Ontogeny of the Haematopoietic System

As for most other tissues and organs, the hematopoietic system is formed during gastrulation and organogenesis. The mesoderm gives rise to hematovascular progenitors which through cellular intermediates go on to differentiate into blood. The hematopoietic system develops in waves: in all vertebrate species, early transitory embryonic waves are gradually replaced by a permanent adult hierarchy which persists throughout the animal’s lifespan. During development, hematopoietic stem cells (HSCs) undergo step-wise differentiation changing their phenotype, functional properties and locations. Complex cell movements, molecular signalling and interplay of transcription factors underlie these processes. This Chapter focuses on tissue origin and mechanisms underpinning the development of the adult hematopoietic system in light of analysis of different vertebrate models.