Chimerism of murine fetal bone marrow by maternal cells occurs in late gestation and persists into adulthood.

Studies of murine severe combined immune-deficient (scid/scid) fetuses gestating in transgene-tagged immune competent dams have established high frequencies of transplacental trafficking of nucleated maternal cells. Maternal cells first appeared in thymus at gestation day (gd) 12.5 and were present in more than 90% of late gestation fetuses. Morphologically heterogeneous maternal cells were located predominantly in bone marrow and thymus and also occasionally in liver, spleen, and nonlymphoid organs. We have now evaluated maternal cell chimerism in offspring with normal lymphoid development. Genetically normal blastocysts from random-bred CD1 mice were transferred to C57BL/6J- lacZ transgene-tagged ROSA26 females. Serial sectioning of fetuses followed by histochemistry for lacZ-expressing cells was used to comprehensively define organs containing maternal cells. Fetuses, sectioned in their entirety, had no detectable maternal cells before gd 16.5. Morphologically homogenous, nucleated maternal cells were first present in fetal bone marrow cavities at gd 16.5 and were evident in all offspring in later gestation. Postnatally, maternal cells were also present in bone marrow cavities into adulthood, as determined by lacZ histochemistry and PCR amplification of the maternal transgene. The frequency of maternally derived cells in postnatal bone marrow was increased compared with late gestation, and occasionally, maternal cells were detected in postnatal spleen. The normalcy of maternal cell transfer to genetically immune competent progeny and their long-term engraftment is suggestive of a functional role for maternal cells in offspring.