Direct observation of hematopoietic progenitor chimerism in fetal freemartin cattle

BACKGROUND: Cattle twins are well known as blood chimeras. However, chimerism in the actual hematopoietic progenitor compartment has not been directly investigated. Here, we analyzed fetal liver of chimeric freemartin cattle by combining a new anti-bovine CD34 antibody and Y-chromosome specific in situ hybridization. RESULTS: Bull-derived CD34(+ )cells were detected in the liver of the female sibling (freemartin) at 60 days gestation. The level of bull-derived CD34(+ )cells was lower in the freemartin than in its male siblings. Bull (Y(+)) and cow hematopoietic cells often occurred in separate clusters. Around clusters of Y(+)CD34(+ )cells, Y(+)CD34(- )cells were typically observed. The thymi were also strongly chimeric at 60 days of gestation. CONCLUSION: The fetal freemartin liver contains clusters of bull-derived hematopoietic progenitors, suggesting clonal expansion and differentiation. Even the roots of the hematopoietic system in cattle twins are thus strongly chimeric from the early stages of fetal development. However, the hematopoietic seeding of fetal liver apparently started already before the onset of functional vascular anastomosis.