During chronic injury, regeneration of the adult liver becomes impaired. In this context bipotent Hepatic Progenitor Cells (HPCs) become activated and can regenerate both cholangiocytes and hepatocytes. Notch and Wnt signalling during hepatic ontogeny are described, but their roles in HPC mediated liver regeneration are unclear. Here we show in human diseased liver and murine models of the ductular reaction with biliary and hepatocyte regeneration that Notch and Wnt signalling direct HPC specification within the activated myofibroblasts and macrophages HPC niche. During biliary regeneration, Numb is downregulated in HPCs, Jagged1 promotes biliary specification within HPCs. During hepatocyte regeneration, macrophage derived canonical Wnt signalling maintains Numb within HPCs, and Notch signalling is reduced promoting hepatocyte specification. This dominant Wnt state is stimulated through engulfment of hepatocyte debris by niche macrophages and can directly influence the HPCs. Macrophage Wnt3a expression in turn facilitates hepatocyte regeneration – thus exemplifying a novel positive feedback mechanism in adult parenchymal regeneration.

We propose that high Delta1 expression by epidermal stem cells has three effects: a protective effect on stem cells by blocking Notch signalling; enhanced cohesiveness of stem-cell clusters, which may discourage intermingling with neighbouring cells; and signalling to cells at the edges of the clusters to differentiate. Notch signalling in epidermal stem cells thus differs from other progenitor cell populations in promoting, rather than suppressing, differentiation.

During gastrulation, epiblast cells are pluripotent and their fate is thought to be constrained principally by their position. Cell fate is progressively restricted by localised signalling cues from areas including the primitive streak. However, it is unknown whether this restriction accompanies, at the individual cell level, a reduction in potency. Investigation of these early transition events in vitro is possible via the use of epiblast stem cells (EpiSCs), self-renewing pluripotent cell lines equivalent to the postimplantation epiblast. Strikingly, mouse EpiSCs express gastrulation stage regional markers in self-renewing conditions. Here, we examined the differentiation potential of cells expressing such lineage markers. We show that undifferentiated EpiSC cultures contain a major subfraction of cells with reversible early primitive streak characteristics, which is mutually exclusive to a neural-like fraction. Using in vitro differentiation assays and embryo grafting we demonstrate that primitive streak-like EpiSCs are biased towards mesoderm and endoderm fates while retaining pluripotency. The acquisition of primitive streak characteristics by self-renewing EpiSCs is mediated by endogenous Wnt signalling. Elevation of Wnt activity promotes restriction towards primitive streak-associated lineages with mesendodermal and neuromesodermal characteristics. Collectively, our data suggest that EpiSC pluripotency encompasses a range of reversible lineage-biased states reflecting the birth of pioneer lineage precursors from a pool of uncommitted EpiSCs similar to the earliest cell fate restriction events taking place in the gastrula stage epiblast.