2. The Differential Adhesion Hypothesis (DAH) was proposed by Malcolm Steinberg in 1964 to
explain how cells sort themselves out relative to one another during development. He proposed this
hypothesis based on experiments conducted by Johannes Holtfreter. Later, modern techniques were
used to measure specific cellular properties which confirmed predictions of the model. More recently,
proteins were manipulated to uncover a molecular explanation for the predictions of the hypothesis.
First, describe one of Holtfreter's experiments (and the results) using embryos that led Steinberg to
propose the DAH. Second, describe a measurable cellular property that the DAH proposes to explain
the results of Holtfreter's experiments. What are the predictions based on this property? Lastly,
indicate how a particular class of molecules has provided a molecular mechanism underlying the
property indicated by the second part of your answer. Describe experimental evidence using these
molecules to support your answer.
