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SUMMARY OF KEY POINTS

Hemopoiesis SUMMARY OF KEY POINTS

  • Pluripotent stem cells for blood cell formation, or hemopoiesis, occur in the bone marrow of children and adults.

  • Progenitor cells, committed to forming each type of mature blood cell, proliferate and differentiate within microenvironmental niches of stromal cells, other cells, and ECM with specific growth factors.

  • These progenitor cells are also known as CFUs and the growth factors are also called CSFs or cytokines.

  • Red bone marrow is active in hemopoiesis; yellow bone marrow consists mostly of adipose tissue.

  • Erythropoietic islands or cords within marrow contain the red blood cell lineage: proerythroblasts, erythroblasts with succeeding developmental stages called basophilic, polychromatophilic, and orthochromatophilic that reflect the cytoplasmic transition from RNA-rich to hemoglobin-filled.

  • At the last stage of erythropoiesis cell nuclei are extruded, producing reticulocytes that still contain some polyribosomes but are released into the circulation.

  • Granulopoiesis includes myeloblasts, which have large nuclei and relatively little cytoplasm; promyelocytes, in which lysosomal azurophilic granules are produced; myelocytes, in which specific granules for one of the three types of granulocytes are formed; and metamyelocytes, in which the characteristic changes in nuclear morphology occur.

  • Immature neutrophilic metamyelocytes called band (stab) cells are released prematurely when the compartment of circulating neutrophils is deleted during bacterial infections.

  • Monoblasts produce monocytes in red marrow, but lymphoblasts give rise to lymphocytes primarily in the lymphoid tissues in processes involving acquired immunity.

  • Megakaryocytes, large polyploid cells of red bone marrow, produce platelets, or thrombocytes, by releasing them from the ends of cytoplasmic processes called proplatelets.

  • All these formed elements of blood enter the circulation by crossing the discontinuous endothelium of sinusoids in the red marrow.

INTRODUCTION

Mature blood cells have a relatively short life span and must be continuously replaced with new cells from precursors developing during hemopoiesis (Gr. haima, blood + poiesis, a making). In the early embryo, these blood cells arise in the yolk sac mesoderm. In the second trimester, hemopoiesis (also called hematopoiesis) occurs primarily in the developing liver, with the spleen playing a minor role (Figure 13–1). Skeletal elements begin to ossify and bone marrow develops in their medullary cavities, so in the third-trimester marrow of specific bones becomes the major hemopoietic organ.

FIGURE 13–1

Shifting locations of hemopoiesis during development and aging.

Hemopoiesis, or blood cell formation, first occurs in a mesodermal cell population of the embryonic yolk sac, and shifts during the second trimester mainly to the developing liver, before becoming concentrated in newly formed bones during the last 2 months of gestation. Hemopoietic bone marrow occurs in many locations through puberty, but then becomes increasingly restricted to components of the axial skeleton.

Throughout childhood and adult life, erythrocytes, granulocytes, monocytes, and ...

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