Adult stems cells - undifferentiated cells located in differentiated tissue such as the liver, muscles or bone marrow. Adult stem cells can divide almost to infinity, and can also transform into other tissue cell types if needed (with certain limitations).

Allogeneic stem cell transplantation - the transplantation of stem cells from one person to another.

Autologous stem cell transplantation - re-transplantation of the body's own stem cells; unlike allogeneic transplantation, no foreign donor is needed.

Blastocyst - term used for the embryo between the fourth and seventh day after fertilisation. At this point, the blastocyst consists of approximately 110-150 cells. The outer cell group later develops into the placenta, the inner cell mass continues to develop into the foetus. Removing embryonic stem cells from the inner cell mass will destroy the blastocyst.

CFU test - CFU is the abbreviation of "Colony Forming Units", and describes a method of determining the number of cells or "colony-forming cells". For this, the isolated cells are first scattered on a culture plate. The colonies formed can be counted after a few days. This type of counting can also be used to investigate how many of the isolated cells are viable and vital. Damaged or dead cells lose their ability to form colonies.

Differentiation - the process of cell specialisation. The characteristics of the unspecialised stem cells change during the differentiation process. They lose the ability to divide infinitely, and gain for example the ability to transport electrical impulses should they differentiate into nerve cells.

Differentiation potential - stem cells can be classified according to their differentiation potential. Stem cells are found at every stage of an organism's development, from the fertilised egg cell through to the mature organism. However, their differentiation potential varies. Stem cells are therefore classified as totipotent, pluripotent or multipotent.

DNA - DNA (deoxyribonucleic acid) is a molecule found in all plant and animal organisms. DNA carries genetic information; it contains all information necessary for the formation of an entire organism.

Flow cytometry - method for the automated counting and differentiation of cells. This method facilitates identification of the number, vitality and nature of the available cells. The information obtained can therefore assist in the diagnosis and observation of the development of certain diseases, such as leukaemia (blood cancer) and immunodeficiency diseases (HIV infections).

Embryo - the term "embryo" refers to a living being in the earliest stages of its development. In humans, this stage begins with the fertilised egg and concludes at the end of the 8th week. From the 9th week until birth, the organism is described as a "foetus".

Embryonic stem cells - pluripotent stem cells from an early stage of the embryo's development. For research purposes, these stem cells are usually extracted from the blastocyst.

Antifreeze - molecule which prevents cells from being mechanically damaged by the formation of ice crystals during freezing. Usually dimethyl sulphoxide (DMSO) is added to the cell solution prepared for freezing; however, in some people this can trigger characteristic side-effects (drop in blood pressure, cardiac arrhythmia, respiratory distress, nausea/vomiting and allergic reactions).

Gene - a gene is a section of DNA containing information for the creation of a certain protein. Once it has been created, this protein carries out a specific function in the body.

GMP - Good Manufacturing Practice. Guidelines for the quality control of manufacturing processes, particularly for the manufacture of pharmaceutical products or foodstuffs. The health authorities only issue a license if it can be proved that the GMP guidelines are complied with.

GVHD - Graft Versus Host Disease. Powerful immune reaction against foreign body tissue arising from allogeneic transplantations.

HLA typing - HLA is the abbreviation of "human leukocyte antigen", and describes certain surface markers on body cells. These are individual to each person; identical HLA characteristics are only found in identical twins. The risk of rejecting transplantations of foreign cells decreases with increasing similarity between the HLA characteristics of donor and recipient, which is why these characteristics are determined for both persons before allogeneic transplantations take place (HLA typing).

Germ layer - describes a layer of cells which arises as a result of the initial differentiation of embryonic stem cells. The first differentiation of the embryo produces three germ layers:

• Ectoderm (outer layer, producing for example nerve or skin cells)

• Mesoderm (middle layer, producing for example muscle cells and red blood cells)

• Endoderm (inner layer, producing for example pancreatic or thyroid cells)

Bone marrow - sponge-like tissue inside the bones, well-supplied with blood vessels. The bone marrow is the sole source of blood formation; the haematopoietic (blood-forming) stem cells in the bone marrow continually renew the red and white blood cells in the body.

Culture medium - the nutrient solutions which keeps cells alive outside the organism. The composition of ingredients varies depending on the cell type, and may also vary depending on the aim of the experiment. It is for example possible to add certain growth substances or medications to the culture medium in defined concentrations.

Cord blood - the blood of a child remaining in the umbilical cord and placenta after birth.

Mesenchyme - embryonic connective tissue which mostly arises from the middle germ layer of the embryo (the mesoderm); however, the other two germ layers also contribute to the formation of the mesenchyme.

Mesenchymal stem cells - cells in the mesenchyme with are pluripotent and which develop into bone, cartilage, tendons and all other connective tissue structures during the course of differentiation.

Multipotent stem cells - unlike totipotent or pluripotent stem cells, the extent to which multipotent stem cells can specialise is extremely limited. For a long time, it was assumed that haematopoietic stem cells in the bone marrow could only produce the various blood cells, for example. However, the results of subsequent research indicate that these stem cells also have significantly greater differentiation potential than initially assumed.

PCR - the Polymerase Chain Reaction is a method of reproducing DNA. PCR can be used to replicate even the smallest quantities of genetic material, thus ensuring that enough is available for further investigations such as the creation of genetic fingerprints.

Pluripotent stem cells - embryonic stem cells are pluripotent (literally: "able to do many things"), i.e. they can differentiate into all cells derived from the three germ layers.

Regenerative medicine - type of treatment in which stem cells are stimulated to differentiate into specific cell types, with the objective of repairing damaged or deceased cells.

Somatic stem cells - alternative term for adult stem cells (soma = from the Greek word for "body").

Totipotent stem cells - the stem cells created shortly after the fusion of egg and sperm cells are described as totipotent (literally: "able to do everything"). A whole person could develop from any one of these cells. However, cells lose their totipotence after the third division (the 8-cell stage).

Transdifferentiation - the transformation of cells in a germ layer to cell types in another germ layer, i.e. the transformation of stem cells in one body tissue (e.g. neuronal stem cells) to cell types from a tissue with different developmental origins (e.g. muscle cells).

Undifferentiated cells - cells which have not yet become specialised. This means that they have not yet taken over any specific function in the body.

Cell culture - the cultivation of plant or animal cells outside the organism in a suitable nutrient medium.