Cell differentiation---nothing less than a miracle of nature---is a process in which a less specialized living cell becomes more specialized, becoming functionally and structurally different than its original state. The process is most common in growing organisms and starts as soon as a female egg is fertilized and cell multiplication is initiated. This results in the formation of a blastocyst---a sphere of cells. This blastocyst is ready to attach to the uterine wall where it continues differentiating. Cell differentiation, therefore, is a process in which simple cells multiply to form specialized cells. It is a steady process that plays a vital role at each individual stage of embryo development.
Formation of a Zygote
A blastocyst differentiates by continually dividing and specializing to form a zygote. The zygote attaches itself to the womb and draws nourishment from a mother's body. Its cells continually multiply and the zygote increases in size. The process of cell differentiation forms different organs within the body of the zygote, and single cells multiply into daughter organ cells, performing entirely different tasks.
As the body grows in size, different organs form when their function is required. The original cell that triggers cell differentiation is called pluripotent. The prototypic embryonic stem cells---functionally specialized cells---form from pluripotent cells.
The gene structure is potentially the most important factor involved in the process of cell differentiation. Every living gene contains an essential set of information, which determine the physical attributes of the host. The genes decide the type of growth and shape of cells to be formed later, as well as a set of instructions that determine how the cell will function. This genetic structure holds vital information that determines how and when cells will differentiate. A slight change in the genetic structure would lead to an entirely different organism altogether.
Gene transformation and cell differentiation is affected by a number of factors with the environment playing a vital role in genetic evolution. Certain protein within cells transmit information and trigger hormones that carry forward the information for cell growth/differentiation. The release of these hormones is affected by environmental factors, such as temperature changes and the supply of oxygen---both of which affect gene expressions.
Cell Differentiation Reversal
Sometimes the term "cell differentiation" also refers to the process taking place in reverse. In case of species such as starfish and worms, specialized, differentiated cells reconvert back to their original embryonic cell. When a worm is severed, or a starfish loses a limb, cellular multiplication is reinitialized and all vital organs previously contained in that severed part are reproduced by differentiation.