Stem cells and potency
A stem cell is an unspecialised cell able to undergo mitosis and produce cells that either remain as stem cells or develop into specialised cells.
Differentiation is the process by which an unspecialised cell becomes specialised. Potency describes the range of specialised cell types that a stem cell can produce.
Totipotent cells have the widest developmental potential. They can produce any body-cell type and can also give rise to extra-embryonic cells involved in structures such as the placenta.
The zygote formed at fertilisation is totipotent, as are the early embryonic cells up to the 16-cell morula stage. Totipotency is therefore present for only a limited period of early mammalian development.
Differentiation and gene expression
During development, totipotent cells use only part of the genetic information in their DNA. Particular genes are active while others are inactive, so different cells produce different sets of gene products.
This selective gene expression causes cells to acquire different structures and functions, producing specialised cells that form tissues. As differentiation proceeds, the cells lose the ability to form the full range of cell types.
Exam Tip: When explaining differentiation, link selective gene expression to the production of different gene products and then to cell specialisation.