DIFFERENTIATION

Different cells contain the same DNA. Differentiation is accompanied by successive activation of new genes and consecutively by production of different specific proteins: (e.g. new receptors - the cell gets the ability to respond to new signals; enzymes - ability to catalyse new intracellular reactions; structural proteins - cell gets new morphology; adhesive molecules - enable adhesion between the cells of the same type and formation of a tissue/epithelium). A cell can change the expression of its genes in response to external signals (e.g. differentiation factors). DNA methylation seems to provide an important mechanism for distinguishing genes that are active from those that are not.

Differentiation progresses in one direction only. Under normal conditions, once a cell has differentiated, it cannot revert to undifferentiated state (tumour can revert = dedifferentiation).

Transdifferentiation = transit of a differentiated cell into a different differentiated cell; e.g. rarely in lower vertebrates, cells of the pigment epithelium of the retina can transdifferentiate into cells of the lens or into cells of the neural retina.

Differentiation may occur during proliferation (by asymmetric division).

Terminal differentiation (the maximum specialization of a cell) occurs in G₀ phase, i.e. the cell does not proliferate (as seen in the neuron during axonogenesis or synaptogenesis). The expression of genes in a differentiated cell is stable; the cell retains its phenotype (e.g. during cultivation).