Researchers examine how embryonic cells know where to grow

For an embryo to develop, new cells of different types must know precisely where to place themselves and in what direction to grow. How the cells are able to do this has been an unexplained question for more than a century.

Recently, an international team of researchers explored this question. The pre-print of their work appeared on Dec. 31, 2020, on arXiv, an open-access collection of electronic preprints. The research team was led by William Bialek, a theoretical physicist at Princeton University. 

Using the embryonic development of a fruit fly (drosophila) as an example, the researchers looked at the four gap genes involved in accurately determining the position of the striped patterns of pair-rule gene expression.

"The local concentrations of the gap gene proteins provide just enough information to support this level of precision," the researchers said.

The mechanism is that the concentration of transcription factors is measured by response elements that control expression of other genes. Transcription factors have an affinity to bind DNA, which is affected by their concentration.

The usual view is that transcription factors control gene expression. But this new paper examines gene expression level as the cells' readout of transcription factors concentrations.

They ask "how accurately the transcription factor concentrations need to be measured to retain crucial biological information."

The researchers note that embryonic cells "can extract all the available information about their position, but only if the concentration measurements [of transcription factor molecules] approach the physical limits to information capacity." 

They show that "the precise values of these limits" are not important but, rather, that the existence of such limits is significant in terms of biological function. 

The researchers' method is to use decoding maps to model the expression levels of all four gap genes. 

"Extracting all the available information requires that cells have an array of parallel responsive elements," they write. "Further, to be efficient each element must respond not to individual gap genes signals but to combinations of these concentrations, so that the space of four concentrations is shattered into compact regions."