Page 45 - Differential Diagnosis in Small Animal Cytology, The Skin and Subcutis

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6.1 Neutrophilic Inflammation
VetBooks.ir General information

• Neutrophils are involved in the innate cell-mediated immunity. One of their key functions
is to engulf and neutralize infectious agents (mostly bacteria).
Cytological features

• Neutrophilic inflammation is diagnosed when the sample contains a vast predominance of
neutrophils. Purulent inflammation is sometimes used as a synonym.
• Neutrophils may present in different forms:
• Non-degenerate neutrophils
Similar to those observed in the circulatory stream. They have segmented, densely
stained nuclei with clumped chromatin. Their presence is primarily suggestive of a ster-
ile inflammatory process; however, infection may still be present.
• Degenerate neutrophils
• Karyolytic neutrophils: characterized by a pale, swollen nucleus with coarse and
pale chromatin; typical nuclear segmentation and cell borders may be partially
lost. These changes are the result of endotoxins released by the infectious agents
that damage the cellular and nuclear membranes, allowing water influx and
swelling of cellular components. They are often associated with bacterial and fun-
gal infections. Degenerate neutrophils may also be found in association with
severe necrosis.
• Pyknotic neutrophils: the nucleus loses the typical segmentations and becomes
rounder, smaller and denser (apoptosis). This is usually a feature of age-related cell
death.
• Karyorrhectic neutrophils: pyknotic neutrophils following the fragmentation of the
nucleus into small fragments.
• Neutrophils may be associated with other cell types, in particular macrophages. In those
cases, the inflammation is often referred to as mixed or pyogranulomatous.

Causes
• Infectious agents (mostly bacteria).
• Trauma/irritation.
• Tissue necrosis (often associated with underlying, rapidly growing neoplasia).
• Immune-mediated process.

Pearls and Pitfalls
• Care must be taken when diagnosing neutrophilic inflammation in a cytological sample
with a significant degree of haemodilution, as the neutrophils could be blood derived,
especially if the animal has peripheral neutrophilia.
• On cytology, the diagnosis can be limited to ‘neutrophilic inflammation’. In absence of
the architecture of the tissue, the exact localization of the inflammatory process cannot
be determined and the exact characterization of the underlying pathophysiology cannot
always be determined.