Oocyte polarity and cell determination in early mammalian embryos

Knowledge on determination and differentiation in the mammalian embryo has not kept pace with discoveries in other phyla. Current concepts overlook well-established pathways leading to polarity in oocytes and embryos of other phyla, modern principles of totipotency in plants and animals, and axis formation in lower vertebrates. Various models derived from invertebrates and frogs could be relevant to the situation in eutherian mammals, and we explore the nature of strict genetic controls in these species and its implications for early mammalian differentiation. Concepts on totipotency and related phenomena in animal and human embryos are examined and the possibility raised that two cell lines are formed in early human embryos from the 2-4 cell stage. Clinical consequences are assessed, including causes of the high incidence of chromosomal mosaicism in human embryos. Our interpretations are obviously speculative, and must be clarified by experimentation.