Porth's Essentials of Pathophysiology, 4e

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Hematopoietic Function

U N I T 3

to two types of multipotent stem cells: the hematopoietic stem cells, which remain in the bone marrow, and the lymphopoietic stem cells, which migrate to the thymus, lymph nodes, spleen, and mucosa-associated lymphoid tissues. The Bone Marrow and Hematopoiesis The bone marrow consists of hematopoietic or blood- forming cells and stromal tissue that provides support for the blood-forming cells. The blood-forming popula- tion of bone marrow is made up of three types of cells: pluripotent stem cells, multipotent stem cells, and com- mitted progenitor cells that develop into the various types of blood cells 1–4 (Fig. 11-1). The pluripotent stem cells give rise to two types of multipotential stem cells, the common lymphoid and the common myeloid stem cells. The common lymphoid stem cells, in turn, differ- entiate into lineage-specific precursor cells that develop into T lymphocytes (T cells), B lymphocytes (B cells), and natural killer (NK) cells. From the common myeloid stem cells arise precursor cells capable of differentiating along the erythrocyte/megakaryocytic and granulocyte- monocyte pathways.

Several levels of differentiation lead to the develop- ment of committed unipotential cells, which are the progenitors for each of the blood cell types. These cells are referred to as colony-forming units (CFUs). 3 These progenitor cells only have a limited capacity for self- renewal but retain the potential to differentiate into lineage-specific precursor cells. These precursor cells have morphologic characteristics that permit them to be recognized as the first cell of a particular cell line. They have lost their ability for self-renewal but undergo cell division and differentiation, eventually giving rise to mature lymphocytes, monocytes, granulocytes, mega- karyocytes, or erythrocytes. Hematopoietic Growth Factors Under normal conditions, the numbers and total mass for each type of circulating blood cell remain relatively constant. This regulation of blood cells is thought to be at least partially controlled by hormone-like growth factors called cytokines. The cytokines are a family of short-lived mediators that stimulate the proliferation, differentiation, and functional activation of the various blood cells.

Pluripotent stem cell

Lymphoid stem cell

Myeloid stem cell

Monocyte (Committed Stem Cell) (Mature Cell) FIGURE 11-1. Major developmental stages of blood cells. NK, natural killer cell. B cell progenitor B cell T cell progenitor Thymus NK progenitor Monoblast Granulocyte Neutrophil Eosinophill Monocyte Plasma cell T cell NK cell

Megakaryocyte

Erythrocyte

Reticulocyte

Megakaryocyte

Basophil

Platelets

Erythrocyte