Morphology of Hairy Cell Leukemia
Accurate diagnosis of hairy cell leukemia rests upon the recognition of the cells in blood, bone marrow, or spleen.
Morphologic observation of these pathognomonic cells is more art than science.
This trenchant observation of Yam and colleagues is as relevant today as when it was written in 1972 and emphasizes that, despite the ever-growing battery of ancillary studies to assist the hematopathologist, the identification of cytologically characteristic cells remains the diagnostic sine qua non of HCL. Here we will spot the light on hairy cells in peripheral blood.
Hairy cells derive their name from the irregular fine cytoplasmic projections that extend for variable distances from the periphery. It is, however, a combination of characteristics that identifies the HC in Romanowsky preparations (e.g. Wright stain).
The eccentric nucleus with its fine chromatin condensation, the pale slate-blue cytoplasm and the fine surface projections all form part of the initial diagnostic impression.HC surface morphology is best appreciated by phase microscopy of a viable "wet" preparation of moving cells in suspension and here the typical villous nature of the HC are easily appreciated.
In Wright-stained preparations, the hairy cell is 1.5 to 2 times the size of a mature lymphocyte and the nucleus occupies one half to two thirds of the cell's area.
HCL tends to be a disease of monotonous cells with respect to cytologic characteristics and size. Hence, although there may be a moderate degree of cell size variation between patients, an individual patient usually displays a remarkably homogeneous population of hairy cells.
The nuclei can have several configurations, including round, oval, spindled, reniform, horseshoe-shaped, and bilobed. Although the hairy cells in an individual vary in nuclear contours, most patients have a preponderant nuclear shape, most commonly oval.
Despite variation in nuclear contour, there are several consistent nuclear characteristics of HCL that assist greatly in recognition.
Most important, the nuclear membrane is nearly always smooth, imparting a distinct demarcation from the surrounding cytoplasm and lacking the fine surface irregularities that typify many other lymphoproliferative disorders.
As well, the nuclear membrane usually appears thickened. The chromatin of HCL has a partially condensed appearance that is intermediate between a mature lymphocyte and a blast.
Additionally, the chromatin has a uniform granular appearance in contrast to the irregularly clumped chromatin of other disorders, particularly B-cell chronic lymphocytic leukemia (B-CLL) and splenic marginal zone lymphoma (SMZL).
Hairy cells have no evident nucleoli or a single nucleolus. Infrequently, cells with two nucleoli are present.
Generally, patients demonstrate a predominance of nucleolated or nonnucleolated cells.
The nucleoli are nearly always small, round, and without irregularities in contour.
Peripheral blood
The morphological presentation of HCL in the peripheral blood is variable. Some patients present with the appearance of leukaemic peripheral blood, including a moderate lymphocytosis composed of morphologically identifiable hairy cells.
In other cases, the peripheral blood smear is notable only for monocytopenia or neutropenia, with scant to absent circulating hairy cells.
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