A stem cell is an undifferentiated cell, or in other words, a cell that has not undergone differentiation. The stem cell changes, becoming a type of cell that carries out a specific function. We call these cells specialised cells.
When an unspecialised cell becomes a specialised cell, the process is called differentiation.
All multicellular organisms begin life as single cells. A fertilised egg (zygote) divides by mitosis, forming identical daughter cells. Initially, the fertilised egg cell forms a ball of cells called an embryo. These cells will divide repeatedly until they eventually form an individual organism, which is made up of billions of differentiated (specialised) cells.
As stem cells differentiate into specialised cells, they commit to a specific pathway of development. The cells become committed to becoming a specific type of cell e.g. a red blood cell or a guard cell.
Each type of differentiated cell has different subcellular structures, which help the cell carry out different functions.
There are three main types of stem cells that you need to know about:
Any of the cells in an early-stage embryo are capable of differentiating into any type of body cell. These cells are called embryonic stem cells.
Stem cells are mostly found in growing embryos but they can also be found in adults. For example, in the:
Adult stem cells can only differentiate into related cells. For example, stem cells in the bone marrow can become blood or immune system cells, while those in the skin form cells of the skin layers.
Plants contain regions of stem cells called meristems, in the root tips and shoot tips of the plant. These are areas where cells are rapidly dividing.
Stem cells replace cells in the particular organ of the body in which they are found. This happens when new cells are required in tissues that have been destroyed by accidents or disease.
The stem cells in embryos differentiate to give us ALL of the specialised cells found in the adult body. Meristem tissue (found in the shoot tips and root tips of plants) can become specialised cells required for a plant to grow.
Stem cells divide to replenish dying cells and repair damaged tissues. However, in adults, the types of specialised cells they can differentiate into are limited.