The Individual Cells of Hematopoiesis

Immature Red Cell Precursors: Proerythroblasts and Basophilic Erythroblasts

Proerythroblasts are the earliest, least mature cells in the erythrocyte-forming series (erythropoiesis). Proerythroblasts are characterized by their size (about 20 ^m), and by having a very dense nuclear structure with a narrow layer of cytoplasm, homogeneous in appearance, with a lighter zone at the center; they stain deep blue after Romanowsky staining. These attributes allow proerythroblasts to be distinguished from myeloblasts (p. 35) and thus to be assigned to the erythrocyte series. After mitosis, their daughter cells display similar characteristics except that they have smaller nuclei. Daughter cells are called basophilic erythroblasts (formerly also called macroblasts). Their nuclei are smaller and the chro-matin is more coarsely structured.

The maturation of cells in the erythrocyte series is closely linked to the activity of macrophages (transformed monocytes), which phagocytose nuclei expelled from normoblasts and iron from senescent erythrocytes, and pass these cell components on to developing erythrocytes.

Diagnostic Implications. Proerythrocytes exist in circulating blood only under pathological conditions (extramedullary hematopoiesis; breakdown of the blood-bone marrow barrier by tumor metastases, p. 150; or erythroleukemia, p. 100). In these situations, basophilic erythroblasts may also occur; only exceptionally in the course of a strong postanemia regeneration will a very few of these be released into the blood stream (e.g., in the compensation phase after severe hemorrhage or as a response to vitamin deficiency, see p. 152).

Normally erythropoiesis takes place only in the bone marrow a

Macroblast

Fig. 8 Early erythropoiesis. a The earliest recognizable red cell precursor is the large dark proerythroblast with loosely arranged nuclear chromatin (1). Below are two orthochromatic erythroblasts (2), on the right a metamyelocyte (3). b Proerythroblast (1). c Proerythroblast (1) next to a myeloblast (2) (see p. 34); lower region of image shows a promyelocyte (3). Toward the upper left are a metamyelocyte (4) and a segmented neutrophilic granulocyte (5).

Fig. 8 Early erythropoiesis. a The earliest recognizable red cell precursor is the large dark proerythroblast with loosely arranged nuclear chromatin (1). Below are two orthochromatic erythroblasts (2), on the right a metamyelocyte (3). b Proerythroblast (1). c Proerythroblast (1) next to a myeloblast (2) (see p. 34); lower region of image shows a promyelocyte (3). Toward the upper left are a metamyelocyte (4) and a segmented neutrophilic granulocyte (5).

Mature Red Blood Precursor Cells: Polychromatic and Orthochromatic Erythroblasts (Normoblasts) and Reticulocytes

The results of mitosis of erythroblasts are called normoblasts. This name covers two cell types with relatively dense round nuclei and grayish pink stained cytoplasm. The immature cells in which the cytoplasm displays a grayish blue hue, which are still able to divide, are now called "polychromatic erythroblasts," while the cells in which the cytoplasm is already taking on a pink hue, which contain a lot of hemoglobin and are no longer able to divide, are called "orthochromatic erythroblasts." The nuclei of the latter gradually condense into small black spheres without structural definition that eventually are expelled from the cells. The now enucleated young erythrocytes contain copious ribosomes that precipitate into reticular ("net-like") structures after special staining (see p. 11), hence their name, reticulocytes.

To avoid confusing erythroblasts and lymphoblasts (Fig. 9 d), note the completely rounded, very dense normoblast nuclei and homogeneous, unstructured cytoplasm of the erythroblasts.

Diagnostic Implications. Polychromatic and orthochromatic erythroblasts may be released into the bloodstream whenever hematopoiesis is activated, e.g., in the compensation or treatment stage after hemorrhage or iron or vitamin deficiency. They are always present when turnover of blood cells is chronically increased (hemolysis). Once increased blood regeneration has been excluded, the presence of erythroblasts in the blood should prompt consideration of two other disorders: extramedullary production of blood cells in myeloproliferative diseases (p. 114), and bone marrow carcinosis with destruction of the blood-bone marrow barrier (p. 154). In the same situations, the reticulocyte counts (after special staining) are elevated above the average of 25%° for men and 40%° for women, respectively, and can reach extremes of several hundred per mill.

Fig. 9 Nucleated erythrocyte precursors. a Two basophilic erythroblasts with ► condensed chromatin structure (1) and a polychromatic erythroblast with an almost homogeneous nucleus (2). b The erythropoiesis in the bone marrow is often organized around a macrophage with a very wide, light cytoplasmic layer (1). Grouped around it are polychromatic erythroblasts of variable size. Erythroblast mitosis (2). c Polychromatic erythroblast (1) and orthochromatic erythroblast (normoblast) (2).

During increased turnover, nucleated red cell precursors may migrate into the peripheral blood a c d

Polychromatic Red Cell

Fig. 9 d The density of the nuclear chromatin is similar in lymphocytes (1) and erythroblasts (2), but in the erythroblast the cytoplasm is wider and similar in color to a polychromatic erythrocyte (3). e Normal red blood cell findings with slight variance in size of the erythrocytes. A lymphocyte (1) and a few thrombocytes (2) are seen. The erythrocytes are slightly smaller than the nucleus of the lymphocyte nucleus.

Fig. 9 d The density of the nuclear chromatin is similar in lymphocytes (1) and erythroblasts (2), but in the erythroblast the cytoplasm is wider and similar in color to a polychromatic erythrocyte (3). e Normal red blood cell findings with slight variance in size of the erythrocytes. A lymphocyte (1) and a few thrombocytes (2) are seen. The erythrocytes are slightly smaller than the nucleus of the lymphocyte nucleus.

Immature White Cell Precursors: Myeloblasts and Promyelocytes

Myeloblasts are the least mature cells in the granulocyte lineage. Mononuclear, round-to-ovoid cells, they may be distinguished from proerythroblasts by the finer, "grainy" reticular structure of their nuclei and the faintly basophilic cytoplasm. On first impression, they may look like large or even small lymphocytes (micromyeloblasts), but the delicate structure of their nuclei always gives them away as myeloblasts. In some areas, condensed chromatin may start to look like nucleoli. Sporadically, the cytoplasm contains azurophilic granules.

Promyelocytes are the product of myeloblast division, and usually grow larger than their progenitor cells. During maturation, their nuclei show an increasingly coarse chromatin structure. The nucleus is eccentric; the lighter zone over its bay-like indentation corresponds to the Golgi apparatus. The wide layer of basophilic cytoplasm contains copious large azurophilic granules containing peroxidases, hydrolases, and other enzymes. These granulations also exist scattered all around the nucleus, as may be seen by focusing on different planes of the preparation using the micrometer adjustment on the microscope.

Diagnostic Implications. Ordinarily, both cell types are encountered only in the bone marrow, where they are the most actively dividing cells and main progenitors of granulocytes. In times of increased granulocyte production, promyelocytes and (in rare cases) myeloblasts may be released into the blood stream (pathological left shift, see p. 112). Under strong regeneration pressure from the erythrocyte series, too—e.g., during the compensation phase following various anemias—immature white cell precursors, like the red cell precursors, may be swept into the peripheral blood. Bone marrow involvement by tumor metastases also increases the permeability of the blood-bone marrow barrier for immature white cell precursors (for an overview, see p. 112 ff.).

In some acute forms of leukemia, myeloblasts (and also, rarely, promyelocytes) dominate the blood analysis (p. 97).

Round cells with "grainy" reticular chromatin structure are blasts, not lymphocytes a c

Myeloblast Erythroblast Images

Fig. 10 Granulocyte precursors. a The least mature precursor in granulopoiesis is the myeloblast, which is released into the blood stream only under pathological conditions. A large myeloblast is shown with a fine reticular nuclear structure and a narrow layer of slightly basophilic cytoplasm without granules. b Myeloblast and neutrophilic granulocytes with segmented nuclei (blood smear from a patient with AML). c Myeloblast (1), which shows the start of azurophilic granulation (arrow), and a promyelocyte (2) with copious large azurophilic granules, typically in a perinuclear location. d Large promyelocyte (1), myelocyte (2), metamyelocyte (3), and polychromatic erythroblast (4). 35

Fig. 10 Granulocyte precursors. a The least mature precursor in granulopoiesis is the myeloblast, which is released into the blood stream only under pathological conditions. A large myeloblast is shown with a fine reticular nuclear structure and a narrow layer of slightly basophilic cytoplasm without granules. b Myeloblast and neutrophilic granulocytes with segmented nuclei (blood smear from a patient with AML). c Myeloblast (1), which shows the start of azurophilic granulation (arrow), and a promyelocyte (2) with copious large azurophilic granules, typically in a perinuclear location. d Large promyelocyte (1), myelocyte (2), metamyelocyte (3), and polychromatic erythroblast (4). 35

Partly Mature White Cell Precursors: Myelocytes and Metamyelocytes

Myelocytes are the direct product of promyelocyte mitosis and are always clearly smaller than their progenitors. The ovoid nuclei have a banded structure; the cytoplasm is becoming lighter with maturation and in some cases acquiring a pink tinge. A special type of granules, which no longer stain red like the granules in promyelocytes ("specific granules," perox-idase-negative), are evenly distributed in the cytoplasm. Myelocyte morphology is wide-ranging because myelocytes actually cover three different varieties of dividing cells.

Metamyelocytes (young granulocytes) are the product of the final myelocyte division and show further maturation of the nucleus with an increasing number of stripes and points of density that give the nuclei a spotted appearance.The nuclei slowly take on a kidney bean shape and have some plasticity. Metamyelocytes are unable to divide. From this stage on, only further maturation of the nucleus occurs by contraction, so that the distinctions (between metamyelocytes, band neutrophils, and segmented neutrophils) are merely conventional, although they do relate to the varying "maturation" of these cell forms.

Diagnostic Implications. Like their precursors, myelocytes and metamy-elocytes normally appear in the peripheral blood only during increased cell production in response to stress or triggers, especially infections (for an overview of possible triggers, see p. 112). Under these conditions, they are, however, more abundant than myeloblasts or promyelocytes.

Promyelocytes Myelocytes Metamyelocytes

Fig 11 Myelocytes and metamyelocytes. a Early myelocyte. The chromatin structure is denser than that of promyelocytes. The granules do not lie over the nucleus (as can be seen by turning the fine focus adjustment of the microscope to and fro). The blood smear is from a case of sepsis, hence the intensive granulation. b Slightly activated myelocyte (the cytoplasm is still relatively basophilic). c Typical myelocyte (1) close to a segmented neutrophil (2). d This metamyelocyte is distinguished from a myelocyte by incipient lobe formation.

Mature Neutrophils: Band Cells and Segmented Neutrophils

Band cells (band neutrophils) represent the further development of metamyelocytes. Distinguishing between the different cell types is often difficult. The term "band cell" should be used when all nuclear sections of the nucleus are approximately the same width (the "bands"). The beginnings of segmentation may be visible, but the indentations should never cut more than two-thirds of the way across the nucleus.

Segmented neutrophils represent the final stage in the lineage that started with myeloblasts, forming gradually, without any clear transition or further cell divisions, by increasing contraction of their nuclei. Finally, the nuclear segments are connected only by narrow chromatin bridges, which should be no thicker than one-third of the average diameter of the nucleus. The chromatin in each segment forms coarse bands, or patches and is denser than the chromatin in band neutrophils.

The cytoplasm of segmented neutrophilic granulocytes varies after staining from nearly colorless to soft pink or violet. The abundant granules are often barely visible dots.

The number of segments increases with the age of the cells. The following approximate values are taken to represent a normal distribution: 10-30% have two segments, 40-50% have three segments, 10-20% have four segments, and 0-5% of the nuclei have five segments. A left shift to smaller numbers of segments is a discreet symptom of reactive activation of this cell series. A right shift to higher numbers of segments (over-segmentation) usually accompanies vitamin B12 and folic acid deficiencies.

Diagnostic Implications. Banded neutrophilic granulocytes (band neutrophils) may occur in small numbers (up to 2 %) in a normal blood count. This is of no diagnostic significance. A higher proportion than 2 % may indicate a left shift and constitute the first sign of a reactive condition (p. 113). The diagnostic value of segmented neutrophilic granulocytes (segmented neutrophils) is that normal values are the most sensitive diagnostic indicator of normally functioning hematopoiesis (and, especially, of normal cellular defense against bacteria). An increase in segmented neutrophils without a qualitative left shift is not evidence of an alteration in bone marrow function, because under certain conditions stored cells may be released into the peripheral blood (for causes, see p. 111). In conjunction with qualitative changes (left shift, toxic granulations), however, granulocytosis does in fact indicate bone marrow activation that may have a variety of triggers (pp. 110 f.), and if the absolute number has fallen below the lower limit of the normal range (Table 2, p. 12), a bone marrow defect or increased cell death must be considered.

Immature Leukocytes Chart

Fig. 12 Neutrophils (neutrophilic granulocytes). a Transitional form between a metamyelocyte and a band cell. b Copious granulation in a band cell (1) (toxic granulation) next to band cells (2) with Döhle bodies (arrows). c Two band cells. d Band cells can also occur as aggregates. e Segmented neutrophilic granulocytes. f Segmented neutrophilic granulocyte after the peroxidase reaction. g Segmented neutrophilic granulocyte after alkaline leukocyte phosphatase (ALP) staining.

Fig. 12 Neutrophils (neutrophilic granulocytes). a Transitional form between a metamyelocyte and a band cell. b Copious granulation in a band cell (1) (toxic granulation) next to band cells (2) with Döhle bodies (arrows). c Two band cells. d Band cells can also occur as aggregates. e Segmented neutrophilic granulocytes. f Segmented neutrophilic granulocyte after the peroxidase reaction. g Segmented neutrophilic granulocyte after alkaline leukocyte phosphatase (ALP) staining.

Cell Degradation, Special Granulations, and Nuclear Appendages in Neutrophilic Granulocytes and Nuclear Anomalies

Toxic granulation is the term used when the normally faint stippled granules in segmented neutrophils stain an intense reddish violet, usually against a background of slightly basophilic cytoplasm; unlike the normal granules, they stain particularly well in an acidic pH (5.4). This phenomenon is a consequence of activity against bacteria or proteins and is observed in serious infections, toxic or drug effects, or autoimmune processes (e.g., chronic polyarthritis). At the same time, cytoplasmic vacuoles are often found, representing the end stage of phagocytosis (especially in cases of sepsis), as are Döhle bodies: small round bodies of basophilic cytoplasm that have been described particularly in scarlet fever, but may be present in all serious infections and toxic conditions. A deficiency or complete absence of granulation in neutrophils is a sign of severe disturbance of the maturation process (e.g., in myelodysplasia or acute leukemia). The Pelger anomaly, named after its first describer, is a hereditary segmentation anomaly of granulocytes that results in round, rod-shaped, or bisegmented nuclei. The same appearance as a nonheredi-tary condition (pseudo-Pelger formation, also called Pel-Ebstein fever, or [cyclic] Murchison syndrome) indicates a severe infectious or toxic stress response or incipient myelodysplasia; it also may accompany manifest leukemia.

Toxic Vacuole Granulocyte

Fig. 13 Variations of segmented neutrophilic granulocytes. a Reactive state with toxic granulation of the neutrophilic granulocytes, more visibly expressed in the cell on the left (1) than the cell on the right (2) (compare with nonactivated cells, p. 39). b Sepsis with toxic granulation, cytoplasmic vacuoles, and Döhle bodies (arrows) in band cells (1) and a monocyte (2). c Pseudo-Pelger cell looking like sunglasses (toxic or myelodysplastic cause). d Döhle-like basophilic inclusion (arrow) without toxic granulation. Together with giant thrombocytes this suggests May-Hegglin anomaly. continued ►

Fig. 13 Variations of segmented neutrophilic granulocytes. a Reactive state with toxic granulation of the neutrophilic granulocytes, more visibly expressed in the cell on the left (1) than the cell on the right (2) (compare with nonactivated cells, p. 39). b Sepsis with toxic granulation, cytoplasmic vacuoles, and Döhle bodies (arrows) in band cells (1) and a monocyte (2). c Pseudo-Pelger cell looking like sunglasses (toxic or myelodysplastic cause). d Döhle-like basophilic inclusion (arrow) without toxic granulation. Together with giant thrombocytes this suggests May-Hegglin anomaly. continued ►

Nuclear appendages, which must not to be mistaken for small segments, are minute (less than the size of a thrombocyte) chromatin bodies that remain connected to the main part of the nucleus via a thin bridge and consequently look like a drumstick, sessile nodule, or small tennis racket. Of these, only the drumstick form corresponds to the X-chromosome, which has become sequestered during the process of segmentation. A proportion of 1-5% circulating granulocytes with drumsticks (at least 6 out of 500) suggests female gender; however, because the drumstick form is easy to confuse with the other (insignificant) forms of nuclear appendage, care should be taken before jumping to conclusions.

Rarely, degrading forms of granulocytes, shortly before cytolysis or apoptosis, may be found in the blood (they are more frequent in exudates). In these, the segments of the nucleus are clearly losing connection, and the chromatin structure of the individual segments, which are becoming round, becomes dense and homogeneous.

Diagnostic Implications. Toxic granulation indicates bacterial, chemical, or metabolic stress. Pseudo-Pelger granulocytes are observed in cases of infectious-toxic stress conditions, myelodysplasia, and leukemia.

The use of nuclear appendages to determine gender has lost significance in favor of genetic testing.

Nuclear Hypersegmentation

Fig. 13 continued. e Hypersegmented neutrophilic granulocyte (six or more segments). There is an accumulation of these cells in megaloblastic anemia. f Drumstick (arrow 1) as an appendage with a thin filament bridge to the nucleus (associated with the X-chromosome), adjoined by a thrombocyte (arrow 2). g Very large granulocyte from a blood sample taken after chemotherapy. h Segmented neutrophilic granulocyte during degradation, often seen as an artifact after prolonged sample storage (more than eight hours).

Fig. 13 continued. e Hypersegmented neutrophilic granulocyte (six or more segments). There is an accumulation of these cells in megaloblastic anemia. f Drumstick (arrow 1) as an appendage with a thin filament bridge to the nucleus (associated with the X-chromosome), adjoined by a thrombocyte (arrow 2). g Very large granulocyte from a blood sample taken after chemotherapy. h Segmented neutrophilic granulocyte during degradation, often seen as an artifact after prolonged sample storage (more than eight hours).

Eosinophilic Granulocytes (Eosinophils)

Eosinophils arise from the same stem cell population as neutrophils and mature in parallel with them. The earliest point at which eosinophils can be morphologically defined in the bone marrow is at the promyelocyte stage. Promyelocytes contain large granules that stain blue-red; not until they reach the metamyelocyte stage do these become a dense population of increasingly round, golden-red granules filling the cytoplasm. The Charcot-Leyden crystals found between groups of eosinophils in exudates and secretions have the same chemical composition as the eosinophil granules.

The nuclei of mature eosinophils usually have only two segments.

Diagnostic Implications. In line with their function (see p. 5) (reaction against parasites and regulation of the immune response, especially defense against foreign proteins), an increase of eosinophils above 400/^l should be seen as indicating the presence of parasitosis, allergies, and many other conditions (p. 124).

Basophilic Granulocytes (Basophils)

Like eosinophils, basophils (basophilic granulocytes) mature in parallel with cells of the neutrophil lineage. The earliest stage at which they can be identified is the promyelocyte stage, at which large, black-violet stained granules are visible. In mature basophils, which are relatively small, these granules often overlie the two compact nuclear segments like blackberries. However, they easily dissolve in water, leaving behind faintly pink stained vacuoles.

Close relations of basophilic granulocytes are tissue basophils or tissue mast cells—but these are never found in blood. Tissue basophils have a round nucleus underneath large basophilic granules.

Diagnostic Implications. In line with their role in anaphylactic reactions (p. 5), elevated basophil counts are seen above all in hypersensitivity reactions of various kinds. Basophils are also increased in chronic myelo-proliferative bone marrow diseases, especially chronic myeloid leukemia (pp.117,120).

Round granules filling the cytoplasm: eosinophilic and basophilic granulocytes a c e

*

K W

Jj

Fig. 14 Eosinophilic and basophilic granulocytes. a-c Eosinophilic granulocytes with corpuscular, orange-stained granules. d In contrast, the granules of neutrophilic granulocytes are not round but more bud-shaped. e Basophilic granulocyte. The granules are corpuscular like those of the eosinophilic granulocyte but stain deep blue to violet. f Very prominent large granules in a basophilic granulocyte in chronic myeloproliferative disease.

Monocytes

Monocytes are produced in the bone marrow; their line of development branches off at a very early stage from that of the granulocytic series (see flow chart p. 3), but does not contain any distinct, specific precursors that can be securely identified with diagnostic significance in everyday morphological studies. Owing to their great motility and adhesiveness, mature monocytes are morphologically probably the most diversified of all cells. Measuring anywhere between 20 and 40 ^m in size, their constant characteristic is an ovoid nucleus, usually irregular in outline, with invaginations and often pseudopodia-like cytoplasmic processes. The fine, "busy" structure of their nuclear chromatin allows them to be distinguished from myelocytes, whose chromatin has a patchy, streaky structure, and also from lymphocytes, which have dense, homogeneous nuclei. The basophilic cytoplasmic layer varies in width, stains a grayish color, and contains a scattered population of very fine reddish granules that are at the very limit of the eye's resolution. These characteristics vary greatly with the size of the monocyte, which in turn is dependent on the thickness of the smear. Where the smear is thin, especially at the feathered end, monocytes are abundant, relatively large and loosely structured, and their cytoplasm stains light gray-blue ("dove gray"). In thick, dense parts of the smear, some monocytes look more like lymphocytes: only a certain nuclear indentation and the "thundercloud" gray-blue staining of the cytoplasm may still mark them out.

Diagnostic Implications. In line with their function (see p. 5) as phagocytic defense cells, an elevation of the monocyte population above 7% and above 850/^l indicates an immune defense reaction; only when a sharp rise in monocyte counts is accompanied by a drop in absolute counts in the other cell series is monocytic leukemia suggested (p. 101). Phagocytosis of erythrocytes and white blood cells (hemophagocytosis) may occur in some virus infections and autoimmune diseases.

> Monocytosis in cases of infection: always present at the end of acute infections; chronic especially in

- Endocarditis lenta, listeriosis, brucellosis, tuberculosis

> Monocytosis in cases of a non-infectious response, e.g.,

- Collagenosis, Crohn disease, ulcerative colitis

> Monocytoses in/as neoplasia, e.g.,

- Paraneoplastic in cases of disseminating tumors, bronchial carcinoma, breast carcinoma, Hodgkin disease, myelodysplasias (especially CMML, pp. 107 f) and acute monocytic leukemia (p. 101)

Monocytes show the greatest morphological variation among blood cells a c e

May Hegglin Inclusions Monocytes

Fig. 15 Monocytes. a-c Range of appearances of typical monocytes with lobed, nucleus, gray-blue stained cytoplasm and fine granulation. d Phagocytic monocyte with plasma vacuoles. e Monocyte (1) to the right of a lymphocyte with azurophilic granules (2). f Monocyte (1) with nucleus resembling that of a band neutrophil, but its cytoplasm stains typically gray-blue. Lymphocyte (2). g A monocyte that has phagocytosed two erythrocytes and harbors them in its wide cytoplasm (arrows) (sample taken after bone marrow transplantation). h Esterase staining, a typical marker enzyme for cells of the monocyte lineage.

Fig. 15 Monocytes. a-c Range of appearances of typical monocytes with lobed, nucleus, gray-blue stained cytoplasm and fine granulation. d Phagocytic monocyte with plasma vacuoles. e Monocyte (1) to the right of a lymphocyte with azurophilic granules (2). f Monocyte (1) with nucleus resembling that of a band neutrophil, but its cytoplasm stains typically gray-blue. Lymphocyte (2). g A monocyte that has phagocytosed two erythrocytes and harbors them in its wide cytoplasm (arrows) (sample taken after bone marrow transplantation). h Esterase staining, a typical marker enzyme for cells of the monocyte lineage.

Lymphocytes (and Plasma Cells)

Lymphocytes are produced everywhere, particularly in the lymph nodes, spleen, bone marrow, and the lymphatic islands of the intestinal mucosa, under the influence of the thymus (T-lymphocytes, about 80%), or the bone marrow (B-lymphocytes, about 20 %). A small fraction of the lymphocytes are NK cells (natural killer cells). Immature precursor cells (lymph node cytology, p. 177) are practically never released into the blood and are therefore of no practical diagnostic significance. The cells encountered in circulating blood are mostly "small" lymphocytes with oval or round nuclei 6-9 ^m in diameter. Their chromatin may be described as dense and coarse. Detailed analysis under the microscope, using the micrometer screw to view the chromatin in different planes, reveals not the patch-like or banded structure of myeloblast chromatin, or the "busy" structure of monocyte chromatin, but slate-like formations with homogeneous chro-matin and intermittent narrow, lighter layers that resemble geological break lines. Nucleoli are rarely seen. The cytoplasm wraps quite closely around the nucleus and is slightly basophilic. Only a few lymphocytes display the violet stained stippling of granules; about 5% of small lymphocytes and about 3% of large ones. The family of large lymphocytes with granulation consists mostly of NK cells. An important point is that small lymphocytes—which cannot be identified as T- or B-lymphocytes on the basis of morphology—are not functional end forms, but undergo transformation in response to specific immunological stimuli. The final stage of B-lymphocyte maturation (in bone marrow and lymph nodes) is plasma cells, whose nuclei often show radial bars, and whose basophilic cytoplasm layer is always wide. Intermediate forms ("plasmacytoid" lymphocytes) also exist.

Diagnostic Implications. Values between 1500 and 4000/^1 and about 35 % reflect normal output of the lymphatic system. Elevated absolute lymphocyte counts, often along with cell transformation, are observed predominantly in viral infections (pp. 67,69) or in diseases of the lymphatic system (p. 75 ff.). Relative increases at the expense of other blood cell series may be a manifestation of toxic or aplastic processes (agranulocytosis, p. 87; aplastic anemia, p. 148), because these irregularities are rare in the lymphatic series. A spontaneous decrease in lymphocyte counts is normally seen only in very rare congenital diseases (agammaglobulinemia [Bruton disease], DiGeorge disease [chromosome 22q11 deletion syndrome]). Some systemic diseases also lead to low lymphocyte counts (Hodgkin disease, active AIDS).

Mature plasma cells are rarely found in blood (plasma cell leukemia is extremely rare). Plasma-cell-like ("plasmacytoid") lymphocytes occur in viral infections or systemic diseases (see p. 68 f. and p. 74f.).

Lymphocytes are small round cells with dense nuclei and some variation in their appearance f f a b d

Acentric Nucleus Lymphocyte Morphology

Fig. 16 Lymphocytes a-c Range of appearance of normal lymphocytes (some of them adjacent to segmented neutrophilic granulocytes). d In neonates, some lymphocytes from a neonate show irregularly shaped nuclei with notches or hints of segmentation. e A few larger lymphocytes with granules may occur in a normal person. f Occasionally, and without any recognizable trigger, the cytoplasm may widen. g A smear taken after infection may contain a few plasma cells, the final, morphologically fully developed cells in the B-lymphocyte series (for further activated lymphocyte forms, see p. 67).

Fig. 16 Lymphocytes a-c Range of appearance of normal lymphocytes (some of them adjacent to segmented neutrophilic granulocytes). d In neonates, some lymphocytes from a neonate show irregularly shaped nuclei with notches or hints of segmentation. e A few larger lymphocytes with granules may occur in a normal person. f Occasionally, and without any recognizable trigger, the cytoplasm may widen. g A smear taken after infection may contain a few plasma cells, the final, morphologically fully developed cells in the B-lymphocyte series (for further activated lymphocyte forms, see p. 67).

Megakaryocytes and Thrombocytes

Megakaryocytes can enter the bloodstream only in highly pathological myeloproliferative disease or acute leukemia. They are shown here in order to demonstrate thrombocyte differentiation. Megakaryocytes reside in the bone marrow and have giant, extremely hyperploid nuclei (16 times the normal number of chromosome sets on average), which build up by endomitosis. Humoral factors regulate the increase of megakaryocytes and the release of thrombocytes when more are needed (e.g., bleeding or increased thrombocyte degradation). Cytoplasm with granules is pinched off from megakaryocytes to form thrombocytes. The residual naked mega-karyocyte nuclei are phagocytosed.

Only mature thrombocytes occur in blood. About 1-4 ^m in size and anuclear, their light blue stained cytoplasm and its processes give them a star-like appearance, with fine reddish blue granules near the center. Young thrombocytes are larger and more "spread out;" older ones look like pyknotic dots.

Diagnostic Implications. In a blood smear, there are normally 8-15 thrombocytes per view field using a 100x objective; they may appear dispersed or in groups. To someone quickly screening a smear, they will give a good indication of any increase or strong decrease in the count, which can be useful for early diagnosis of acute thrombocytopenias (p. 164 f.).

Small megakaryocyte nuclei are found in the bloodstream only in severe myeloproliferative disorders (p. 171).

Megakaryocytes are never present in a normal peripheral blood smear. Thrombocytes are seen in every field view

Drum Stick Peripheral Blood

Fig. 17 Megakaryocytes and thrombocytes. a Megakaryocytes in a bone marrow smear. The wide cytoplasm displays fine, cloudy granulation as a sign of incipient thrombocyte budding. b Normal density of thrombocytes among the erythrocytes, with little variation in thrombocyte size. c and d Peripheral blood smears with aggregations of thrombocytes. When such aggregates are seen against a background of apparent thrombocytopenia, the phenomenon is called "pseudo-thrombocytopenia" and is usually an effect of the anticoagulant EDTA (see also p. 167).

Fig. 17 Megakaryocytes and thrombocytes. a Megakaryocytes in a bone marrow smear. The wide cytoplasm displays fine, cloudy granulation as a sign of incipient thrombocyte budding. b Normal density of thrombocytes among the erythrocytes, with little variation in thrombocyte size. c and d Peripheral blood smears with aggregations of thrombocytes. When such aggregates are seen against a background of apparent thrombocytopenia, the phenomenon is called "pseudo-thrombocytopenia" and is usually an effect of the anticoagulant EDTA (see also p. 167).

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Responses

  • ELIAS BURNS
    What is the size of a giant neutrophil band?
    6 years ago
  • esmeralda
    What is the difference between Metamyelocyte and band neutrophils?
    6 years ago
  • uta
    What is a neutrophilic band cell and what is not?
    6 years ago
  • kedija
    When left shift occurs in hematology how many bands are there?
    6 years ago
  • yemane
    Where Does Hematopoiesis Occur?
    5 years ago
  • DEREK
    Do normal lymphocytes contain larger azuraphilic granuules?
    5 years ago
  • henrik paarma
    Does mono cause toxic granulation of granulocytes?
    5 years ago
  • mari lehtim
    Do proerythrocytes form myeloblasts?
    5 years ago
  • eglantine
    How do myelocytes represent the precursor cells for thrombocytes?
    5 years ago
  • hana
    Which is a very large cell whose cytoplasm breaks away at the edges to form individual thrombocytes?
    5 years ago
  • jessica
    WHAT DOES A NEUTROPHIL BLAST LOOK LIKE?
    4 years ago
  • atso
    Do promyelocytes divide?
    4 years ago
  • Bungo
    CAN METAMYELOCYTES HAVE VACUOLES?
    3 years ago
  • Mike
    What is asegmented neutrophils &noramal range for 3years child?
    1 year ago

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