15 The Hemangioblast

Abstract

HISTORICAL PERSPECTIVE
The name hemangioblast was first introduced by Murray (1932) to describe discrete cell masses that developed in chick embryo cultures and displayed both hematopoietic and endothelial potential. Although originally used to describe groups of cells, the name hemangioblast is now used exclusively in reference to a single cell, the hypothetical precursor of the hematopoietic and endothelial lineages. The concept that these two lineages share a common precursor arose from observations of early embryos which indicated that the respective precursor populations develop in close spatial and temporal proximity in the yolk sac (Sabin 1920; Haar and Ackerman 1971). Detailed histological analysis revealed that commitment to the hematopoietic and endothelial lineages begins with the proliferation of a single layer of mesodermal cells in the presumptive yolk sac that results in the formation of cell clusters, known as mesodermal cell masses (Haar and Ackerman 1971). Cells within these mesodermal masses differentiate quickly and give rise to angioblasts, precursors of the endothelial lineage, and primitive erythroblasts, the first committed hematopoietic cells. As these populations mature further, the angioblasts generate endothelial cells which rapidly establish the first vascular structure that surrounds the primitive erythroblasts. These clusters of developing endothelial and erythroid cells within the yolk sac, commonly referred to as blood islands, are found by the headfold stage of embryonic development in the mouse (day 8.0 of gestation) and represent the first site of hematopoietic and vascular differentiation (Haar and Ackerman 1971).

Since the hypothesis of the hemangioblast was first put forward, a...