Contrary to what has been written in many books, toxic change in neutrophils is not
necessarily associated with "toxemia". The term derives from the fact that these
abnormalities were first noticed in human patients with gram negative sepsis and
endotoxemia. However, toxic change in neutrophils do not reflect a "toxic effect" of bacteria on neutrophils but are morphologic
abnormalities acquired during maturation under conditions that intensely stimulate neutrophil
production and shorten the maturation time in marrow. This accelerated maturation occurs secondary to cytokine stimulation, which is usually in reponse to inflammation. Animals recovering from bone marrow injury or who are administered hematopoietic cytokines (e.g. granulocyte-colony stimulating factor or G-CSF) can also show accelerated maturation in neutrophils or toxic change. On the standard hemogram, the presence
of toxic change is reported under the WBC morphology section and is subjectively graded as mild, moderate and marked.
|Normally matured segmented and late band neutrophils, shown in panels A and C, have
white cytoplasm with pink granules, long and fairly narrow nuclei and tightly condensed
chromatin. Segmented and band neutrophils with toxic change (examples shown in panels
B and D) have less condensed chromatin than their normal counterparts and bluer cytoplasm
due to retention of ribosomal RNA.
Most of the toxic changes reflect asynchrony of maturation between the nucleus and cytoplasm. During normal
granulocytopoiesis, the lengthening and pinching of the nucleus are coordinated with
progressive condensation of the chromatin and loss of cytoplasmic protein synthetic machinery (RNA in the form of ribosomes and rough endoplasmic reticulum - which imparts a blue color to the cytoplasm with hematologic stains). With accelerated maturation, nuclear divisions may be skipped (resulting in larger cells than normal) and the cells retain immature features, including increased amounts of rough endoplasmic reticulum or ribosomes in the cytoplasm and lighter chromatin than normal. Specific granules may be less visible than in normally
mature cells and, in some species, primary granules (normally inapparent in neutrophils) retain their affinity for the stains used for peripheral blood. The cells can also have frothy or vacuolated cytoplasm.
Thus in a blood smear we look for 5 main features of toxic change:
|Dohle bodies (one is illustrated by the black arrow) in the cytoplasm of a toxic neutrophil from a cat.
- Cytoplasmic basophilia: This is a streaky diffuse irregular blue appearance to the cytoplasm and is due to the presence of polyribosomes and rough endoplasmic reticulum.
- Dohle bodies: These are pale round to linear blue aggregates in the cytoplasm, caused by whorls of rough endoplasmic reticulum (see image to the right). This is often the earliest and first indication of toxic change. However, note that healthy cats can have low numbers of small Dohle bodies and Dohle bodies form in neutrophils with storage (storage-related artifact). Thus, Dohle bodies alone do not always indicate toxic change.
- Cytoplasmic vacuolation: These are indistinct vacuoles in the cytoplasm, giving it a frothy appearance and is due to degranulation of lysosomes. Note that clear punctate vacuoles are usually not attributed to toxic change, but are frequently another storage-related artifact (these vacuoles can occur within 4 hours of blood collection).
- Nuclear immaturity: The nuclear chromatin is lighter (finer) and less coarsely clumped than normal. This is the hardest and most subtle change to see.
- Toxic granulation: These are distinct red granules in the cytoplasm due to the primary granules taking up stain. This is a rarely seen feature in domestic animals, but is most commonly seen in large animals (horses, ruminants, camelids - see image below).
|These images are from an iron deficient alpaca that also had an inflammatory leukogram due to an enteritis.
A: The two cells are a segmented neutrophil (upper cell) and a band neutrophil (black arrow). Toxic change is evident in both cells - cytoplasmic vacuolation and toxic granulation in the segmented neutrophil and cytoplasmic basophilia in the band neutrophil. B: Three segmented neutrophils are shown, all of which are toxic, demonstrating toxic granulation (upper two cells identified with the red arrows) and cytoplasmic vacuolation (all three cells). The red blood cells in the background are hypochromic (have less hemoglobin than normal) which is a cardinal feature of iron deficiency.
Because toxic change usually indicates an inflammatory leukogram, it frequently accompanies a left shift, i.e. presence of immature neutrophils (bands, metamyelocytes, myelocytes), in animals. However, some animals can have a left shift without toxic change (e.g. dogs with immune-mediated hemolytic anemia) and toxic change can be present (albeit rarely) in animals without a left shift. In the latter circumstances, we look for reasons other than inflammation for the toxic change (e.g. marrow dysplasia, storage artifact).
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