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Early in precursor development in the marrow, cells destined to be leukocytes of the granulocytic series, neutrophils, eosinophils, and basophils, synthesize proteins and store them as cytoplasmic granules. The synthesis of primary or azurophilic granules defines the conversion of the myeloblast, a virtually agranular, primitive cell that is the earliest granulocyte precursor identifiable by light microscopy into the promyelocyte, which is rich in azurophilic granules. Synthesis and accumulation of secondary or specific granules follows. The appearance of specific granules marks the progression of the promyelocyte to neutrophilic, eosinophilic, or basophilic myelocytes. Thereafter, the cell continues maturation into an amitotic cell with a segmented nucleus, capable of ameboid motility, phagocytosis, and microbial killing. The mature granulocytes also develop cytoplasmic and surface structures that permit them to attach to and penetrate the wall of venules. The mature granulocytes enter the blood from the marrow, circulate briefly, and move to the tissues to carry out their major function of host defense.

Acronyms and Abbreviations

Acronyms and abbreviations that appear in this chapter include: C3a, serum complement fragment 3a; C5a, serum complement fragment 5a; ECP, eosinophil cationic protein; EDN, eosinophil-derived neurotoxin; EPO, eosinophil peroxidase; Ig, immunoglobulin; IL, interleukin; MBP, major basic protein; PMN, polymorphonuclear neutrophil.

In the normal adult human, the life of granulocytes is spent in three environments: marrow, blood, and tissues. Marrow is the site of differentiation of hematopoietic stem cells into granulocyte progenitors and of proliferation and terminal maturation (Fig. 59–1). Precursor cell proliferation, which consists of approximately five divisions, occurs only during the first three stages of maturation (blast, promyelocyte, and myelocyte). After the myelocyte stage, the cells are no longer capable of mitosis and enter a large marrow storage pool from which they are released into the blood where they circulate for a few hours before entering tissues.

Figure 59–1.

Diagrammatic representation of the stages of maturation of marrow granulocytes (see text for discussion). Of every 100 nucleated cells in marrow, 0.5% are myeloblasts, 5% are promyelocytes, 12% are neutrophilic myelocytes, 23% are neutrophilic metamyelocytes and bands, and 30% are mature neutrophilic cells, yielding a total of approximately 65% of cells representing developing neutrophils in normal human marrow. A similar pattern of maturation occurs in the eosinophil and basophil lineage although the number of these cell types are fewer. There are about 2.0 % eosinophils and 0.1% basophils in normal marrow (see Chap. 3).

(Used with permission from Lichtman’s Atlas of Hematology, www.accessmedicine.com.)

Light Microscopy and Electron Microscopy

The Myeloblast

The myeloblast is an immature cell with a large, oval nucleus, sizable nucleoli, and few or no granules. As the earliest morphologically recognizable precursor in the evolution of the neutrophil from the colony-forming cell, it is an immature cell with a large nucleus and ...

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