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Palisaded Granulomas of the Skin
In a variety of disorders, the response of the connective tissue of the skin to local insults often is a local
recruitment of histiocytes. In the response, the role of histiocytes is to promote the lysis of collagen bundles; in the act, an expanded mucinous matrix compensates for loss in the domain of the collagenous
component. One combination of such changes is spoken of as a palisaded granuloma.
The palisaded granulomas begin as interstitial infiltrates of activated histiocytes (S9C22VA2-1). These are mainly Kp-1 reactive cells, presumably programmed to enzymatically digest, or unbundle, fibrous components of connective tissue. They release enzymes into the tissue and, early on, are victims of their own products. In the area of the histiocytic infiltrates, both cells and connective tissue fibers undergo lysis. Rarely, a single collagen bundle of a classic lesion of granuloma annulare will be isolated by palisaded histiocytes (histiocytic or granulomatous ring-binding of a solitary collagen bundle) (S9C25aP16a-2). In the process of transforming from traceries of histiocytes to palisades of histiocytes, a relative, or absolute, increase in the
amount of mucinous matrix develops in the area of damage (S9C23P14-1-6, S9C24P15-3-5, & S9C25P16-4 & 5). Fibrin deposits may be found in the mucinous matrix. In a mature lesion, it is tempting to propose that the cells, themselves, have
the capacity to directionally elaborate extracellular enzymes; the enzymes are directed into the area of damaged connective tissue without compromising the vitality of most of the cells forming the palisades. In
this act, the cytoplasmic processes of histiocytes may play an important role (S9C22VA2-3, S9C24P15-1 & 2, & S9C25aP16a-1). Some palisaded granulomas show a
preferential digestion of collagenous bundles relative to the effect of the same enzymes on elastic fibers (S9C25P16-1-3). In other examples, such as
actinic granuloma, there appears to be a preference for elastica (S9C25P16-7-9); some of the elastic fibers (usually actinically damaged fibers) are
phagocytized by histiocytes and histiocytic giant cells (S9C22VA2-2). The histologic patterns of lesions in the category of actinic
granuloma often more closely resemble a foreign-body granuloma rather than a palisaded granuloma (with granuloma annulare as a classic example of a palisaded granuloma).
“Interstitial” patterns (i.e., lesions without significant “necrobiosis”), in which histiocytes form traceries
among collagen bundles, may be a feature of early, or late (regressing) lesions of granuloma annulare. In early lesions, collagen bundles are little altered.
In late lesions, the histiocytes are less prominent. In response to a hyperplasia of fibroblasts, the collagen
bundles reform; they tend to be coarse and closely aggregated (these fibrotic areas qualify as sclerosing granuloma annulare) (S9C25P16-6).
Hyalinization in the center of a palisaded granuloma is uncommon in classic granuloma annulare; if found as a feature of a problematic palisaded granuloma, perhaps a diagnosis other than granuloma annulare should be
considered (S9C25aP16a-6).
Some lesions of granuloma annulare (GA) show central areas of acidophilic necrosis with fragments of nuclear
debris (disintegrating inflammatory cells). Such lesions tend to have well-developed granulomatous qualities; they may be mistaken for infectious granulomas. Perforating forms of GA often are characterized by
central areas of necrosis. In this variation, the zone of necrosis extends to the dermal-epidermal interface; the epidermis may be damaged (involved) by the necrotizing process. In some examples of GA, some of the
vessels in, or bordering, a zone of necrosis will show thrombosis and fibrinoid necrosis (S9C25aP16a-7).
In S9C25aaP16aa-1,
the patterns are typical of a classic lesion of GA; there is a central zone of altered, somewhat mucinous connective tissue with a peripheral zone of palisaded histiocytes. The histiocytes of the palisades are
mostly cells that are reactive for Kp-1 (CD-68) (S9C25aaP16aa-2-6).
Elastolysis
(Anetoderma or Macular Atrophy)
The palisaded granulomas serve as an introduction to the elastolytic disorders. Often, by the time elastolytic
disorders are clinically diagnosable, the patterns are fully developed and the process is relatively free of inflammatory infiltrates. In some examples, histiocytes and a few histiocytic giant cells can
be identified in zones showing a reduction, or a loss, of elastica. The elastic tissue alterations are variable in degree. In some examples, a zone of the dermis is relatively free of stainable elastica. In other
examples, there is a reduction in the size of elastic fibers and in the affinity of the fibers for elastic tissue stains (S9C25aaaP16aaa-1-3). In areas of partial elastolysis, the elastic fibers often are small and variably fragmented. The
fibers may stain pale grey rather than black with a Verhoeff van Giesen stain. In the areas of elastolysis,
the character of the collagenous component often is not evaluated; in some examples, the collagen bundles are smaller in diameter and are widely spaced in a clear matrix (S9C25aaP16aa-7 & 8). In such areas, the collagen bundles often are complexly intertwined and many appear to
be cut in cross-section.
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