THE RADIAL UNIT HYPOTHESIS: The Radial Unit Hypothesis: Each proliferative unit produces all the neurons in a single cortical column. But there are different kinds of neurons within a single cortical column. How can DIFFERENT kinds of cells be produced from the same progenitor cells at different points in time. Some kind of external signal (not yet specified) tells the progenitors in a given region of the neural tube to start producing one kind of cell or another.