We examined the contribution of the fetal membranes, amnion and chorion, to human embryonic and fetal hematopoiesis. A population of cells displaying a hematopoietic progenitor phenotype (CD34++/+CD45low cells) of fetal origin was present in the chorion at all gestational ages, associated with stromal cells or near blood vessels, but was absent in the amnion. Prior to 15 weeks of gestation, they lacked hematopoietic in vivo engraftment potential. Differences in the chemokine receptor and β1 integrin expression profiles of progenitors between the 1st and 2nd trimesters suggest that these cells had gestationally-regulated responses to homing signals and/or adhesion mechanisms that influenced their ability to colonize the stem cell niche. Definitive hematopoietic stem cells capable of multilineage and long-term reconstitution when transplanted in immunodeficient mice, were present in the chorion from 15-24 weeks’ gestation, but were absent at term. The 2nd trimester cells also engrafted secondary recipients in serial transplantation experiments. Thus, the human chorion contains functionally mature hematopoietic stem cells at mid-gestation.
- Received April 13, 2016.
- Accepted February 20, 2017.