A dual involvement of Protocadherin 18a in stromal cell development guides the formation of a functional hematopoietic niche.

Hematopoietic stem and progenitor cells emerge from the aorta and migrate to the caudal hematopoietic tissue (CHT) of zebrafish larvae, the hematopoietic equivalent of the mammalian fetal liver, for their proliferation and differentiation. We previously reported that somite-derived stromal cells were a key component of the CHT niche. Here, we found that the cell adhesion protein Protocadherin 18a (Pcdh18a) is expressed in the stromal cell progenitors (SCPs) emigrating from somites toward the future CHT. Deletion of most of the Pcdh18a intracellular domain caused a decrease in the number of SCPs, the directionality of their migration, and the cell-contact mediated repulsion that normally occurs between migrating SCPs. These defects were followed by abnormal morphogenesis of the venous plexus that forms the CHT framework, and the inability of the CHT to function as a niche for hematopoietic stem and progenitor cells. Finally, we found that the extracellular domain of Pcdh18a mediates trans heterophilic adhesion of stromal cells to endothelial cells in vivo and thereby the reticular versus perivascular fate of SCPs. Thus, Pcdh18a expression in SCPs is essential for the proper development of the hematopoietic niche.