1. Stem Cells

2. Objectives
Explain what is meant by the terms stem cells, pluripotency and totipotency and discuss the way in which society uses scientific knowledge to make decisions about the use of stem cells in medical therapies (e.g. regulatory authorities relating to human embryo research, ability of stem cells to develop into specialised tissues, potential sources of stem cells, who could benefit from the therapies, procedures to obtain stem cells and their risks).
Explain how cells become specialised through differential gene expression, producing active mRNA leading to synthesis of proteins which in turn control cell processes or determine cell structure in animals and plants (details of transcription factors are not required at AS).

3. Stem Cells
All mulicellular organisms are made up of many types of cell that are specialised for their function
These cells all originated from one type of cell known as stem cells
Stem cells are unspecialised cells which have the ability to develop into any type of cell by dividing by mitosis and undergoing differentiation

4. Human stem cells
Stem cells are found in the embryo where they can differentiate into all the cells in the human body and also the extraembryotic cells which form the placenta and umbilical cord– these cells are known as totipotent cells as they have the ability to form all cell types
Some adult tissues also contain stem cells like the bone marrow. These cells are known as pluripotent cells as they can produce all the specialised cells but not the extraembryotic cells

5. Totipotency
Totipotent cells are only present in humans in the early life of an embryo. They will differentiate into extraembryotic cells and pluripotent cells which will then go on to differentiate into specialised cells.

6. Development
After the human zygote has undergone three complete cell cycles, it consists of 8 identical cells.
Each of these cells is totipotent
By five days after conception, a hollow ball of cells called the blastocyst has formed.
The outer blastocyst layer forms the placenta
The inner mass of 50 cells or so forms the embryo
These cells are known as pluripotent embryonic stem cells which cannot give rise to all 216 cell types that make up the adult body

8. Human Stem Cells

9. Plant tissue culture
Plants also have stem cells in places where the plant is growing like in the roots and shoots.
All plant stem cells are totipotent
They can be used to produce new plants asexually using plant tissue culture

10. Specialisation of Stem Cells
1. Stem cells contain all the same genes but not all are expressed because not all are active
2. Under the right conditions some genes are activated and others are inactivated by a stimulus
mRNA is transcribed from active genes
mRNA from active genes is translated into proteins
These proteins modify the cells – the determine cell structure and control cell processes (including gene activation)
Changes to the cell by these proteins cause the cell to differentiate and become specialised

11. Differentiation

12. Specialisation of Stem Cells
Red blood cells are produced from a type of stem cell in the bone marrow.
They contain a lot of haemoglobin and have no nucleus
The stem cell produces a new cell with the genes for haemoglobin activiated.
Other genes, such as those used for removing the nucleus are activated too and this results in specialisation of the cell.