There are two main types of stem cells – embryonic stem cells and adult stem cells (also known as tissue stem cells). More recently scientists have created a new type of stem cell, known as induced pluripotent stem cells.
Keep reading to learn more about each of these types of stem cells
Embryonic stem cells: what are they are where do we find them?
Embryonic stem cells are cells found in the embryo when it’s just a few days old. They are pluripotent cells, meaning they are capable of differentiating into all the different types of specialised cells in the body.
In order to understand more about what these cells are and where we find them, it is useful to understand human development, from the time an egg is first fertilised.
Day 1: Fertilisation occurs when a male sperm and female egg fuse. The fertilised egg is referred to as a zygote.
Days 2-4: The zygote divides (cleaves) to form 2 cells, then divides again to form 4 cells. Over the next few days the cells keep dividing and by day 4 a ball of ~32 cells has been formed. This ball of cells is called a morula.
Days 3-8: The cells begin to specialise and form a hollow ball of cells called a blastocyst. The outer layer of cells is referred to as the trophoblast and contains extra-embryonic cells which will go on to form structures which will support the embryo as it grows, including the placenta. Within the blastocyst is a small cluster of cells, called the inner cell mass. These are the pluripotent embryonic stem cells which will go on to form the embryo. Scientists isolate embryonic stem cells from the inner cell mass of the blastocyst around days 5-6.
The zygote is a totipotent cell, meaning it can go on to produce all the cells in the body, plus extra-embryonic cells of the trophoblast. When the blastocyst forms the cells have begun to specialise and the cells of the inner cell mass can no longer form the trophoblast. As a result of they are referred to as pluripotent cells.
Adult stem cells: what are they and where do we find them?
Adult stem cells, also known as tissue stem cells, are multipotent stem cells found in many tissues in the body. They are also found in the foetus during development, as well as in the umbilical cord and placenta.
Their role is to allow the constant renewal of cells throughout life. For example, it is estimated that 50-70 billion cells dies every day in the adult human; adults shed 30-40,000 skins cells shed per hour, and need 100,000 new blood cells needed every day. Adult stem cells help replenish these cells. They also help the body heal following injury or disease.
Adult stem cells have a more limited differentiation potential than pluripotent embryonic stem cells, meaning they can become fewer cells types (hence the term multipotent). You can find more about stem cell potency here.
As a result there are different types of adult stem cells within the body. For example:
- Haematopoietic stem cells, which can form blood cells
- Mesenchymal stem cells, which can form mesenchymal cell types, such as bone cells (osteoblasts), cartilage cells (chondrocytes), muscle cells (myoblasts), fat cells (adipocytes), etc.
- Neural stem cells, which can form the different types of cells found in the nervous system, such as neurons, astrocytes and oligodendrocytes.
These multipotent stem cells are found within various tissues and organs in the body in specialised locations (microenvironments) called ‘stem cell niches’. The niche is thought to control and regulate stem cells proliferation and differentiation, allowing maintenance of a pool of stem cells throughout life and allowing stem cells to be activated when needed. How this works is not fully understood and is an area of interest for scientists.
The most commonly described stem cell niche is in bone marrow, which contains both haematopetic stem cells and mesenchymal stem cells. However other tissues, such as fat, contain a population of cells similar to mesenchymal stem cells. Numerous other stem cell niches have been identified within the body and contain populations of multipotent cells.
Induced pluripotent stem cells: what are they are where do we find them?
Induced plurpipotent stem cells or IPS cells, do not occur naturally in the body. They were created in a lab by ‘reprogramming’ adult skin cells (fibroblasts) into pluripotent cells, similar to embryonic stem cells. This was achieved by insertion of genes which are known to be important for both self-renewal and pluripotency of embryonic stem cells.
Mouse IPS cells were first developed by Prof Shinji Yamanaka (Japan) in 2006 and a year later he achieved the same thing with human cells. As a result of this work Prof Yamanaka was awarded a Nobel Prize in 2012.
Cellular reprogramming of adult cells into IPS cells was originally achieved through gene transfection. However this technique had a number of problems, including low efficiency (only a tiny proportion of cells was effectively reprogrammed) and the risk of tumour formation, as some of the genes inserted were oncogenes (genes which have the potential to cause cancer)
Scientists around the world are working on ways of improving the technique and these cells have a wide range of potential applications, including understanding genetic diseases and drug testing. If a safe and reproducible method of creating IPS cells can be developed these cells will also have huge potential for tissue engineering and regenerative medicine.