Simply put, reprogramming a cell means changing its identity and function using tools and methods that don't naturally occur in the context of and organism during its development and growth.
Until recently, the dogma said that the process of cellular specification, or differentiation, could not be altered. It was thought that during the development of a particular individual, a cell could only move in one direction, from undifferentiated – with the potential to become any tissue - to terminally differentiated and permanently committed to a particular role in a specific tissue and/or organ. In other words, an hepatocyte will always be a cell of the liver until the death of the cell or the individual, whatever comes first.
In 1962 John Gurdon demonstrated that cells could indeed be forced to move from more to less differentiated. He showed that a cell from the intestine can be ‘reprogrammed’ to become a whole new individual. The most notable example of reprogramming using only a few genes is the work of Takahashi and Yamanaka in 1996 that showed how to reprogram a fibroblast into a pluripotent stem cell.
Other examples of cellular reprogramming include those of changes in cellular lineage. The first one showed by Helen Blau and colleagues in 1983 by fusing cells of totally different tissues and even of different species and later in 1987 by Andrew Lassar and colleagues showing that the introduction of one single gene into fibroblasts is sufficient to reprogram them into muscle cells.
Today we know of four methods to reprogram cells – here called ‘target cell’; 1) by introducing into the target cell specific DNA or RNA or microRNA, 2) by fusing the target cell with pieces of cytoplasm from the cell one is seeking to obtain, 3) by exposing the target cell to small molecules known to activate or silence specific genes and 4) by directly taking the nucleus of the target cell and introducing it into a cytoplasm of the cell we want. The scientific community is aggressively pursuing all four approaches with the goal of generating cells for transplantation.