Types of stem cell
Embryonic stem cells
Embryonic stem cells are taken from fertilised eggs that have reached the blastocyst stage, typically around five days after fertilisation. Within the hollow ball of cells that makes up the blastocyst is the inner cell mass, a group of embryonic stem cells that are pluripotent - that is, they can become any type of specialised cell. The outer cells of the blastocyst (the trophoblast) ultimately form the placenta.
Embryonic stem cells can be grown in Petri dishes in the laboratory by a process called ‘cell culture’. If the stem cells are still growing without having differentiated after six months or more in culture, they are known as embryonic stem cell lines, which are widely used by scientists for research.
Adult stem cells
Stem cells aren’t restricted to pre-birth stages of development, but are found in a number of the body’s tissues and organs. These cells help maintain and repair the tissues. Among the tissues thought to contain adult stem cells are the brain, bone marrow, skin, skeletal muscle and blood.
Adult stem cells are multipotent, which means that they can produce a certain type of cells, particular to a given tissue. For example, blood stem cells can become all types of blood cell, including platelets, red blood cells and white blood cells, and neural stem cells can become the nerve cells and supporting cells of the brain and spinal cord. It remains unclear exactly how many types and sources of adult stem cells exist.
Fetal stem cells
These cells, taken from umbilical cord blood, have the ability to develop into a limited number of specialised cell types. Some organisations offer people the chance to ‘bank’ the blood, in the hope that it will provide a source of stem cells for the donor in the future. Currently, the technology to use umbilical cord blood in this way does not exist.
Image: Scanning electron micrograph of inner cell mass cells; Yorgos Nikas, Wellcome Images.
These cells can be isolated and grown in culture to give rise to embryonic stem cell lines, which are widely used in research.
