Umbilical cord blood produces small megakaryocytes after transplantation.

Delayed platelet engraftment is a major complication of umbilical cord blood (CB) transplantation. Megakaryocytes derived from CB in vitro are smaller than megakaryocytes derived from bone marrow (BM) or mobilized peripheral blood from adults. Small megakaryocyte size may contribute to delayed platelet engraftment.

Developmental differences in megakaryocyte maturation are determined by the microenvironment.

Historically, physicians have attributed delayed platelet engraftment following umbilical cord blood transplant to decreased numbers of stem cells in cord blood compared with adult bone marrow. However, recent studies suggest that delayed platelet engraftment may be caused by an intrinsic inability of neonatal stem cells to produce mature, polyploid megakaryocytes. We tested this hypothesis by transplanting adult bone marrow and newborn liver hematopoietic stem and progenitor cells from transgenic mice expressing green fluorescent protein into myeloablated wild-type recipients and comparing the size and ploidy levels of megakaryocytes that developed in adult transplant recipients.