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Abstract
To review the process of blood-cell formation in the murine and human yolk sac.
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The yolk sac is the first site of blood-cell production during murine and human ontogeny.
Primitive erythroid cells originate in the yolk sac and complete their maturation,
including enucleation, in the bloodstream. Though species differences exist, the pattern
of hematopoietic progenitor cell emergence in the yolk sac is similar in mouse and
man. In both species, there is a stage of development where both primitive red blood
cells and definitive erythroid progenitors are produced in the yolk sac. An "embryonic"
hematopoietic stem cell that engrafts in myeloablated newborn but not adult mice can
be detected in the murine yolk sac and embryo. Stem-cell activity in the human yolk
sac has not been reported.
The yolk sac is the sole site of embryonic erythropoiesis. However, definitive erythroid,
myeloid, and multipotential progenitors also originate in the yolk sac. The relationship
between these progenitors and the "embryonic" hematopoietic stem cell has not been
elucidated. Yolk sac-derived progenitor cells may seed the developing liver via the
circulation and serve as the immediate source of the mature blood cells that are required
to meet the metabolic needs of the rapidly growing fetus.