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LICHEN STRIATUS, AS A MODEL FOR THE LICHENOID REACTION (tier 2)
The definition of stages in the life history of lichen planus-like processes is compromised by the basic requirements that a specific lichen planus-like variant must
be given recognition; the histologic requisites are satisfied only in the established phase of such a reaction. In defining lichen planus, we recognize only one stage in the life-history of a lichenoid reaction.”
The evidence that the definition defines a specific disease is unconvincing; it defines a common stage in the multi-staged evolution of a basic reaction pattern.
Of all the lichenoids of lichen planus-like type, perhaps lichen striatus is most suitable as a model for defining stages in the evolution of a lichenoid reaction (3).
Lichen striatus, a distinctive clinical disorder, most common in children, is characterized by linearity (S3C14P3C-1). It is manifested histologically in a wide variety of patterns (S3C13P2-1);
the wide range of patterns complicates any attempt to rigidly define the histologic patterns. In an example with fully developed patterns, the epidermis, as well as the skin appendages, can be involved; the patterns
are lichenoid. Those of involved skin appendages take on lympho-epithelial qualities, but can be accommodated in the framework of lichenoid reactions (S3C13P2-3 & 4, & S3C15P4-1-3). The follicular component may show
features of follicular mucinosis. The sweat gland component has immunoproliferative qualities. Lymphoid cells surround sweat glands and migrate across basement membranes to intermingle among the epithelial cells.
The patterns are immuno-proliferative; the glandular epithelium undergoes hyperplasia. In addition, there is some degree of nuclear atypia in the altered glandular component. The changes are reminiscent of the
lympho-epithelial lesions of salivary glands in Sjogren’s syndrome.
In many lesions of lichen striatus, changes that might be characterized as a primary stage of a lichenoid reaction is represented; it is characterized by psoriasiform epidermal hyperplasia, and by pityriasic patterns of lymphoid infiltrates in the epidermis and the dermis. The psoriasiform qualities include focal parakeratosis, variable hyperplasia of the superficial unit of the epidermis, and regular hyperplasia of the basal unit of the epidermis with elongation of rete ridges (a common feature of an epidermis showing hyperplasia of the basal unit). Inflammatory changes are variable; they often are spotty in distribution (S3C16P5-1-3 & 6-8, & S3C17P6-5). Patterns of inflammation in both the dermis, and epidermis provide the pityriasic qualities. In characterizing the patterns in lesions
of lichen striatus as often showing a pityriasis-like quality, recognition is given to epidermal patterns in which lymphoid infiltrates are prominent in the hyperplastic basal unit; in addition, those variants of
pityriasic patterns which might also be characterized as lichenoid, rather than spongiotic (S3C14P3-3), are to be emphasized. In this approach,
the lymphoid
infiltrates are perivenular in the reticular dermis, and tend to be band-like in the papillary dermis. Actually, the infiltrates tend to fill dermal papillae. In some examples, the infiltrates are spotty in regard to the involvement of dermal papilla (S3C14P3-1). In other lesions, the infiltrates fill neighboring papillae without skip areas; in a localized area in which this variant pattern is
represented, all of the papillae within the confines of a lesion are filled. Lymphoid cells migrate in continuity into the hyperplastic basal unit and, generally, do so to the level of the interface between the
basal and the superficial units. The spaces among keratinocytes of the basal unit are widened and mucinous. At this stage, lytic defects in the basal unit are not a prominent feature.
In the next, or established, stage of progression of histologic changes, the patterns take on a lichen planus-like quality; the lymphoid infiltrates produce lysis,
and coagulation of keratinocytes of the basal unit. Focally, these alterations result in the formation of lytic defects at the dermal-epidermal interface. In response to the alterations in the basal layer,
compensatory changes develop in the superficial unit. The superficial unit shows compact hyperkeratosis (orthokeratosis), an expanded granular layer, and hyperplasia and hypertrophy of the keratinocytes; the
hyperplasia perhaps is more of a hypertrophy of individual keratinocytes than a significant increase in the number of cells, or cell layers (S3C14P3-4).
Individual cells become enlarged; they are angulated with their long axis parallel to the surface of the skin; they show increased amounts of acidophilic cytoplasm, and increased prominence of tonofibrils. In
documenting and emphasizing these alterations, we might propose that the functionality of these changes has to do with a defense process; the alterations are accompanied by closure of inter-cellular spaces (closure
is effected by the formation of an inter-cellular lipid membrane derived from cellular organelles, the Odland bodies or keratinosomes); such an alteration would restrict the migration of lymphocytes into the
interstitial spaces of an epidermis whose basal unit has already been seriously damaged; the integrity of the epidermis is in this manner preserved. Activated aggressive T cells would be denied would be denied
access to a superficial unit having such characteristics. On the other side of the altered superficial unit, a compact layer of orthokeratin at the surface would be rather resistent to injuries emanating from the
environment.
In the migrations of lymphocytes and histiocytes into the epidermis in lichenoid reactions of lichen planus-like type, the integrity of the basement membrane is
compromised. Defects in the basement membrane are found in the same regions as are lytic defects in the basal unit of the epidermis. Reduplicated basement membrane material also found in the same areas.
In lichenoid reactions, the insults to the basal unit are cyclic; lytic phases give way to partial repair only to again give way to lysis and coagulation of the
regenerating keratinocytes. In the process, each attempt at repair is accompanied by a newly formed basement membrane. In the lichen planus-like disorders, the lytic defects in the basal unit are more commonly
repaired by inlays of new fibrous tissue, a process that might be characterized asaccretive growth of the papillary dermis in the region of the defects. These phenomena all combine to: 1.) entrap remnants of the
original membrane in the newly formed fibrous tissue; and 2.) form a new fibrous surface along which a new membrane material is formed. The result is a progressive widening of the fibrous matrix of an altered
papillary dermis, and reduplication of the basement membrane. In the process of inlaying new fibrous tissue in the lytic defects in the basal unit of the epidermis, cellular debris, including dead, dyskeratotic
keratinocytes (apoptotic debris included) become entrapped; it would then appear that the dead cells have dropped into the dermis when, in fact, the dermis has invaded the epidermal domain, claimed a portion of it,
and deposited fibrous tissue about epidermal cellular debris.
In the senescent phase, the inflammatory infiltrates, and the associated epidermal insults remit. The epidermis shows the effects of the earlier insults; the rete
patterns are effaced, or they are partially preserved as blunt peg-like extensions. The basal unit is poorly represented; at the dermal-epidermal interface, basement membrane material is reduplicated. Often the
superficial unit will persists in a hypertrophied state for a period during the process of resolution. The papillary dermis will be hyperplastic, widened and fibrotic (S3C14P3-5 & 6, & S3C17P6-4 & 6). Vessels of the papillary
dermis will be dilated, and may be increased in number. The patterns are such that they would correlate with those of lesions whose clinical appearance might be characterized as poikilodermatous; poikiloderma is the
clinical correlate of a senescent lichenoid reaction.
In lichen striatus, the follicles may be affected by the same sequential events which characterize the alterations in the epidermis (S3C16P5-4 & 5, & 9 & 10, & S3C17P6-1 & 4). Primary, established, and senescent patterns may be represented. In the primary stages, the affected follicles will show
increased prominence of the intercellular spaces; in addition, the widened spaces often are mucinous; in some examples, the mucinous changes may progress to the formation of mucin-filled defects; the primary stage,
as expressed in the follicular components, often results a pattern that histologically qualifies as follicular mucinosis (S3C13P2-5 & 6).
Clearly, not all examples of follicular mucinosis qualify as a marker for cutaneous T-cell lymphoma. It should be noted that the lesions of the epidermis also show an increased prominence of mucinous matrix among
keratinocytes of the basal unit (in addition to a component of infiltrating T lymphocytes (S3C14P3-2 a&b).
In the established phase, the basal layer of follicular epithelium undergoes lysis, and the superficial unit shows hypertrophy; the patterns take on the qualities of
lesions of lichen planopilaris with the potential for a similar evolution; the follicle may undergo atrophy.
In the senescent phase, as it affects the follicular components of lichen striatus, the follicular basal unit is poorly developed; the perifollicular sheath is
widened. The superficial follicular unit may also be thin. The lumen of an affected follicle, in a senescent phase, often is dilated, and contains compact keratinized debris.
Sweat glands may also be affected. Not all examples of lichen striatus will show a sweat gland component, but the changes, when
manifested, are characteristic. If the sweat gland components are combined with both epidermal lichenoid patterns and follicular lichenoid patterns, the combinations take on the qualities of diagnostic patterns. The
sweat gland changes belong in the category of primary lichenoid reaction, with the stipulation that the target population of cells are those of the sweat gland apparatus, rather than the epidermal keratinocytes. The
patterns, when well-developed, qualify as a lymphoepitheliosis; lymphocytes and histiocytes migrate into sweat glands, and sweat duct epithelium. They provoke an immuno-stimulatory response; the glandular epithelium
becomes stratified. Lumina may be obliterated, or may be duplicated. The epithelial cells show crowding of nuclei, and variations in nuclear size, orientation, and staining; often, the nuclei are hyperchromatic with
uniformly dense chromatin (S3C13P2-7 & 8, & S3C17P6-2 & 3).
The changes in sweat gland epithelium are comparable to those of affected epithelial structures in Sjogren’s syndrome.
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