Cells differ from each other in morphology (structure), and this difference is a reflection of physiological activities and biochemical functions that are ultimately under the control of genes. The differences that can be seen grossly, observed in the microscope, and detected by biochemical and molecular procedures together comprise what is known as differentiation.

Differentiation is associated with embryology. The undifferentiated cells of a zygote, morula, and blastula give rise to progressively more differentiated cells until the adult forms, which is a mosaic of many highly differentiated cells. Ordinarily, differentiated cells have lost the competence to give rise any other kind of cell. For example, muscle cells never give rise to organ cells, and vise versa. Moreover, contained within the mosaic of terminally differentiated cells are a number of stem cells. A stem cell is a less than fully differentiated cell that has retained the competence to...