Identification of bone marrow derived nonhematopoietic cells by double labeling with immunohistochemistry and in situ hybridization.

Stem cell migration/trafficking is a field of interest that is shared by pathologists, histologists, clinical transplantation teams, cardiologists, neurologists, and many other members of different disciplines. Until the findings of a successful combination of in situ methods, the origin of chimeric parenchymal cells was a dilemma. These doublelabeling techniques have brought insight to our new concept of stem cell biology. It has been extremely helpful in the detection of the origin of terminally differentiated, including hematopoietic and nonhematopoietic, cells appearing following allogeneic stem cell transplantation. It has also become a standard approach for evaluation of repopulation following tissue injury in solid organ transplant patients or experimental models. Although very useful, this technique has its advantages and pitfalls. It requires expertise in application and interpretation. Suitable selection of specific markers against parenchymal cells and preferably a cocktail of antibodies targeting infiltrating inflammatory cells are mandatory. One pitfall of this method is its restriction to sex-mismatched pairs. The spectrum of labels for X and Y chromosomes are suitable for combination. To prevent misinterpretation, the precautions needed are defined in this chapter.