Hans-Willem Snoeck, MD, PhD

ES cells are derived from the inner cell mass of the blastocyst and can be maintained in a pluripotent state in defined conditions in both human and mouse. The capacity of embryonic stem (ES) cells to differentiate and generate diverse cell types in culture together with the access to virtually unlimited numbers of tissue-specific progenitors in these differentiation cultures provides a novel source of cells for cell replacement therapy. Furthermore, the recent discovery that adult, somatic cells can be reprogrammed using a relatively simple procedure to a pluripotent state (induced pluripotent cells, or iPS cells) opens the way for the generation of patient-specific ES cells, which would overcome rejection problems associated with transplantation of ES cell-derived tissues. The thymus is required for the development of T cells from very early hematopoietic precursors, which continuously seed this organ from the bone marrow. Before and during adolescence, the thymus begins to involute, and the production of naive T cells decreases. While thymic involution in young individuals is likely under evolutionary selection and therefore probably beneficial, further thymic involution in old individuals may be detrimental to health, and perhaps to longevity. It is widely hypothesized that improving thymic function in the elderly will increase health, and perhaps extend life span. We are therefore developing approaches to generate functional thymic tissue from embryonic stem cells.