Characteristics of the foetal liver stromal population during embryonic haematopoietic and vascular development

Abstract

Murine haematopoiesis progresses step-wise through a number of defined stages that includes the yolk sac, aorta-gonad-mesonephros (AGM), foetal liver (FL), placenta and finally the bone marrow. The number of haematopoietic stem cells (HSCs) increases dramatically in the FL during mid-gestation. The cellular and signalling processes that govern HSC expansion and maturation in the FL are not entirely understood. Mesenchymal stem cells (MSC) are known to provide a niche for HSC maintenance in the adult bone marrow. Therefore, we hypothesised that FL MSCs may also play a role in the maturation and expansion of HSCs during embryonic development.

Due to the lack of bona fide markers for FL MSCs, we first estimated the number of FL MSCs between E11.5 and E18.5 using colony forming unit- fibroblast (CFU-F) assays. FL CFU-Fs demonstrate serial clonogenicity, long-term self-renewal, and in vitro multipotency and their number progressively increases during FL development. Using two germ layer specific Cre transgenic mouse lines, I discovered that the majority of FL MSCs are derived from MesP1 mesoderm but that at E18.5 but not at E13.5, the FL also contains MSCs derived from Wnt1 neuro-ectoderm.

HSC expansion in the FL occurs during E11.5-E16.5. To evaluate the contribution of FL MSCs to HSC expansion, further investigations were focused on the E13.5 time point with E18.5 FL used as a control. MSCs in the E13.5 and E18.5 FL can be significantly enriched by purifying cells expressing PDGFRα. PDGFRα+ cells reside near the peritoneal surface of the hepatic lobes and in the peri-sinusoidal and perivascular regions. Lineage tracing studies coupled with confocal microscopy revealed that PDGFRα+ cells directly contribute to hepatic mesothelial and endothelial cells during FL development and thus to the structural integrity and vasculature of the developing liver. Genome-wide expression analysis revealed that PDGFRα+ cells up- regulate genes and pathways involved in haematopoiesis. E13.5 PDGFRα+ population was also shown to support the ex vivo culture of haematopoietic stem and progenitor cells in a competitive transplantation assay.