Fates of blood : Studies on stem cell differentiation potential and B lymphocyte generation in chimeric cattle

Stem cells exist in most adult tissues. Some of these somatic stem cells may be more plastic than expected, not limited to generating new cells for the tissue of their origin. While such findings suggest a revolution in regenerative medicine, they remain controversial. Most stem cell research is based on transplantation experiments of isolated and often cultured cells in mice, and the results may be affected by the manipulation of the animals and the transplanted cells. Here, naturally chimeric twin cattle were used to investigate the differentiation potential of stem cells in a non-manipulated large mammal. Due to conjoined placental circulations, blood of the bovine twins is mixed for most of the fetal period, and circulating stem cells are effectively exchanged. We developed powerful methods for tracing their progeny in various tissues of freemartin cattle, females born as a twin to a bull. While from 10% to 90% of the hematopoietic system in freemartins was donorderived, donor contribution to non-hematopoietic tissues was in most cases minor. Thus, hematopoietic stem cells and other cell types circulating in fetal blood do not generate significant numbers of non-hematopoietic cells in the development, growth and physiological turnover of bovine tissues. However, they may be important in tissue repair and regeneration, as suggested by increasing numbers of donor-derived cells in newly forming granulation tissue. Chimeric cattle were also used to investigate the generation of bovine B lymphocytes, which occurs differently than in the commonly studied human and mouse, and is poorly understood. The results indicate that the ileal Peyer’s patch determines the peripheral B cell pool in young cattle, and is likely responsible for the production of the preimmune antibody repertoire by post-rearrangement strategies. To facilitate a direct analysis of bovine stem cells, the first antibodies against bovine CD34 were generated. The CD34 glycoprotein is commonly used as a marker for hematopoietic progenitors and endothelial cells in the human and mouse. CD34 mRNA was found to be alternatively spliced and widely expressed in cattle tissues. Using the new antibody, the protein was detected in most blood vessel endothelia, primitive hematopoietic cells and some non-hematopoietic cell types.