Reconstituted thymus organ culture is based on fetal thymus organ culture (FTOC). Purified thymocyte populations, from genetically modified mice or even from other species, are cultured in vitro with thymic lobes depleted of their endogenous thymocytes (by 2'-deoxyguanosine treatment) to form a new thymus. This potent and timesaving method is distinct from FTOC, which assesses development of unmodified thymic lobes, and reaggregate thymic organ culture, in which epithelial cells are separately purified before being aggregated with thymocytes.