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fibroblastic variant, particularly on the elbow or jaw. Differential diagnoses for such
lesions include various forms of chronic infectious lymphangitis/lymphadenitis (e.g.
glanders, epizootic lymphangitis and cutaneous histoplasmosis) and metastasising
carcinomas.
Why biopsy?
Histopathology usually permits confirmation of a clinical diagnosis of equine sarcoid
and elimination of the considerable list of differential diagnoses. However, nonexcisional
biopsy may provoke transition of a quiescent sarcoid into a rapidly
growing aggressive one. For this reason, many equine practitioners are loath to
interfere, particularly with large or poorly demarcated lesions or when a suitable
method of treatment is unlikely to be available once the diagnosis is confirmed.
Diagnostic histological features of equine sarcoids
Histologically, most sarcoids are characterised by dermal fibroblastic proliferation
and intimately associated, pseudoepitheliomatous hyperplasia of the epidermis.
There is usually some degree of hyperkeratosis and there may be mild parakeratosis.
Long, narrow, anastomosing rete pegs of hyperplastic and at least focally acanthotic
epidermis extend into the proliferating spindle cell component in the dermis.
The infiltrative and poorly demarcated dermal component is usually of moderately
high cell density, with the superficial portion tending to be more cellular than the
deep part. The dermal fibroblasts are slender to plump fusiform cells with poorly
defined cytoplasmic boundaries. The nuclei may be slender and elongate or enlarged
and of irregular shape. Mitotic figures are usually infrequent (0-1 per high power
field) but may be more numerous (up to 2-3 per high power field) in superficial than
in deep areas of sarcoids and adjacent to areas of ulceration. Mitoses and
fibroblastic nuclear pleomorphism, anisokaryosis and nucleolar enlargement may be
observed in rapidly growing or recurrent sarcoids; cellular anaplasia is, however, rare.
The fibroblasts are typically arrayed in tight whorls, interweaving bundles, haphazard
tangles and occasionally herringbone and basket weave patterns, with a small to
moderate amount of intercellular collagen. In recently developed sarcoids, the
volume of intercellular collagen may be minimal. In chronic lesions, the collagen
content increases and the dense fibres may appear hyalinised. Plump dermal
fibroblasts usually directly abut the epidermis and, in a high percentage of sarcoids,
are characteristically oriented perpendicular to the epidermal basement membrane in
a picket-fence arrangement. Hair follicles enveloped by fibroblasts may undergo
cystic dilation.
Histological variants
Prominent hyperplasia of the epidermis with formation of long rete pegs is not
invariably present in sarcoids. It is usually observed in verrucous and mixed forms
and at the margins of ulcerated areas in fibroblastic sarcoids. In occult sarcoids, the
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ACVSC Proceedings Dermatology Chapter Science Week 2005