How does a complex organism such as a human develop from a single cell? It all starts from a single fertilized egg which gives rise to a vast array of cell types, such as nerve cells, muscle cells, and epithelial cells that characterize the adult? Throughout development and adulthood, cellular differentiation leads cells to assume their final morphology and physiology. Differentiation is the process by which unspecialized cells become specialized to carry out distinct functions.

A zygote is a totipotent stem cell with very high differentiation potential that can form all cell types and produce a whole new multicellular organism. As the zygote undergoes successive cell divisions, it generates pluripotent stem cells (PSCs) with more restricted differentiation potential. PSCs give rise to a few adult stem cells that further divide and become more terminally differentiated. When a cell differentiates, it may undertake significant changes in size, shape, metabolic activity, and overall function.

Beginning with the fertilized egg, all the cells in the body contain the same DNA, but each type of cell only "reads" the portions of DNA that are relevant to its function. Several factors like hormones or signaling molecules trigger cell differentiation and, in response, turn on the expression of specific genes while turning off others. The primary mechanism by which genes are turned "on" or "off" is through transcription factors. A transcription factor is a class of proteins that bind to specific genes on the DNA molecule and either promote or inhibit their transcription. The expression of specific genes mediates the synthesis of necessary proteins required for a particular cell function, aiding cellular differentiation.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 3.6: Section Title Cellular Differentiation.