Department Colloquium | Physics Problems in Early Embryonic Development

Speaker: William Bialek (Princeton University)

One of the most beautiful phenomena in nature is the emergence of a fully formed, highly structured organism from a single undifferentiated cell, the fertilized egg. Biologists have shown that in many cases the “blueprint” for the body is laid out with surprising speed and is readable as variations in the expression levels of particular genes. As we try to understand how this happens, we face a number of physics problems: How can spatial patterns in the concentration of these molecules scale with the size of the egg, so that organisms of different sizes have similar proportions? What insures that the spatial patterns are reproducible from one embryo to the next? Since the concentrations of all the relevant molecules are small, does the random behavior of individual molecules set a limit to the precision with which patterns can be constructed?

Although the phenomena of life are beautiful, one might worry that these systems are just too complicated and messy to yield to the physicists' desire for explanation in terms of powerful general principles. For the past several years, a small group of us have been struggling with these problems in the context of the fruit fly embryo. To our delight, we have been able to banish much of the messiness, and to reveal some remarkably precise and reproducible phenomena. In particular, the first crucial step in the construction of the blueprint really does involve the detection of concentration differences so small that they are close to the physical limits set by the random arrival of individual molecules at their targets. This problem may be so serious that the whole system for constructing the blueprint has to be tuned to maximize how much signal can be transmitted against the inevitable background of noise, and this idea of optimization is a candidate for a more general principle from which the behavior of such biological systems can be derived.