The cytoplasm of eggs, somatic and pluripotent cells, or ectopically expressed factors, can reprogram the nucleus of many kinds of somatic cell, so that gene expression (of these nuclei) is switched to that characteristic of the initial cytoplasmic cell type. Mechanisms of reprogramming include chromatin decondensation and remodelling, DNA demethylation, histone modifications and changes in the rate of transcription of many genes (including those required for pluripotency). As cells become more differentiated, their nuclei become increasingly resistant to reprogramming. Resistance seems to depend on the acquisition of a combination of several epigenetic factors, each of which contributes to the stability of the differentiated state. Eggs, oocytes, somatic cells or ES-cell-specific factors are incompletely efficient at reversing these stabilising factors.
