Differentiation, Dedifferentiation and Redifferentiation

As cells produced by root apical, shoot apical and lateral (cambial) meristems move away from the actively dividing zone, most of them undergo differentiation -- a set of structural and biochemical changes to the cytoplasm and cell wall that equips a once-generalised meristematic cell to carry out one specific function, and that, in doing so, usually costs the cell its own further capacity to divide. The differentiation of a tracheary element (a tracheid or vessel member) of the xylem is a clear example: the cell elongates considerably, its wall becomes thickened and impregnated with lignin in a distinctive secondary pattern, and its living protoplasm eventually breaks down entirely, leaving behind a hollow, lignified, dead conduit perfectly suited to conducting water under tension but incapable of dividing ever again. Sieve tube elements of the phloem differentiate along a different, gentler path -- they lose their nucleus but remain living, their end walls perforate into sieve plates, and companion cells differentiate alongside them to support their ongoing metabolic activity.

Remarkably, some cells that have already differentiated and, by the usual rule, lost the capacity to divide, can under the right internal or external conditions regain that capacity -- a reversal called dedifferentiation. The formation of the vascular cambium from otherwise mature, non-dividing interfascicular parenchyma cells lying between the vascular bundles of a young dicot stem is the textbook example: these cells, which had already differentiated into ordinary storage parenchyma, resume active division and organise themselves into a continuous meristematic ring. The formation of the cork cambium (phellogen) from mature cortical or epidermal parenchyma of an ageing stem or a wounded tissue is a second, equally important example of the same phenomenon, and is also the basis on which a plant heals a wound by dividing up mature cells at the injury site to seal it over. …