Normal WBC maturation sequence begins with the blast form, derived from hematopoietic stem cells that, in turn, are thought to be derived from tissue reticulum cells (Fig. 6-1). In the myelocytic (granulocytic or neutrophilic) series, the blast is characterized by a large nucleus with delicate very uniform-appearing light-staining chromatin and with one or more nucleoli. Typically, a blast has relatively scanty basophilic cytoplasm without granules,* but the French-American-British (FAB) group (Chapter 7) describes a category of blasts with cytoplasm that may contain a few “azurophilic” granules. Next in sequence is the progranulocyte (promyelocyte), which is similar to the blast but has a variable number of cytoplasmic granules. The promyelocyte gives rise to the myelocyte. Myelocyte nuclear chromatin is more condensed, there is no nucleolus, and the nucleus itself is round or oval, sometimes with a slight flattening along one side. The cytoplasm is mildly basophilic and is granular to varying degrees, although sometimes granules are absent. Often there is a small, localized, pale or clear area next to the flattened portion (if present) of the nucleus, called the myeloid spot. Next, the nucleus begins to indent; when it does, the cell is called a metamyelocyte (juvenile). As the metamyelocyte continues to mature, the nucleus becomes more and more indented. The nuclear chromatin becomes more and more condensed, clumped, and darkly stained, and the cytoplasm becomes progressively less basophilic. The entire cell size becomes somewhat smaller, with the nucleus taking up increasingly less space. Finally, the band (stab) neutrophil stage is reached. There is some disagreement as to what constitutes a band as opposed to a metamyelocyte or band as opposed to an early mature polymorphonuclear leukocyte. Basically, a band is distinguished from a late metamyelocyte when the nucleus has indented more than one half its diameter and has formed a curved rod structure that is roughly the same thickness throughout. As the band matures, nuclear indentation continues and may also occur in other areas of the nucleus. When at least one area of nuclear constriction becomes a thin wire, the cell has reached the final stage of maturity, called the polymorphonuclear (poly) or segmented neutrophil. The nucleus has segmented into two or more lobes, at least one of which is connected only by a threadlike filament to the next. The nuclear chromatin is dense and clumped. The cytoplasm is a very slightly eosinophilic color, or at least there is no basophilia. There usually are small irregular granules, which often are indistinct.

Maturation sequence of granulocytic (myelocytic) series

Fig. 6-1 Maturation sequence of granulocytic (myelocytic) series. A, Blast; B, promyelocyte; C, myelocyte (top, early stage; bottom, late stage); D, metamyelocyte (top, early stage; bottom, late stage) E, band granulocyte (top, early stage; bottom, late stage) F, segmented granulocyte (top, early stage; bottom, hypersegmented late stage).

Terminology of blood cells

Table 6-1 Terminology of blood cells

In some cases there may be a problem differentiating bands from segmented neutrophils when a bandlike nucleus is folded over itself in such a way as to hide the possibility of a thin wirelike constricted area (Fig. 6-2). The majority of investigators classify this cell as a segmented form. However, many laboratorians consider these cells bands; unless the reference range takes into account the way these cells will be interpreted, the number of bands reported can differ considerably between persons or between laboratories and could lead to incorrect diagnosis. When there is multiple nuclear segmentation and the lobes connected only by a thin wire number more than five, the cell is termed hypersegmented. Some investigators believe that hypersegmentation is present if more than 5% of the neutrophils have five lobes. Naturally, there are transition forms between any of the maturation stages just described (Fig. 6-1).

Folded segment versus folded band

Fig. 6-2 Folded segment versus folded band. A, One end folded (“mushroom” effect). B, Nuclear fold closer to center. a, True band; b, segment with hidden constriction.

Monocytes are often confused with metamyelocytes or bands. The monocyte tends to be a larger cell. Its nuclear chromatin is a little less dense than chromatin of the myeloid cell and tends to have a strandlike configuration of varying thickness rather than forming discontinuous masses or clumps. The nucleus typically has several pseudopods, which sometimes are obscured by being superimposed on the remainder of the nucleus and must be looked for carefully. Sometimes, however, a monocyte nuclear shape that resembles a metamyelocyte is found. The monocyte cytoplasm is light blue or light gray, is rather abundant, and frequently has a cytoplasm border that appears frayed or has small irregular tags or protrusions. The granules of a monocyte, when present, usually are tiny or pinpoint in size, a little smaller than those of a neutrophil. In some cases, the best differentiation is to find undisputed bands or monocytes and compare their nucleus and cytoplasm with that of the cell in question.

The reference range for peripheral blood WBCs is 4,500-10,500/mm3 (4.5-10.5 Ч 109/L). Most persons have WBC counts of 5,000-10,000/mm3, but there is significant overlap between normal and abnormal in the wider range, especially between 10,000 and 11,000/mm3. The mean WBC count in African Americans may be at least 500/mm3 (0.5 Ч 109/L) less than those in Europeans, with some investigators reporting differences as much as 3,500/mm3. This difference would be important, since it would produce a greater than expected incidence of apparent leukopenia and less than expected leukocyte response to infection and inflammation. However, not all reports agree that there is a consistent difference between the two racial groups. Normal WBC differential values are listed here:

The value for each cell type traditionally is known as the cell “count” (i.e., band count), although the findings are expressed as a percentage of 100 WBCs rather than the actual cell number counted.

Some investigators report a diurnal variation for neutrophils and eosinophils. Neutrophil peak levels were reported about 4 P.M. and lowest values reported about 7 A.M., with an average change of about 30%. Eosinophil levels roughly paralleled serum cortisol levels, with highest values about 7 A.M. and lowest values about 4 P.M. The average change was about 40%. The remainder of this chapter describes anomalies of WBC morphology or count and associated disease states.