An embryo develops from a fertilised egg. All of the cells in an embryo start off identical and undifferentiated. These cells are called embryonic stem cells and can become specialised to form any type of cell. They do this by switching genes on and off. For example, if one of the embryonic stem cells formed a muscle cell, it would switch on the genes to turn it into a muscle cell and switch off the genes that would cause it to become a different type of cell.
The embryonic stem cells receive signals from other cells so that they turn the correct genes on and off. Most cells in a human embryo start to become specialised when the embryo has eight cells. This is very early on in the development of the embryo. A whole new organism with skin, eyes, heart, liver and all of the organs it needs, will develop from the embryonic stem cells. This allows the organism to have all the tissues it needs with the correct functions to be able to survive.
Some stem cells remain in the bodies of adults. Adult stem cells are found in limited numbers at certain locations in the body, including the:
Adult stem cells are unspecialised but can become specialised much later than embryonic stem cells. They can differentiate into related cell types only. For instance, adult stem cells in the bone marrow can differentiate into blood cells and cells of the immune system, but not other cell types.
Cell division in plants occurs in regions called meristems.
Cells of the meristem divide by mitosis and produce unspecialised cells. These cells can differentiate to produce all types of plant cells at any time during the life of the plant.
The main meristems are close to the tip of the shoot, and the tip of the root.
In a growing shoot, new cells are being produced continuously near the tip. As the cells become older, further away from the tip, they become differentiated - they enlarge and develop vacuoles.