The hematopoietic system is comprised of many different cell types that fulfill
important physiological functions throughout embryonic and adult stages of mouse
development. As the mature blood cells have a limited life-span, the pool of blood cells
needs constant replenishing. At the basis of the hematopoietic system reside the
hematopoietic stem cells (HSCs). HSCs are characterized by the ability to self-renew and to
produce all the different mature hematopoietic cell types. These HSCs differentiate
to the common lymphoid and myeloid progenitors (CLP and CMP) which in tum give rise
to more lineage restricted precursor cells that then differentiate into mature hematopoietic
cells (Figure 1 p.8). Mature hematopoietic cells can be subdivided into the lymphoid
compartment, comprising T and B lymphocytes which form the adaptive immune (defense)
system, and the myeloid compartment which is comprised of the oxygen transporting
erythrocytes (red blood cells), blood clot-mediating platelets, immune response-mediating
neutrophils, eosinophils and basophils and the macrophages, of which the main function is
to remove dead and foreign cells. Although the mature blood cells fulfill the physiological
functions of the hematopoietic system, the hematopoietic stem cells are essential within the
adult hematopoietic hierarchy since these cells are responsible for the life long production
of the different mature hematopoietic cells
