Development of Hematopoietic Stem Cells in the Human Embryo

Haematopoietic stem cells (HSCs) develop intra-embryonically in the aorta-gonad-mesonephros (AGM) region, more specifically in the ventral domain of the embryonic dorsal aorta. This process is highly conserved across vertebrate species. Key transcription factors and signalling pathways involved in HSC development have been identified using various model organisms. As a mammal, the mouse represents an excellent model for understanding HSC development in the human embryo. However, in utero development significantly reduces accessibility of the embryo for experimentation, which hampers analysis of dynamic developmental processes.

We have overcome this hurdle by establishing AGM in vitro systems that can recapitulate in vivo HSC development. The generation of HSCs during embryonic development is a highly specific process. So far any significant expansion of HSCs from adult sources using solely external molecular cues have been largely unsuccessful. This is despite the fact that there is a pressing need to supply clinically relevant HSCs to some groups of patients. The generation of HSCs from pluripotent stem cells (ES/iPS cells) holds great hopes for regenerative medicine, however, this goal has not been achieved without genetic manipulations due to poor understanding of mechanisms acting in the embryo.

Using a dissociation-reaggregation AGM culture system enabled developmental hierarchy of developing mouse HSCs to be reconstructed. On co-aggregation with AGM stroma or OP9 cells, embryonic precursors mature into adult definitive HSCs, which can provide multi-lineage long-term repopulation upon transplantation into adult irradiated recipients. We have established that during development, mouse HSCs mature through upregulation CD41, CD43 and CD45 haematopoietic markers. The analysis of the AGM niche revealed a highly heterogeneous signalling landscape that can regulate HSC development in a concerted manner.

During human development, HSCs also emerge first in the ventral domain of the embryonic dorsal aorta and reside in a population, which express markers known from mouse HSC development. These first HSCs emerge in small numbers, possess an enormous regenerative/ self-renewing potential and can generate hundreds of fully functional daughter HSCs. However, it appears that establishing human AGM cultures, which recapitulate HSC development because of yet unclear reasons, is problematic. This makes it difficult to study the developing human HSC hierarchy and AGM stromal factors driving HSC development.

In this talk, commonalities and differences in mouse and human development as well as routes to the solution of this hurdle will be discussed.