Cell specialization, also known as cellular differentiation, is the process by which a stem cell changes from one type to a more specialized type of cell. This process happens multiple times, and there are multiple levels of cell potency, which is the cells ability to differentiate into other cell types. The different levels of cell potency include:

• Totipotent cells: Cells that can differentiate into all cell types, including the placental tissue. In mammals, only the zygote and subsequent blastomeres are totipotent, while in plants, many differentiated cells can become totipotent with simple laboratory techniques.

• Multipotent cells: Cells that can differentiate into multiple different, but closely related cell types.

• Oligopotent cells: Cells that are more restricted than multipotent cells, but can still differentiate into a few closely related cell types.

• Unipotent cells: Cells that can differentiate into only one cell type, but are capable of self-renewal.

Each specialized cell type in an organism expresses a subset of all the genes that constitute the genome of that species. Each cell type is defined by its particular pattern of regulated gene expression. Cell differentiation is thus a transition of a cell from one cell type to another and it involves a unique combination of proteins that determine which functions can be carried out. Specialized cells with unique roles are organized into tissues, which work together to help the organism function. Examples of specialized cells include neurons, cardiomyocytes, osteocytes, and erythrocytes.