Meristems & Stem Cells
(a) Cellular differentiation is the process by which a cell develops more specialised functions by expressing the genes characteristic for that type of cell.
Differentiation into specialised cells from meristems in plants; embryonic and tissue (adult) stem cells in animals. Stem cells are relatively unspecialised cells in animals that can continue to divide and can differentiate into specialised cells.
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Stem cells are often in the news because of potential medical use and because of the ethical debate around human embryonic stem cells. 
Undifferentiated cells that are capable of cell division are known as stem cells. In animals stem cells are found all over the body, but in plants specialised areas called meristems contain cells capable of division. Through cell division, a stem cell can generate further stem cells and a large clone of differentiated cells that will not be capable of cell division.
Embryonic Stem Cells
During embryonic development, specialised cells (e.g., muscle or immune cells) arise from a common stem cell that differentiates. Embryonic stem (ES) cells are harvested from embryos and used to produce cell types useful for therapeutic purposes, such as transplantation. This technology raises a significant ethical concern because most ES cells arise from human embryos.
These cells have an almost unlimited capacity for division when grown in culture whilst still retaining the potential to differentiate into all cell lineages in the body. However, to get them, it is necessary to destroy preimplantation embryos of 100–200 cells. Many people believe that life begins at conception and that performing research on cells derived from the destruction of humanembryos is morally unacceptable. Another important consideration is that transplanted ES cells can trigger host immune rejection.
This may be overcome by the finding that adult body cells (somatic cells) can be reprogrammed to a pluripotent state, called induced pluripotent stem (iPS) cells. iPS stems cells that arise from somatic cells because they can be recovered from an adult source. Furthermore, if cells are recovered from a human donor who will also later receive the differentiated cells, this method removes any immune reaction encountered during transplantation.
The ability to reprogram somatic cells into iPS cells that are pluripotent and can self-renew has transformed the fields of developmental biology and regenerative medicine.
