1.5 - Stem Cells

What a stem cell is

• A stem cell is an unspecialised cell - unlike a nerve cell or muscle cell, it hasn't yet taken on a particular form or job.
• Stem cells can do two things:
  1. Self-renewal - Keep dividing by mitosis to produce more cells of the same type (more stem cells), so a supply of them is maintained.
  2. Specialisation - Depending on the instructions they're given, their daughter cells can become specialised, developing into a particular cell type such as a blood cell, a nerve cell or a muscle cell.

How they differ from specialised cells

• In most fully developed tissues the cells have already become specialised and can no longer turn into other types of cell.
• A stem cell stays unspecialised and keeps the ability to divide and to give rise to specialised cells.
• Because of this, stem cells are the source of the new specialised cells an organism needs as it grows and develops, and of the new cells that replace worn-out or damaged ones.

Embryonic and adult stem cells

• There are two kinds of stem cell in animals. They differ in how many types of cell they can turn into.
• After a number of divisions, animal stem cells develop into specialised cells that are adapted for particular functions, such as nerve cells.

Embryonic stem cells

• Obtained from an embryo at a very early stage, or from the umbilical cord.
• Can form the full range of cell types found in the body - any kind of cell at all.
• This makes sense because every one of the many specialised cell types has to come from those few early cells - which is why stem cells are so important for the growth and development of an organism.

Adult stem cells

• Found in the body throughout life, in certain places such as the bone marrow.
• Less versatile than embryonic stem cells: they form only a limited range of cell types, usually those of the same general type as the tissue they came from.
  • For example, the stem cells in bone marrow form the different types of blood cell.
• Used for growth and for repair, replacing cells that are damaged or worn out.

Meristems - Stem cells in plants

• In plants, stem cells are found in meristem tissue (meristems).
• Meristems sit in the parts of a plant that are actively growing - mainly the tips of the roots and the tips of the shoots.
• These are the only plant cells that divide by mitosis.

What meristem cells can do

• The cells produced in meristems are unspecialised, like animal stem cells.
• They can divide and become any type of plant cell the plant needs - for example root cells, leaf cells, or the cells of the transport tissues.
• Unlike most animal stem cells, meristem cells keep this ability for the whole life of the plant. This is part of how a plant can carry on growing and producing new parts throughout its life.
• Because they can be grown into any type of plant cell, meristem cells can also be used to produce new plants in cloning techniques.