Myeloblasts and Promyelocytes (Fig. 7 a-h)
Myeloblasts are the earliest precursors of granulocytopoiesis that can be identified by light microscopy. Today it is believed that they also function as precursors of monocytes, i.e., as myelomo-noblasts. They range from 12 to 20 im in diameter (Fig. 7a-c). The cytoplasmic rim is basophilic but may show a range of hues from soft pale blue to dark blue. The cytoplasm is agranular on ordinary panoptic staining, although older cells frequently show incipient granulation signifying transition to the promyelocyte stage (Fig. 7 d). The peroxidase reaction is usually negative, but there is no question that agranular, peroxidase-positive myeloblasts exist. The nucleus shows a very fine, dense chromatin structure with as many as six nucleoli, which generally are distinct and pale blue.
Promyelocytes evolve directly from the myeloblasts by incorporating azurophilic granules into their cytoplasm in a concentric pattern surrounding the clear zone in the nuclear indentation (Gol-gi zone), which itself is devoid of granules. Initially there are few granules, but these subsequently thicken and spread to fill the cytoplasm. At first the cytoplasm is basophilic but gradually lightens until it acquires the typical myelocytic tinge. The variable staining properties of the cytoplasm have led some authors to subdivide the promyelocytes into mature and immature forms, groups I and II, etc. As in myeloblasts, the promyelocyte nucleus is finely structured and contains up to six nucleo-li. The cells are peroxidase-positive. First to appear are the primary or azurophilic granule, which contain peroxidase; they are joined later by the specific secondary granules (peroxidase-negative), which increase as maturation proceeds. The cells are 20 - 25 im in diameter, making promyelocytes the largest cell of the granulocyto-poietic and erythropoietic lines. In absolute terms, mitoses are more frequent than in the myeloblasts (Fig. 7d-h).
The offspring of the promyelocytes are the myelocytes (Fig. 8a-e). Generally these cells are somewhat smaller than their immediate precursors, with diameters ranging from 14 to 20 im. As the coarse promyelocytic granules become more sparse, the typical fine neutrophilic granularity becomes predominant. The basophilic cytoplasm lightens from the nucleus outward, becoming acidophilic, while the nuclear chromatin acquires a coarser structure. Nucleoli are rarely visible. The myelocyte is the most plentiful granulo-cytopoietic cell type found in the bone marrow.
As maturation proceeds, the nuclear chroma-tin becomes even coarser and more dense. The nucleus becomes indented or horseshoe-shaped, while the cytoplasm and granules remain essentially the same as in mature myelocytes. The cells at this stage are called metamyelocytes or juvenile forms (Fig. 8 d). A few may be found in the peripheral blood. Metamyelocytes are no longer capable of division.
The band neutrophil (Fig. 8 e) is distinguished from the metamyelocyte by its smaller and more coarsely structured nucleus. Its cytoplasm and granules are like those of metamyelocytes. Constrictions begin to appear in the nucleus, but the cell is not classified as a segmented form until the bridge between two nuclear lobes is filiform or less than one-third the width of the adjacent lobes.
The nuclear lobes of the segmented neutrophil (Fig. 8 b, c, g) present a coarse, clumped chromatin structure. Most of these cells contain 3-5 nuclear lobes usually joined by thin, short chroma-tin filaments. A cell containing more than 5 lobes is said to be hypersegmented. This is especially common in pernicious anemia but is not pathog-nomonic for that condition. The band and segmented forms range from 10 to 15 im in size.
Eosinophilic granulocytes (Fig. 8 b, e, f) develop in much the same way as neutrophils, but the two cell types are always distinguishable from the pro-myelocyte stage onward. The nuclei have struc tures similar to the corresponding maturation stages. The typical large eosinophilic granules almost completely obscure the cytoplasm. Conspicuously large granules, sometimes of a deep blue color, maybe found among these mature eo-sinophilic granules in the early stages of promye-locytes and myelocytes (Fig. 8 b, e) but are no longer present in the mature stages. Another typical feature of eosinophils is their nuclear segmentation, with most cells possessing two nuclear lobes and a smaller number having three. Mitoses of eosinophils are also occasionally found in bone marrow. All eosinophils are peroxidase-positive.
Basophilic granulocytes (Fig. 8 g) also have a developmental pathway similar to neutrophils. Typically they contain large basophilic granules that obscure the cytoplasm and even cover the nucleus. In segmented basophils the nucleus consists of multiple lobes and often presents a clover-leaf shape. The cells tend to be somewhat smaller than neutrophils and eosinophils. They are usually peroxidase-negative when standard technique is used.
Mast cells (tissue basophils, Fig. 8 h) are classified into two types based on nuclear size and shape and granule density. Mastoblasts and pro-mastocytes have a relatively large nucleus with an indistinct structure and sparse granules. Masto-cytes are large cells (diam. 15 - 30 im) with a round, compact nucleus showing structural similarities to the lymphocyte or plasma cell. Acid mucopolysaccharides are responsible for the typical metachromasia of the granular stain pattern and can be demonstrated cytochemically with to-luidine blue stain (for method, see p. 7). A strong naphthol AS-D chloroacetate esterase reaction is also typical (Fig. 8 h, right). Mast cells produce heparin and histamine in a ratio of 3: 1. Isolated mast cells may be found in fragments of normal bone marrow. The mast cell population in bone marrow is increased in severe inflammatory disorders, panmyelophthisis, non-Hodgkin lymphoma, and especially in Waldenstrom disease (lym-phoplasmocytoid immunocytoma) and hairy cell leukemia. Atypical mast cells may be found in mastocytoses (Figs. 128, 129).
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