Tint maps to mouse chromosome 6 and may interact with a notochordal enhancer of Brachyury.

At the proximal part of mouse chromosome 17 there are three well-defined genes affecting the axis of the embryo and consequently tail length: Brachyury, Brachyury the second, and the t-complex tail interaction (T1, T2, and tct). The existence of T1 and tct in fact defines the classical "t-complex" that occupies approximately 40 cM of mouse chromosome 17. Their relationship to each other and various unlinked interacting genes has been enigmatic. The tint gene was the first of the latter to be identified. We report here its genetic mapping using a microsatellite scan together with outcrosses to Mus spretus and M. castaneous followed by a subsequent testcross to T, T1, and T2 mutants. Surprisingly, tint interacts with T2 but not with T1. The implications of our data suggest that T2 may be part of the T1 regulatory region through direct or indirect participation of tint.

Development of the hemangioblast defines the onset of hematopoiesis in human ES cell differentiation cultures.

The onset of hematopoiesis in the mouse embryo and in the embryonic stem (ES) cell differentiation model is defined by the emergence of the hemangioblast, a progenitor with both hematopoietic and vascular potential. While there is evidence for the existence of a hemangioblast in the mouse, it is unclear if this progenitor develops during the establishment of the human hematopoietic system. In this report, we have mapped hematopoietic development in human ES cell (hESC) differentiation cultures and demonstrated that a comparable hemangioblast population exists. The human hemangioblasts were identified by their capacity to generate blast colonies that display both hematopoietic and vascular potential. These colony-forming cells express the receptor tyrosine kinase KDR (VEGF receptor 2) and represent a transient population that develops in BMP-4-stimulated embryoid bodies (EBs) between 72 and 96 hours of differentiation, prior to the onset of the primitive erythroid program. Two distinct types of hemangioblasts were identified, those that give rise to primitive erythroid cells, macrophages, and endothelial cells and those that generate only the primitive erythroid population and endothelial cells. These findings demonstrate for the first time the existence of the human hemangioblast and in doing so identify the earliest stage of hematopoietic commitment.

Sequential development of hematopoietic and cardiac mesoderm during embryonic stem cell differentiation.

The ability to generate a wide spectrum of differentiated cell types from ES cells in culture offers a powerful approach for studying lineage induction and specification and a promising source of progenitors for cell replacement therapy. Although significant efforts are being made to optimize culture conditions for the generation of different cell populations from ES cells, the identification and efficient isolation of specific progenitors for many lineages within these cultures remains a major challenge. By specifically tracking hematopoietic and cardiac development, we demonstrate here that these two lineages arise from distinct mesoderm subpopulations that develop in sequential waves from pre-mesoderm cells. Access to these populations provides a unique approach to isolate and characterize the earliest progenitors of these lineages.

Haploinsufficiency of Runx1 results in the acceleration of mesodermal development and hemangioblast specification upon in vitro differentiation of ES cells.

The AML1 gene (recently renamed Runx1), which encodes the DNA-binding subunit of a transcription factor of the core binding factor (CBF) family, is required for the establishment of definitive hematopoiesis. We have previously demonstrated that Runx1 is expressed in yolk sac mesodermal cells prior to the establishment of the blood islands and in the embryoid body (EB)-derived blast-colony-forming cells (BL-CFCs), the in vitro equivalent of the hemangioblast. Analysis of Runx1-deficient embryonic stem (ES) cells demonstrated that this gene is essential for the generation of normal numbers of blast colonies, the progeny of the BL-CFCs. In the present study, we analyzed the potential of Runx1(+/-) ES cells to determine if heterozygosity at the Runx1 locus impacts early developmental events leading to the commitment of the BL-CFCs. Our results indicate that Runx1 heterozygosity leads to an acceleration of mesodermal commitment and specification to the BL-CFCs and to the hematopoietic lineages in EBs.