|
Membranous Lipodystrophy
Injury to lipocytes produces a variety of responses, chief of which is a granulomatous. This type of response is
a reaction in response to the release of free lipid into the tissue; it usually is lipogranulomatous (symmetrically rounded, clear spaces are represented among the histiocytes forming the granulomas). Lipid-filled
microcysts, that are outlined by convoluted hyaline membanes, are another common, non-specific feature of damaged adipose tissue; this type of reaction is characterized as membranous lipodystrophy (S10C8P6-1 & 2).
In some lipogranulomatous reactions, the lipid droplets are small and vary in size (S10C8P6-3 & 4; S10C9P7-1-4); histiocytes containing droplets of this type might be mistaken for lipoblasts.
Lobular Panniculitis, unclassified
The classification of the panniculitides is imprecise; the characterization of variants on the basis of the
distribution of the inflammatory infiltrates is a most general approach. Lobular variants, in which lipogranulomatous patterns are admixed with lymphoplasmacytic infiltrates, are common. For some examples, areas of
necrosis are common and, in the areas of necrosis, small vessels often contain fibrin thrombi. Some of the lesions showing these features will prove to be examples of lupus profundus (panniculitis). The
commonness of vascular components in many variants of panniculitis suggests that the vascular changes are of prime importance; immune complexes may have an important role in the pathogenesis of many (most?) forms of
panniculitis.
If emphasis is placed on the character of the inflammatory infiltrates in the classification of the
panniculitides, examples will be encountered in which an assignment to the category of either neutrophilic or lymphohistiocytic panniculitis cannot be made. Depending on the field selected for examination, the
process may be one or the other; the patterns are not mutually exclusive.
The character of the infiltrate may, in part, depend on the stage in the evolution of the process in the area selected for examination. In areas, the process may be hemorrhagic, again emphasizing a role for a vascular component in the evolution of the process. Not only will there be regional variations in the character of the inflammatory infiltrates, but both septal and lobular components may be represented (S10C10P8-1-4).
Not all infectious processes, such as infections with acid fast organisms, or fungi, involving the subcutaneous
tissue are characterized by significant granulomatous components. Some forms of atypical mycobacterial infections, such as those related to M. chelonii, can be characterized by diffuse, poorly organized infiltrates
with areas of suppuration (S10C10P8-5 & 6).
Fat Necrosis of the Newborn
In the category of lobular panniculitides, fat necrosis of the newborn is distinctive. It is almost purely
histiocytic with relatively little in the way of a lymphoid component. The infiltrates are diffuse and confluent. They are distinguished by small clusters of radially arranged histiocytes. Extensions of cytoplasm
project into a central defect to produce a distinctive spicular pattern (S10C11P9-1-6). The defect
apparently represents the site from which lipid crystals have been removed during the processing of tissue for histologic examination. This distinctive pattern is irregularly interrupted by symmetrically round
defects that vary in size; these rounded defects are lipid-filled cysts, produced by the breakdown of lipocytes.
Substitutive Fibrosis
(with Morphea as an example)
The interface between adipose and the fibrous tissue of the dermis is at best a tentative boundary. In a variety
of inflammatory disorders, including morphea, necrobiosis lipoidica, and lupus profundus, connective tissue cells, particularly in the deep 1/3 of the dermis, are activated; the activated cells contribute collagen
fibrils to pre-existing collagen bundles. The collagen bundles become coarser and, peculiarly, appear to be more uniform in distribution. They come to lie parallel to one another and to the surface of the skin; they
are streaked in a direction that is parallel to the surface of the skin (this process qualifies as an interstitial, sclerosing collagenosis). In addition, connective tissue cells among lipocytes (in the zone near,
and in continuity with, the deep margin of the dermis) are activated in a productive fashion; they lay down a new, delicate fibrous matrix among the lipocytes. Neighboring lipocytes become entrapped in this matrix;
as it expands. As the new matrix matures into dense fibrous tissue, the entrapped lipocytes undergo progressive atrophy. In this manner, a domain, somewhat similar to that of the pre-existing dermis (a domain
defined by densely fibrous matrix) will expand at the expense of a portion of the domain of the pre-existing adipose tissue; the process qualifies as a substitutive collagenosis (S10C12P10-1-3).
|
