Page 28 - Clinical Hematology Atlas

12 SECTION TWO HEMATOPOIESIS

ematopoiesis is a vigorous process of blood cell production and maturation that in the
Hadult occurs primarily in the bone marrow. The process begins with the pluripotential
hematopoietic stem cell (multipotent progenitor), which is capable of proliferation, replica-
tion, and differentiation. In response to cytokines (growth factors), the pluripotential stem
cell will differentiate into a common myeloid or common lymphoid progenitor. Both the
myeloid and lymphoid progenitors maintain their pluripotential capacity. The lymphoid
progenitor proliferates and differentiates into T, B, and natural killer cells. The myeloid pro-
genitor proliferates and differentiates into granulocyte, monocyte, erythrocyte, and mega-
karyocyte lineages. To this point in maturation, none of these stem cells can be morpho-
logically identified, although it is postulated that they appear similar to a small resting
lymphocyte. The blue shaded area in Figure 2-1 highlights the stem cell populations. Each
lineage and maturation stage will be presented in detail in the following chapters.
Hematopoiesis is a dynamic continuum, that is, cells gradually mature from one stage to
the next and may be between stages when viewed through the microscope. In general, the
cell is then identified as the more mature stage. General morphological changes in blood
cell maturation are demonstrated in Figure 2-2.
Figures 2-3 and 2-4 illustrate cell ultrastructure. A review of organelles will facilitate cor-
relation of morphological maturation with cell function. This topic is explored in depth in
hematology textbooks. Table 2-1 delineates the location, appearance, and function of indi-
vidual organelles.