Stem cells make it possible to study the molecular mechanisms of a disease under laboratory conditions. Stem-cell based models are therefore used to research diseases. Disease models allow us to investigate how a disease develops and how active substances function in the human body. Potentially, they can also enable us to map the origin and course of specific diseases and their underlying molecular processes. The use of induced pluripotent stem cells (iPS cells) has enormous advantages in terms of creating disease models. Due to the simple production process for iPS cells, it is possible to meet the high demand for pluripotent cells for these kinds of models more easily and specifically than with human embryonic stem cells. In addition, iPS cells can be used to produce patient-specific models to simulate the course of a disease. The individual impact of specific treatments on a patient can then be traced with the help of tissue models, if these have been produced using iPS cells, which in turn were produced from the non-pluripotent somatic cells of the patient. These kinds of processes are used for example in the fight against cancer.

The example of Alzheimer’s disease makes the advantage of models based on human embryonic stem cells clear. A study published in 2014 in the scientific journal Nature was able to examine hypotheses based on animal models regarding the origin of the disease using nerve cells grown in a petri dish. Researchers at Harvard Medical School in Boston were able to grow a nerve plexus using a laboratory model that showed features of familial Alzheimer’s dementia. Using more advanced models, not only is it possible to study the mechanisms of the disease, but it is also possible to test medication on them. 

Somewhat more complex is referring to stem cells in cancer research because up until now the role played by stem cells in the development of cancer has been the subject of controversy. To date, two models have been discussed by researchers for investigating the formation of tumours: the stochastic model and the cancer or tumour stem cell model. According to the stochastic model, every cancer cell has the ability to either divide or differentiate. Whether a cancer cell renews itself or differentiates is thus down to chance. According to a significantly newer model, cancer stem cells play a key role in the development of cancer. The tumour stem cells possess abilities that are very similar to those of stem cells, i.e. self-renewal and a high potential for differentiation. Therefore, in a tumour, the stem cells form the basis of an organised system which produces new cells and determines the formation of cancerous tissue. Due to these properties, cancer stem cells are a major factor in the formation of tumours, even if the resulting tumour later consists largely of differentiated cancer cells that no long have any further potential for differentiation. With the aid of this model, the problems of traditional cancer treatment can be explained more clearly. The tendency of tumours, despite apparently successful chemotherapy, to return in a more aggressive form and to form metastases could be due to the fact that the tumour stem cells which determined the development of the cancer have not been fought successfully. Yet both models of cancer development do not mutually exclude one another. Tumour stem cells can certainly become the object of the processes described in the stochastic model. It seems reasonable to conclude that cancer stem cells are formed, among other things, by the transformation of normal stem cells caused by mutation and that autologous stem cells can therefore be highly significant in the formation of cancer.

How tissue stem cells degenerate and which signalling paths, hormones and transmitters contribute to the formation of cancer should provide us with important information on how to treat these diseases. Specific signalling paths that control important steps in embryonic development can also be reactivated during the formation of a tumour. Tumour cells have abilities to proliferate which in stem cells of normal tissue are necessary for maintenance, repair and regenerative processes. Of particular interest in this context are the molecular and cellular mechanisms of self-renewal, the significance of the cell niche and the influence of oncogenes and tumour suppressor genes on stem cells. Normal stem cells, like cancer cells, have the capacity for life-long self-renewal. The self-renewal of stem cells is a physiological mechanism, which maintains a small pool of stem cells that are able to proliferate indefinitely, but are also able to produce a variety of differentiated cells which maintain the function of the body.

Researchers at Harvard Stem Cell Institute under Dr Khalid Shah, to cite just one example from the field of clinical research, are loading stem cells with cell toxins which as part of "homing" (migrating to specific tissue in the body) seek out an existing tumour and deposit themselves on the cell surface. For a currently incurable brain tumour, a glioblastoma, which grows rapidly and mostly leads to the rapid death of the patient, a mouse model has been developed which in a few years is due to also be trialled on humans as part of a clinical study. Using this stem-cell specific mechanism gets round the problem up until now of the tumour being relatively difficult to access and that the toxin either killed the inserted cell itself or damaged the healthy surrounding tissue.