Early human development: new data raise important embryological and ethical questions relevant for stem cell research

Recent research has considerably changed our views about the developmental biology of early mammalian embryos compared with the ideas that were predominant throughout the previous 30 years or so. New data obtained recently suggest that the mammalian embryo uses traits of axes determination mechanisms that are not too different from the modes used by other vertebrates. In particular, it appears that asymmetry cues derived from the oocyte cytoplasm and modified/specified during sperm penetration appear to be crucial in normal embryogenesis, rather than the environmental influences exerted, e.g. at embryo implantation in the uterus. On the other hand, recent advances in research on the equivalents of a Spemann-Mangold organizer in species other than amphibia (including mammals) provide a background for new discussions of early embryonic patterning (axis formation) processes in the embryonic disc. In combination, these new views appear to be of considerable interest in the debate on the developmental properties and the ethical status of embryonic stem cells. The present review focuses specifically on the new aspects of axis determination and pattern formation processes in early mammalian embryos and relates this to questions about the developmental potential of embryonic stem cells (totipotency vs pluripotency/omnipotency), i.e. facts that appear to be worth considering in the recent debate about the ontological status of the early human embryo as well as of human embryonic stem cells.