Hematopoietic stem cells (HSCs) develop from the hemogenic endothelium in cluster structures that protrude into the embryonic aortic lumen. Although much is known about the molecular characteristics of the developing hematopoietic cells, we lack a complete understanding of their origin and the three-dimensional organization of the niche. Here, we use advanced live imaging techniques of organotypic slice cultures, clonal analysis, and mathematical modeling to show the two-step process of intra-aortic hematopoietic cluster (IACH) formation. First, a hemogenic progenitor buds up from the endothelium and undergoes division forming the monoclonal core of the IAHC. Next, surrounding hemogenic cells are recruited into the IAHC, increasing their size and heterogeneity. We identified the Notch ligand Dll4 as a negative regulator of the recruitment phase of IAHC. Blocking of Dll4 promotes the entrance of new hemogenic Gfi1+ cells into the IAHC and increases the number of cells that acquire HSC activity. Mathematical modeling based on our data provides estimation of the cluster lifetime and the average recruitment time of hemogenic cells to the cluster under physiologic and Dll4-inhibited conditions.

中文翻译：

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Notch配体Dll4损害细胞募集至主动脉簇并限制血液干细胞的产生。

造血干细胞（HSC）由造血性内皮形成，簇状结构突出到胚胎主动脉腔中。尽管对发育中的造血细胞的分子特征了解很多，但我们对它们的起源和利基的三维组织缺乏全面的了解。在这里，我们使用器官切片培养，克隆分析和数学建模的先进实时成像技术来显示主动脉内造血簇（IACH）形成的两步过程。首先，有血源性祖细胞从内皮中萌芽并分裂，形成IAHC的单克隆核心。接下来，周围的血液细胞被募集到IAHC中，从而增加其大小和异质性。我们确定Notch配体Dll4为IAHC募集阶段的负调节剂。Dll4的阻滞促进了新的造血Gfi1 +细胞进入IAHC，并增加了获得HSC活性的细胞数量。根据我们的数据进行数学建模，可以估算在生理和Dll4抑制条件下簇的寿命和造血细胞向簇的平均募集时间。