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Types of Stem cells
Human embryonic stem cells
Human embryonic stem cells are derived from the cells found within an embryo when it is a hollow ball called a blastocyst and before it attaches to the womb. The blastocyst is made up of an outer layer of cells that form the placenta during implantation and clump of cells on the inside (called the inner cell mass) from which the embryo proper develops. At this stage the embryo can fit on a pin head and consists of 50 – 80 cells.
Source material for human embryonic stem cells
Embryos are donated by couples undergoing assisted conception and are surplus to their clinical needs. The procedures are licensed and closely monitored by the Human Fertilisation and Embryology Authority.
Making human embryonic stem cells
An embryonic stem cell line is a self-perpetuating population of cultured cells derived from the inner cell mass of a single blastocyst. The process of making such a line usually begins with isolating the inner cell mass from the outer layer of cells. These cells are then maintained in a culture dish with nutrient liquid (culture medium) that supports their proliferation. Eventually, when the cells outgrow the capacity of the culture dish, they are gently removed from the surface and divided between additional culture dishes - a process called passage. After several passages, a sufficient number of cells are generated such that the cell line can be characterized (to verify that it is a bona fide human embryonic stem cell line) and a bank of frozen cell samples made to provide a stock for future use.
Properties of human embryonic stem cells
In culture, human embryonic stem cells have both the ability to self renew (i.e. to make more of themselves) and to differentiate to form other cell types. In fact, many of the different cell types that are found in the embryo, foetus and adult body can be produced by human embryonic stem cells - a property called pluripotency. Cells that are pluripotent not only provide a valuable research tool for investigating the mechanisms that control developmental processes, but they can also be used to generate therapeutics.
Induced Pluripotent Stem Cells
Recently, a major advance in stem cell technology (gaining the 2012 Nobel Prize in Physiology or Medicine for John Gurdon (UK) and Shinya Yamanaka (Japan)) has allowed adult human cells of one type to be transformed to another type. For instance adult human skin cells could be made into heart cells. The transformation is not direct but via conversion of the skin cell into a pleuripotent cell, ‘a cell that can be anything’, and then directing it to become a heart cell.
In the labs at the UCL Institute of Ophthalmology, we have been able to demonstrate that it is possible to transform human skin cells into Retinal Pigment Epithelial (RPE) cells, a very critical cell type central to sight. We have also been able to produce a rudimentary multilayer retina. If the skin is taken from someone with a retinal dystrophy, we can then recreate eye tissue in the lab that is a perfect copy of that person’s disease. This ability to create perfect models of human diseases in the cell type the disease affects, is an important breakthrough both in terms of understanding the nature of the disease itself and to study potential treatments. We also hope to create perfect copies of missing eye cells from patients' skin cells to make a transplant that will not be rejected, for future therapy of disease such as AMD.