Not in the Genes: Embryonic Electric Fields
Many biologists believe that embryo development is controlled by a genetic program encoded in DNA. Other biologists maintain that development cannot be reduced to a genetic program. Although DNA is involved in specifying the amino acid sequences of proteins, other sources of information are needed to specify the three-dimensional structure of the embryo. One such source is a system of spatial coordinates communicated, in part, by an endogenous electric field -- that is, an electric field generated by the embryo itself.
In the 1950s, Danish chemist Jens Skou discovered that the membranes of living cells contain complex channels that pump out three sodium ions for every two potassium ions they pump in. Since sodium and potassium ions each carry an electric charge of +1, the interior of the cell becomes electrically negative relative to the outside, resulting in an endogenous electric field.
In the 1970s, American biophysicists Lionel Jaffe and Richard Nuccitelli invented a probe that could measure endogenous electric fields. Jaffe, Nuccitelli and others went on to show that endogenous fields are generated by frog and chick embryos (among others), and that such fields can control cell behavior during development. In 1995, Purdue University biomedical engineers Richard Borgens and Riyi Shi proposed that endogenous electric fields provide spatial coordinates for the establishment of embryonic pattern.
Recently, Tufts University biologist Dany S. Adams and colleagues discovered a never-before-seen pattern of bioelectric signals in frog embryos that plays a role in face development. When Tufts University biologist Michael Levin and colleagues manipulated those signals artificially, the affected embryos produced eyes outside their head regions.
Although genetic program advocates might argue that such endogenous electric fields are fully specified by information in DNA, I would argue that they are not. Experiments in single-celled organisms indicate that membrane patterns are determined by pre-existing membranes, not by the DNA. So although the molecular components of individual sodium-potassium channels may be encoded in DNA sequences, the three-dimensional arrangement of those channels -- which determines the form of the endogenous electric field -- constitutes an independent source of information in the developing embryo.
Photo credit: Mouse embryo; Vincent Pasque, University of Cambridge. Wellcome Images.