Sucrose synthase, which cleaves sucrose in the presence of uridine diphosphate (UDP) into UDP-glucose and fructose, is thought to be a key determinant of sink strength of heterotrophic plant organs. To determine the roles of the enzyme in carrot, we characterized carrot sucrose synthase at the molecular level. Two genes (Susy*Dc1 and Susy*Dc2) were isolated. The deduced amino acid sequences are 87% identical. However, the sequences upstream of the translation initiation codons are markedly different, as are the expression patterns of the two genes. Susy*Dc2 was exclusively expressed in flowers. Transcripts for Susy*Dc1 were found in stems, in roots at different developmental stages, and in flower buds, flowers and maturing seeds, with the highest levels in strong utilization sinks for sucrose such as growing stems and tap root tips. Expression of Susy*Dc1 was regulated by anaerobiosis but not by sugars or acetate. The carrot sucrose synthase protein is partly membrane-associated and this insoluble form may be directly involved in cellulose biosynthesis. Tap roots of the carrot cultivar used accumulated starch in the vicinity of the vascular bundles, which correlated with high sucrose synthase transcript levels. This finding suggests that soluble sucrose synthase in tap roots channels sucrose towards starch biosynthesis. Starch accumulation appears to be transient and may be involved in sucrose partitioning to developing tap roots.

This study was undertaken to evaluate the kinetics of engraftment after in utero transplantation of murine fetal liver and human cord blood stem cells in the nonobese diabetic/severe combined immunodeficient (NOD/SCID) mouse model. NOD/SCID fetuses were injected with murine fetal liver or human cord blood CD34+ cells at day 13.5 of gestation. Frequencies of donor cells were analyzed by flow cytometry up to 48 h post transplantation and 4-16 weeks postnatally. Hematopoietic multilineage reconstitution capacity was assessed. Both types of donor cells home rapidly. However, the frequency of human cord blood stem cells rapidly diminished while the murine fetal liver stem cells expanded over time, resulting in multilineage hematopoietic reconstitution. Differences in long-term reconstitution of allogeneic versus xenogeneic donor cells were ascribed to the inability of the human cells to self-renew and differentiate in the fetal mouse environment, demonstrating the limitations of this commonly used xenograph.

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Background: Allogeneic hematopoietic stem cell transplantation in utero has been successfully used for the prenatal treatment of severe combined immunodeficiency syndrome. However, this therapy has not been successful in the treatment of other conditions in which the fetus is immunologically competent. Material and Methods: We reviewed the currently explored strategies to overcome these problems, including prenatal gene therapy using ex vivo transduced autologous hematopoietic cells or direct gene targeting in utero. Results: Some of the strategies such as stromal cell co-transplantation have been shown to be successful in preclinical studies. Similarly, prenatal gene transfer has been shown to be feasible in the fetal sheep model; however, safety concerns regarding transduction of fetal germ cells or maternal cells remain. Conclusion: Progress is being made in the exploration of new modalities of in utero transplantation although the procedure remains experimental and long-term clinical efficacy needs to be proven. In utero gene therapy seems feasible, but more animal studies are needed in order to assess its safety.