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Hematopoietic stem cells (HSCs) have played a major role in stem cell biology, providing many conceptual ideas and models. Among them is the concept of the "niche", a special bone-marrow microenvironment that by exchanging cues regulates stem-cell fate. The HSC niche also plays an important role in HSC transplantation. Successful engraftment of donor HSCs depends on myeloablative pretreatment to empty the niche. The concept of the stem-cell niche has now been extended to the generation of organs. We postulated that an empty "organ niche" exists in a developing animal when development of an organ is genetically disabled. This organ niche should be developmentally compensated by blastocyst complementation using wild-type primary stem cells (PSCs). We proved the principle of organogenesis from xenogeneic PSCs in an embryo unable to form a specific organ, demonstrating the generation of functionally normal rat pancreas by injecting rat PSCs into pancreatogenesis-disabled mouse embryos. This principle has held in pigs. When pancreatogenesis-disabled pig embryos underwent complementation with blastomeres from wild-type pig embryos to produce chimeric pigs, the chimeras had normal pancreata and survived to adulthood. Demonstration of the generation of a functional organ from PSCs in pigs is a very important step toward generation of human cells, tissues, and organs from individual patients' own PSCs in large animals.

The placenta plays numerous important roles to support fetal development such as gas exchange, nutrient supply, and hormone production. Placental defects underlie many aspects of pregnancy losses and complications; thus understanding and regulating gene function during placentation is of high clinical relevance. However, the lack of a facile and efficient method for placenta-specific gene manipulation has hampered study of the placenta. We have previously shown that transduction of fertilized mouse eggs with lentiviral (LV) vectors efficiently generates transgenic animals; however, transgene expression occurred in both the fetus and the placenta. In the present study, we transduced zona-free blastocysts with LV vectors expecting placenta-specific gene expression, since most placental cells differentiate from trophoblast cells that form the outermost layer of the blastocyst. Transgene expression was observed in trophoblast cells from preimplantation stages and in the placenta throughout gestation. All the analyzed placentas carried the transgene, while none of the fetuses became transgenic. By applying this method, embryonic lethality caused by placental defects in several knockout animal models was substantially rescued. This technology provides a powerful system for gene manipulation exclusively in placental organogenesis with implications for the treatment of placental dysfunction.

Gene targeting in ES cells is a powerful tool to generate mice bearing predesigned mutations in the germ line. However, these mice carrying such constitutional mutations are often lethal and, therefore, we cannot study other functional aspects of the gene at later stages or in particular tissues. To inactivate the target gene in particular tissues or at particular stages of development, conditional gene inactivation based on the Cre-loxP recombination system of bacteriophage P1 is considered one of the applicable techniques. To express the Cre gene in a cell-type or developmental stage specific manner, either transgenic or adenoviral technology is most considerable technique in vivo.