Stem Cells, Meristems and Differentiation

Growth needs both more cells and different cells

A multicellular organism develops from a single cell into an organism containing many kinds of cell. Cell division supplies more cells, but increasing cell number alone is not enough. Cells must also become suited to different jobs, such as carrying signals or absorbing substances.

Cell differentiation is the process by which a cell becomes specialised for a particular function. Differentiation allows cells to form tissues with particular functions, so that different parts of an organism can carry out different jobs.

How switching genes changes a cell

Genes are sections of DNA containing instructions for making proteins. Almost all body cells in an organism contain the same genetic information, but they do not use all of those instructions in the same way.

During differentiation, particular genes are switched on and others are switched off. This changes which proteins the cell makes, affecting its structure and how it works. Different patterns of gene activity therefore allow cells with the same genetic information to develop into different cell types.

For example, a developing nerve cell becomes elongated, allowing it to make connections over long distances. Its specialised structure suits its role in carrying signals. Differentiation connects genetic instructions to cell structure and, ultimately, to the function of a tissue.

Stem cells supply cells that can specialise

A stem cell is an unspecialised cell that can divide to produce more stem cells and can also give rise to specialised cells through differentiation. Maintaining a supply of stem cells matters: if every cell became specialised immediately, there would be no continuing source of unspecialised cells.

In animals, embryonic and adult stem cells have different roles and different ranges of possible cell types.

Embryonic stem cells are found in early embryos. They can give rise to any type of body cell. As an embryo develops, cell division increases the number of cells and differentiation produces the varied specialised cells needed to build its tissues. Together, these processes allow a complex organism to develop.

Adult stem cells remain in certain tissues, such as bone marrow. They can normally differentiate into only a limited range of cell types. Their main role is to maintain tissues by replacing cells and helping to repair damage. For example, stem cells in bone marrow produce new blood cells. Most specialised animal cells no longer have the broad ability to differentiate that embryonic stem cells have.

Meristems allow plants to keep growing

A meristem is plant tissue containing stem cells. Meristems are found in growing regions, including root and shoot tips.

Cells in a meristem divide, supplying new unspecialised cells. These cells can then differentiate into the specialised cell types the plant needs. For example, cells produced near a root tip can differentiate into root hair cells, which help the plant absorb water and mineral ions.

Plant stem cells retain the ability to give rise to different types of plant cell throughout the plant's life. This continuing supply of cells allows roots and shoots to keep growing. As in animals, division supplies additional cells, while differentiation gives those cells the structures needed for their functions.

Differentiation

• Genes are switched on and off → different proteins are made → cells develop specialised structures and functions.
• Specialised cells form tissues with particular functions.
• Division increases cell number; differentiation changes what cells do.

Animal stem cells

• Embryonic: early embryos; can form any body cell type; build tissues during development.
• Adult: certain tissues, such as bone marrow; form a limited range of cell types; replace cells and repair tissues.

Plant meristems

• Found in growing regions, including root and shoot tips.
• Stem cells divide to supply new cells, which then differentiate.