HEMOPOIETIC CHIMERISM IN IMPORTED AND LABORATORY-BRED MARMOSETS

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Abstract
The reproductive physiology of the marmoset, a small New World primate, is characterized by a high incidence of fraternal twinning and development of placental vascular anastomoses between the twin fetuses. Hemopoietic chimerism resulting from the interaction of these two phenomena was determined in imported and laboratory-bred marmosets. Two methods based on cell sex specificity were used: the neutrophil-bearing drumstick and chromosome analyses. The drumstick neutrophil offers presumptive evidence for chimerism of this cell type, but the variability of the appearance of this marker and the lack of correlation with lymphocyte chimerism, as determined by sex chromosome analysis, prevent its use in any quantitative estimation of percentage of chimerism. Chimerism was found in all heterosexual co-twins and in all hemopoietic tissues examined: lymph node, spleen, bone marrow, blood leukocytes, and thymus. The proportion of heterosexual cells in one twin was the same as that in its co-twin. To explain this similarity, it is suggested that a “central pool” of mixed hemopoietic stem cells is established in the placenta; from this site, an equivalent distribution of cells to each fetus takes place. An interesting observation was the apparent selectivity for male cells in all marmosets, male or female; the mean ratio of male to female cells was 1.36. Heterosexual blood chimerism was also found in twins of like sex and in single-born animals. Three pathways were suggested to account for these phenomena: 1) transplacental passage of hemopoietic stem cells from the maternal to the fetal system, (2) exchange of cells from one fetus to another (or another pair), followed by death and resorption of one fetus, or (3) early aggregation of the blastomeres of twins culminating in a single-born chimeric young.

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