Stem Cells
- A stem cell is a cell that can divide (by mitosis) an unlimited number of times
- Each new cell (produced when a stem cell divides) has the potential to remain a stem cell or to develop into a specialised cell such as a blood cell or a muscle cell (by a process known as differentiation)
- This ability of stem cells to differentiate into more specialised cell types is known as potency
- There are three main types of potency:
- Totipotency – totipotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo, as well as extra-embryonic cells (the cells that make up the placenta and umbilical cord)
- Pluripotency – pluripotent stem cells are embryonic stem cells that can differentiate into any cell type found in an embryo but are not able to differentiate into cells forming the placenta and umbilical cord
- Multipotency – multipotent stem cells are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent
Totipotent cells
- Totipotent cells can divide and produce any type of body cell
- Totipotent cells exist for a limited time in early mammalian embryos
- The zygote formed when a sperm cell fertilises an egg cell is totipotent
- The embryonic cells up to the 16-cell stage of human embryo development (around the fourth day after fertilisation) are also totipotent
- These cells are still in the form of a solid ball of cells known as a morula
- Initially, the totipotent cells in the embryo are unspecialised
- During development, totipotent cells begin to translate only part of their DNA, which results in cell specialisation
- There are no totipotent cells present in the later stages of development as cells lose their ability to differentiate into any cell type
Pluripotent cells
- By around the fifth day after fertilization, the embryonic cells have divided further and formed a structure known as a blastocyst, which has an outer layer of cells and an inner mass of cells (that are located inside the outer layer)
- The outer layer of cells will later form the placenta
- The inner mass cells are no longer totipotent (they have lost some of their ability to differentiate)
- They can still differentiate into any cell type found in an embryo but are not able to differentiate into cells forming the placenta and umbilical cord
- These cells are known as pluripotent embryonic stem cells
Multipotent cells
- Stem cells are also found in some adult tissues but they are much less potent than embryonic stem cells (i.e. they can only specialise into certain types of cells)
- For example, intestinal stem cells specialise into intestinal epithelial cells to replace those that are constantly being lost
- This form of potency is known as multipotency – multipotent stem cells are adult stem cells that have lost some of the potency associated with embryonic stem cells and are no longer pluripotent
- Plants also contain stem cell in areas of growth, such as their shoots and roots
Stem cells can be totipotent, pluripotent or multipotent