Mitotic cell division in unicellular organisms such as bacteria or yeast produces identical sister cells that are also identical to the mother cell. But in multicellular plants, sister cells are different from each other and usually also from the mother cell that produced them. These differences result from variations of gene expression in cells that are genetically identical. (The alternative hypothesis, that differentiation depends on differences in gene content in different cell types, can be discounted because differentiated cells isolated from the plant and placed in sterile culture on suitable nutrient medium regenerate entire plants that contain all the expected cell types.) The fifty or so specialized cell types of higher plants result from the operation of three developmental processes: cell polarity, asymmetric cell divisions, and positional information.

Origin of Cell Polarity

Polarity is the condition in which opposite ends of a structure are...