Making Blood: The Haematopoietic Niche throughout Ontogeny.

Approximately one-quarter of all cells in the adult human body are blood cells. The haematopoietic system is therefore massive in scale and requires exquisite regulation to be maintained under homeostatic conditions. It must also be able to respond when needed, such as during infection or following blood loss, to produce more blood cells.

Gastrokine-2 is transiently expressed in the endodermal and endothelial cells of the maturing mouse yolk sac.

The yolk sac (YS) is a thin, bi-layered membrane encompassing the developing mammalian embryo in utero. The outer layer of the YS is composed of visceral endodermal cells derived from the primitive endoderm. The inner mesenchymal layer is highly vascularised and the first source of haematopoietic cells.

Hemogenic endothelium: a vessel for blood production.

Blood cell production, or hematopoiesis, is critical to the survival of the developing mammalian embryo. The origins of hematopoietic stem cells, capable of giving rise to all blood cell types, are being revealed. During embryogenesis, hematopoietic stem and progenitor cells are generated from a unique population of vascular endothelium termed hemogenic endothelial cells.