2. • All blood cells arise from a type of cell called the
hematopoietic stem cell (HSC)
• Stem cells are cells that can differentiate into other
cell types; they are self-renewing—they maintain their
population level by cell division.
• Hematopoiesis, the formation and development of red
and white blood cells.
• A hematopoietic stem cell is multipotent, or
pluripotent, able to differentiate in various ways and
thereby generate erythrocytes, granulocytes,
monocytes, mast cells, lymphocytes, and
megakaryocytes.
3. • In the embryonic yolk sac during the first weeks
of development. Here, yolk-sac stem cells
differentiate into primitive erythroid cells that
contain embryonic hemoglobin.
• In the third month of gestation, hematopoietic
stem cells migrate from the yolk sac to the fetal
liver and then to the spleen; these two organs
have major roles in hematopoiesis from the third
to the seventh months of gestation.
• After that, the differentiation of HSCs in the bone
marrow becomes the major factor in
hematopoiesis and by birth there is little or no
hematopoiesis in the liver and spleen.
4. • Early in hematopoiesis, a multipotent stem cell
differentiates along one of two pathways, giving
rise to either a common lymphoid progenitor cell
or a common myeloid progenitor cell.
• During the development of the lymphoid and
myeloid lineages, stem cells differentiate into
progenitor cells, which have lost the capacity for
self-renewal and are committed to a particular
cell lineage.
5. • Common lymphoid progenitor cells give rise to
B, T, and NK (natural killer) cells and some
dendritic cells.
• Myeloid stem cells generate progenitors of red
blood cells (erythrocytes), many of the various
white blood cells (neutrophils, eosinophils,
basophils, monocytes, mast cells, dendritic
cells), and platelets
