Stem Cells
Stem Cells
• Stem cells are unspecialised cells that can develop into others types of cell.
• Stem cells divide by mitosis to become new cells, which then become specialised, through differential gene
expression.
• All multicellular organisms have some form of stem cell.
• The process by which a cell becomes specialised is called differentiation.
Totipotent Stem Cells
• Can divide and differentiate into any type of cell.
• Only present in mammals in the first few cell divisions of an embryo.
Pluripotent Stem Cells
• Can divide and differentiate into any type of cell except the cells that make up the placenta and umbilical
cord.
• Can be used to replace cells and treat human disorders.
(Multipotent Stem Cells can divide and differentiate into a limited number of cell types. Unipotent stem cells can only
divide into a single type of cells.)
Development of the early embryo
• The human zygote undergoes 3 complete cell cycles, so consists of 8 totipotent identical cells.
• A blastocyst (hollow ball of cells) forms ~5 days after conception.
• The outer layer of the blastocyst, named the trophoblast, forms the placenta.
• The inner mass of ~50 cells form the tissue of the embryo. At this point, the embryonic stem cells become
pluripotent.
• The cells of the inner mass go on to develop into adult stem cells. Adult stem cells are multipotent and can
only produce cells with a related function.
Differentiation
A cell’s genome is its entire set of DNA, including all the genes it contains. However a cell doesn’t express (i.e. make
proteins from) all the genes in its genome. Stem cells become specialised because different genes in their DNA
become active and get expressed:
1. Stem cells all contain the same genes, but not all of them are expressed because not all of them are active.
2. Under the right conditions, some genes are activated and others are inactivated.
3. mRNA is only transcribed from the active genes.
4. The mRNA from the active genes is then translated into proteins.
5. These proteins modify the cell: they determine the cell structure and control cell processes (including the
activation of more genes, which produces more proteins).
6. Changes to the cell produced by these proteins cause the cell to become specialised (differentiate). These
changes are difficult to reverse, so once a cell has specialised it stays specialised.
Specialised cells need to be able to perform specialised functions in the body; hence they require specific structures.
Cell differentiation allows stem cells to differentiate into cells with the required structures to carry out the necessary
functions.
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