The purpose of cell differentiation is to provide organisms with the many different types of specialized cells needed to perform specific functions. Organisms start from a single cell that contains all the required genetic information, or instructions, which reproduces into many stem cells with the potential to become any type of specialized cell. First, it is decided which genetic information will be expressed, or carried out, thereby indicating the type of cell that is to be formed. Then, through differentiation, those instructions are carried out and specialized cells are formed.
The human body has about 200 different types of specialized cells that perform various functions and make up the tissues and organs of the body. One of the properties of a stem cell is that it is unspecialized — it lacks the ability to perform any of the specific functions of the body. Its primary function is to create specialized cells, and it contains the genetic information needed to form any type of cell, like a blood cell or a nerve cell. The process of forming these specialized cells is called cell differentiation.
In the earliest stages of development of a human embryo, the differentiation occurs depending on the location of the stem cells. For example, those on the outer layer differentiate to form skin cells. During the differentiation process, cells develop specific shapes, structures, and characteristics needed for performing particular functions in the body.
The process usually consists of several steps, with the cell becoming more specialized at each. These steps are triggered by signals that scientists do not yet understand, but it is thought that some of the signals come from the environment and from nearby cells. Other signals are internal and come from the genetic material, or DNA, inside the cell. Once the cell is differentiated, it develops epigenetic markers that restrict what DNA information can be expressed; these are passed on when the cell reproduces so that future daughter cells will also be specialized.
In adults, cell differentiation also occurs for the same purpose. Adult stem cells also need to form the specialized cells needed to perform vital functions. Adult stem cells, however, can be limited to forming certain categories of cells; these limits are referred to as cell potency. Some, called totipotent or pluripotent, have the potential to form any kind of specialized cell. Others, called multipotent, are more limited. For example, the blood forming adult stem cells in bone marrow can only generate different types of blood cells.