Transplanted cell expressing the photo pigment rhodopsin (red)

What are stem cells?
All living organisms consist of cells. Very simple organisms like bacteria or amoebae consist of one cell only. Complex organisms like people consist of billions of cells and hundreds of different cell types, each performing a specific task. Over time many cells die and most of these cells are replaced by new young cells. These new cells arise when specialised cells, named stem cells, divide into two smaller cells that grow out until they are full sized. One of the ‘daughter cells’ remains a stem cell, the other daughter cell becomes the same cell type as the cell it is replacing.

Adult versus embryonic stem cells
Many people will associate stem cells with embryo research. This is not unreasonable, because embryos are a common source of stem cells; they are not the only source of stem cells though. Many tissues in the human body contain stem cells that perform the renewal of dying cells in that tissue, as described above.

Embryonic stem cells have slightly different properties than adult stem cells; e.g. embryonic stem cells have the ability to become any cell type in the body, whereas adult stem cells usually can only become a cell type of the tissue the stem cell is located in.

Sources of adult stem cells
The most widely known source of stem cells is the bone marrow. This tissue contains the adult blood stem cells that produce all the various cell types in the blood stream. The cells in the blood stream generally do not survive for long, and the bone marrow has to replace them many times over a life time. Like blood cells, most cell types are replaced efficiently by stem cells that are present in that tissue, but there are exceptions. Most cells in the retina are not replaced when they are lost. For most forms of retinitis pigmentosa, but also for the various forms of macula degeneration, the loss of nerve cells in the retina is the cause of blindness. The cells in the retina (most prominently the light-sensitive cells, the photoreceptor cells) should survive for the life time of the person, as replacement of lost cells does not occur. If the cells in the retina are unhealthy, e.g. through a hereditary disease such as retinitis pigmentosa, the cells die more rapidly than usual, resulting in a progressive loss of vision. In the long run no cells and no vision remain. For this reason it was always assumed that the retina does not contain stem cells. However, in recent years a population of retinal stem cells has been identified and these cells might be used to repair the retina.

This page last modified 18 December, 2012 by xxx