ABSTRACT
The organs and tissues of a body are composed of many cell types with distinct characteristics. Differentiation initiates in early embryogenesis when pluripotent cells become restricted to different developmental pathways [1,2]. Specification of each cell lineage involves successive stages of signal induction followed by changes in gene expression profiles, as the cell induction signal is transduced to a nuclear transcription signal. The cell function, morphology, and motility properties that these differentiating cell types acquire are defined by the transcriptome, the set of genes transcribed from the genome [3]. This selective expression of the genetic information is established by gene regulatory networks. Although transcription factor network feedback loops can maintain a form of transcriptional memory [4], developmental signals are typically transitory, and DNA-bound transcription factor associations do not usually persist through DNA replication. The stable transmission of such gene expression patterns through cell division depends on epigenetic mechanisms that can propagate multiple gene expression traits in the absence of changes in the genomic DNA sequence [5]. The resulting clonal variegation of cellular properties creates cell lineages that can further differentiate as tissues develop.
