Activation and deactivation of different genes commands the process of cell differentiation. Gene expression responsible for cell differentiation is controlled by intrinsic and extrinsic signals. This regulated signalling from inside and outside the cell is responsible for embryonic development.
Environment around the cell, such as small molecules, proteins, temperature and oxygen control the gene expression. Cellular communication takes places to decide the fate of particular cell by interplay of signals between the proteins synthesized around the cell. These proteins can be morphogens, growth factors or cytokines. This extrinsic signalling sets off intercellular signalling that stimulates expression of genes. Alteration in gene expression by turning gene on or off, regulates the production of gene product.
Gene expression is regulated intrinsically by modifying DNA. DNA and chromatin are altered chemically. Change in chromatin effects gene expression by controlling the binding of genes to transcription factors. These epigenetic chemical modifications are known as DNA methylation and histone modification. Chromatin modification plays important role in gene expression during cell development. For example, proteins responsible for chromatin modification play important role in muscle cell differentiation. Transcription factors MyoD and MEF regulate enzymes responsible for chromatin modification, such as histone acetyltransferases and deacetylases.
Chromatin modification helps in continuous gene expression which is required during cell differentiation. The gene silencing involved in embryogenesis, promotes cell development into mature cell types. This silencing of gene is done by making gene inaccessible to transcription machinery, and when genes are needed again in adult cell type the chromatin modification opens DNA and make it available for transcription.
Embryonic cell types have particular regions for chromatin modification that regulate gene expression required for embryonic development. These regions can modify gene expression by activating or silencing genes.