How organisms develop from a single cell
'Unspecialised' stem cells can develop into any other type of cell. Stem cells are found in animal zygotes (very young embryos), and in plants of all ages.
A zygote is a structure that forms when a sperm fertilises an egg.
The zygote then divides many times by mitosis to form an embryo. The first division of the zygote forms two cells, the next four, the next eight, and so on.
Up to the eight-cell stage, all of the cells are identical. They are called embryonic stem cells. It is possible for embryonic stem cells to develop into any other specialised type of cell that the growing embryo needs - for example, nerve cells, blood cells and muscle cells. However, once the embryonic stem cells become specialised, they can't change into any other type of cell.
The specialised cells can form all the different types of tissue that the embryo needs.
Cells become specialised because the genes that are not required are switched off. Only the genes needed to make a particular type of cell work are switched on. So muscle cells only have the genes needed to make muscle cell proteins switched on. All the other genes, such as those needed to make blood cell proteins and nerve cell proteins, are switched off.