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Summary In vivo hematopoietic generation occurs in waves of primitive and definitive cell emergence. Differentiation cultures of pluripotent embryonic stem cells (ESCs) offer an accessible source of hematopoietic cells for blood-related research and therapeutic strategies. However, despite many approaches, it remains a goal to robustly generate hematopoietic progenitor and stem cells (HP/SCs) in vitro from ESCs. This is partly due to the inability to efficiently promote, enrich, and/or molecularly direct hematopoietic emergence. Here, we use Gata2Venus (G2V) and Ly6a(SCA1)GFP (LG) reporter ESCs, derived from well-characterized mouse models of HP/SC emergence, to show that during in vitro differentiation they report emergent waves of primitive hematopoietic progenitor cells (HPCs), definitive HPCs, and B-lymphoid cell potential. These results, facilitated by enrichment of single and double reporter cells with HPC properties, demonstrate that in vitro ESC differentiation approximates the waves of hematopoietic cell generation found in vivo, thus raising possibilities for enrichment of rare ESC-derived HP/SCs., Highlights • Gata2 reports waves of hematopoietic cell potential during ESC differentiation • Ly6aGFP expression distinguishes a late wave of ESC hematopoietic differentiation • Fluorescent reporters enrich ESC-derived cells with hematopoietic potential • Double reporter ESCs verify waves of hematopoietic progenitor generation, Kauts et al. demonstrate that Gata2Venus, Ly6aGFP, and double reporter expression in differentiating mouse embryonic stem cells (ESCs) discriminates and facilitates enrichment of most hematopoietic progenitors generated in vitro. Sequential waves of ESC-derived reporter cell emergence show increasing hematopoietic lineage potency and approximate the in vivo waves of hematopoietic cell generation found in the mouse embryo.

Summary Estrogen stimulates breast development during puberty and mammary tumors in adulthood through estrogen receptor-α (ERα). These effects are proposed to occur via ERα+ luminal cells and not the mammary stem cells (MaSCs) that are ERαneg. Since ERα+ luminal cells express stem cell antigen-1 (SCA-1), we sought to determine if SCA-1 could define an ERα+ subset of EpCAM+/CD24+/CD49fhi MaSCs. We show that the MaSC population has a distinct SCA-1+ population that is abundant in pre-pubertal mammary glands. The SCA-1+ MaSCs have less stem cell markers and less in vivo repopulating activity than their SCA-1neg counterparts. However, they express ERα and specifically enter the cell cycle at puberty. Using estrogen-deficient aromatase knockouts (ArKO), we showed that the SCA-1+ MaSC could be directly modulated by estrogen supplementation. Thus, SCA-1 enriches for an ERα+, estrogen-sensitive subpopulation within the CD24+/CD49fhi MaSC population that may be responsible for the hormonal sensitivity of the developing mammary gland., Highlights • SCA-1+ delineates ER-positive cells in the CD24+ CD49fhi mammary stem population • SCA-1+ cells have lower repopulation activity • SCA-1+ cells are estrogen responsive, Mouse mammary stem cells are thought to be estrogen-receptor negative and receive hormonal influence via estrogen-receptor-positive luminal neighbors. In this article, Britt and colleagues describe a population within the mammary stem cell-enriched compartment that is estrogen-receptor positive and directly responsive to estrogens. This has implications for understanding how aberrant hormone exposure affects breast cancer risk.

Summary Mast cells are tissue-resident immune cells. Their overgrowth/overactivation results in a range of common distressing, sometimes life-threatening disorders, including asthma, psoriasis, anaphylaxis, and mastocytosis. Currently, drug discovery is hampered by use of cancer-derived mast cell lines or primary cells. Cell lines provide low numbers of mature mast cells and are not representative of in vivo mast cells. Mast cell generation from blood/bone marrow gives poor reproducibility, requiring 8–12 weeks of culture. Here we report a method for the rapid/robust production of mast cells from pluripotent stem cells (PSCs). An advantageous Gata2Venus reporter enriches mast cells and progenitors as they differentiate from PSCs. Highly proliferative mouse mast cells and progenitors emerge after 2 weeks. This method is applicable for rapid human mast cell generation, and could enable the production of sufficient numbers of physiologically relevant human mast cells from patient induced PSCs for the study of mast cell-associated disorders and drug discovery., Graphical Abstract, Highlights • Efficient mast cell production is achieved with novel Gata2-reporter PSCs • 14-day Gata2-reporter PSC culture produces mast cells and self-renewing progenitors • Gata2-reporter mast cells have high protease content and degranulate • Gata2-reporter iPSC method advances prospects for human mast cell research, In this article, Dzierzak and colleagues show the rapid and robust production of physiologically relevant mast cells from mouse ESCs via an innovative approach using a Gata2 reporter. This method is applicable to human mast cell production and provides the opportunity for research into patient-specific mast cell disorders.