|
Current and revised models of
hematopoeisis.
(A) Current Model. Hematopoietic stem
cells (HSCs) in the bone marrow (yellow) give rise to
all blood lineages and have the ability to self-renew.
Their direct progeny are multipotent progenitors (MPPs)
that give rise to all blood lineages but have lost the
ability to self-renew. The originally proposed model
predicted a split between the lymphoid lineages, arising
from a common lymphoid progenitor (CLP), and the myeloid
and erythroid lineages, both arising from a common myeloid
progenitor (CMP). T cells developing in the thymus (grey)
were proposed to be derived from CLPs, which travel
from the bone marrow to the thymus through the blood
(red).
(B) Revised model. Direct examination
of blood revealed that the only circulating cells with
T potential were HSCs and MPPs, not CLPs. The so-called
CLP, therefore, does not give rise to T cells, but instead
remains in the bone marrow to make B and NK cells. Instead
HSCs or MPPs must enter the thymus and give rise to
early T lineage progenitors (ETPs) and thus T cells.
|
