the early posterior primitive streak of mammals. Vasa RNA is expressed in somatic cells of gastrula to early somite stages, and Vasa RNA and protein are expressed in PGCs of later embryos. Taken together the evidence strongly suggests that turtles, and other reptiles (lacertoid lizards) with the same location of PGCs in embryos, use the inductive mode of germ cell specification. Phylogenetic analysis of the available evidence supports the following hypotheses: (1) the inductive mode is basal among reptiles, indicating that this mode was maintained as basal tetrapods evolved to amniotes, (2) the predetermined mode arose twice within reptiles, and (3) the induced mode may be used in several lepidosaurs whose PGCs are located in an unusual pattern distributed around the embryo.