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Histologic Definitions (TIER 2)
Epidermal Units:
Conceptually, the epidermis consists of superficial and basal units (3). Between these two units, the morphologic boundary is not sharply defined. Among other functions, the regeneration of cells and the preservation
of a basement membrane are functions of the basal unit. Hyperplasia of the basal unit is manifested by elongated rete ridges (psoriasiform hyperplasia) and a watery distention of the intercellular spaces. Processes
which are psoriasiform are usually characterized by increased cytoplasmic basophilia of the cytoplasm of keratinocytes in the affected basal unit (S3C13P2-2).
The superficial epidermal unit
is concerned with the formation of an impervious barrier between the skin and the environment. In part, the product of keratinocytes of the superficial unit is the keratin layer. Additionally, keratinocytes of the superficial unit progressively obliterate the intercellular spaces. Hyperplasia of the superficial unit is best illustrated in a lesion of lichen planus; it is characterized by hypertrophy (enlargement) of keratinocytes, increased cytoplasmic acidophilia, and prominent cytoplasmic tonofilaments. The long axes of the hypertrophied cells parallel the surface of the skin. The keratinized product is orhtokeratotic and densely compacted. This epidermal pattern qualifies as established lichenoid hyperplasia. Lichenoid hyperplasia and psoriasiform hyperplasia (i.e., expansion of the basal unit, including the rete ridges) are not mutually exclusive phenomena; they are commonly combined in a lesion such as lichen simplex chronicus. Hyperplasia of both units is evident in the pictorials related to pityriasis rubra pilaris (see SECTION 2). In lichenoid hyperplasia of lichen planus-like type, the epidermis beyond the damaged basal unit is almost exclusively dedicated to terminal differentiation. This, in part, dedication is, in large part, a response to the extensive damage to the basal unit. A lesion showing this combination of alterations has compensated for damage to the basal unit by compensatory hyperplasia of the superficial unit. The lytic process is thus denied access to the superficial unit; the integrity of the impervious barrier is not only preserved, it is augmented.
Epidermal patterns in a primary lichenoid reaction:
In some lichenoid reactions, or as a phase in the evolution of some lichenoid reactions, the epidermal pattern is psoriasiform; cytolytic defects in the epidermis are minimal. The defects that are encountered in
spongiotic and psoriasiform processes are the result of an expansion in the volume of the intercellular matrix, rather than evidence of significant cytolysis.
In lichenoid reactions, lymphocytes closely intermingle with keratinocytes (S3C11VA1-1-3). This intermingling is particularly prominent in primary
lichenoid reactions. It is the typical (common) pattern in lesions of lichen striatus; it occasionally encountered as a feature in the early stages of lichen planus, or might be encountered at the edge of a lesion
of lichen planus. In this primary (initial) hyperplasia, an hyperplastic basal unit has elongated rete ridges and widened, mucoid, inter-cellular avenues. The psoriasiform epidermal pattern, commonly encountered
in lesions of pityriasis lichenoides et varioliformis acuta, is a variation of a primary lichenoid reaction (3). It often includes a rather uniform parakeratotic cap; in this disorder, the patterns of a psoriasiform process are combined with those of a lichenoid process to produce combined pattern; in the resulting combination, the patterns are pityriasic.
Epidermal patterns in an established lichenoid reaction:
This type is characterized by well developed lytic defects affecting the basal layer at the dermal-epidermal interface, or by defects within the basal unit. The defects contain lymphocytes, histiocytes, dendritic
cells, and coagulated keratinocytes. Bordering well developed, lytic defects, the surviving superficial epidermal unit usually is hyperplastic. The keratinocytes in the hyperplastic superficial unit undergo
hypertrophy. Their dimensions are increased; their cytoplasm is eosinophilic, and their tonofilaments are prominent. This pattern of epidermal hyperplasia characterizes most of the established lichenoid reactions;
it histologically defines a lesion of lichen planus, but does not establish lichen planus as something more than a distinctive reaction pattern.
Epidermal patterns in a senescent or resolving lichenoid reaction:
If preceded by an established pattern, senescence is characterized by an inlay of fibrous tissue in lytic defects in the basal epidermal unit. The lichenoid reaction, during its life, may be repeated several times in
the same general area. By repetition of the lichenoid reaction, basement membrane material is reduplicated, first along the surface of the newly inlaid fibrous tissue (17). In time, much of the reduplicated basement
membrane material comes to lie within the newly inlaid fibrous tissue, as accretive fibrosis progresses in increments. A portion of epidermal domain is lost in the process of inlaying the fibrous tissue in the
defects of the epidermal domain.
As a consequence of the sequential, accretive growth of fibrous tissue, the fibrous component in what appears to be the domain of the papillary dermis is progressively widened. Effete, coagulated keratinocytes (“apoptotic cells”) within the clefts at the dermal-epidermal interface become incorporated into the newly formed fibrous tissue (S3C11VA1-1-3). Immunoglobulins deposit upon the surface of the trapped, dead keratinocytes to produce the so-called colloid or Civatte bodies.
Senescence is also accompanied by a progressive reduction in the density of the inflammatory infiltrates in the papillary dermis, especially at the dermal-epidermal interface. In the late stages of a senescent
lichenoid reaction, the papillary dermis is widened and fibrotic. Rete ridges in the overlying epidermis are effaced, or are irregular in outline (saw-tooth pattern). Melanophages and colloid bodies remain in the
fibrotic papillary dermis as markers for the earlier interplay between aggressive lymphocytes and their target cells (S3C12P1-5). The senescent
histologic pattern relates to clinical patterns which are poikilodermatous.
It is implied in the concept of primary, established, and senescent lichenoid reactions that one phase follows the other in sequence (S3C11VA1-2).
There is histologic evidence that the lichenoid reaction may evolve into the senescent phase without a significant, intervening established phase.
DESCRIPTION OF DISEASES
(“True Lichens”)
LICHEN PLANUS
Lichen planus is the prototype of lichenoid diseases of the skin, particularly the cell-rich type. Clinically, its distinctive papules are flat topped, angular, scaly, and violaceous. They may spread, or coalesce, to
form plaques. The papules vary in size and distribution. They rarely are solitary or linear. Some linear lesions may be the result of the isomorphic phenomenon (18). In some examples, the lesions are vesicular or,
rarely, bullous. In others, they are hypertrophic, atrophic, or follicular. Flexural areas and oral mucosa are common sites.
Histopathology:
Lichen planus is characterized histologically by marked compensatory hyperplasia of the superficial epidermal unit, prominent clefts in the epidermal domain at the dermal-epidermal junction, and the accretion of fibrous inlays in the epithelial defects (S3C12P1-1-6). The epidermal changes typically show compact hyperkeratosis with a prominent granular layer. Angulated, hypertrophic keratinocytes
are oriented with their long axis parallel to the skin surface. Focal or diffuse lysis of keratinocytes produces attrition of the basal epidermal unit with the formation of defects. Degenerating keratinocytes,
histiocytes, dendritic cells, and lymphoid cells intermingle in the areas of lysis. In the dermis, a band-like infiltrate of lymphocytes fills a widened papillary dermis and hugs the epidermis. Infiltrates of
inflammatory cells extend from the dermal infiltrates into the basal unit of the epidermis. Lysis of keratinocytes results in attrition of the epithelial domain of the rete ridges. Each eroded rete ridge
acquires a narrow, pointed outline (saw-toothing).
With time, fibrous tissue is inlaid in the defects of the epidermal domain. Necrotic keratinocytes in the defects are entrapped in the fibrous inlays to become Civatte bodies (19). Irregular fragments of basement
membrane are also entrapped in the inlays. They result primarily from incomplete epithelial regeneration in an area of ongoing lysis of cells and repair; the resulting patterns qualify as accretive fibrosis.
Hair follicles may be involved by the lytic process. The result may be involution and eventual atrophy of the inflamed follicles. The end-result is a scarring alopecia (Graham-Little syndrome).
The cellular kinetics of lichen planus are not clearly defined. Proliferating keratinocytes have been characterized as moving from lateral positions into the epidermis over the lytic clefts (20).
Clinicopathologic Correlations: Several, distinctive and recognizable, clinical variants of lichen planus also have distinctive, pathologic correlates.
Lichen planopilaris is a variant of lichen planus with variable but, occasionally, prominent follicular changes. The inflammatory infiltrates in the perifollicular connective tissue sheaths affect the follicular
epithelium in a manner similar to that affecting the epidermis of non-follicular lichen planus. The keratinized product in the lumen of the affected follicle usually is dense orthokeratin. The lumen of the follicle
is dilated (the follicle is plugged), and the granular layer of the infundibulum is accentuated. The hair follicle reverts to a catagen phase from which it does not recover. The perifollicular inflammation and
fibrosis produces progressive attrition of the follicular epithelium. The follicle eventually atrophies, leaving, as a marker, a linear band of fibrous tissue, the remnant of the perifollicular connective tissue
sheath.
In bullous and vesicular lichen planus, the clefts, by confluence, produce detachment of the damaged epidermis. The potential cleft, or the space through which cleavage occurs has been referred to as the "Max
Joseph" space. The vesicle or bulla may be difficult to distinguish from other vesicular diseases such as bullous pemphigoid. The papillary dermis beneath the vesicle may show a typical band of inflammatory
cells but, often, is relatively free of inflammatory cells . At the margin of the vesicle, the infiltrates in the papillary dermis often are characteristic; they tend to be band-like and to hug the epidermis. The
angle formed by the roof and the floor of the vesicle is usually acute. An important feature in the histologic interpretation is found in the intact roof of the vesicle, where the epidermis shows compact
hyperkeratosis, hypergranulosis, and hypertrophy of individual keratinocytes (i.e., persistent markers for the established reaction which preceded the separation). Clusters of necrotic keratinocytes will be found
along the floor of the sub-epidermal defect.
Hypertrophic lichen planus is a variant that is excessively elevated and crusted. It is more common in certain anatomic sites, such as the lower extremities.The epidermis is hyperplastic with irregularly elongated
rete ridges, sometimes associated with pseudoepitheliomatous hyperplasia. The papillary dermis is widened and fibrotic. It often contains aggregates of torouous, thick walled vessels of the type seen in stasis
dermatitis, or angiodermatitis. Often, the inflammatory infiltrates are spotty rather than diffuse. In some areas, the infiltrations may hug the elongated rete ridges or follicular extremities. The characteristic
changes of lichen planus tend to be focal at the extremities of these epithelial structures. Eosinophils and plasma cells are occasional features of the infiltrates in hypertrophic lichen planus.
SEE S3C11VA1-1-8: fourth item down in cluster of beige navigation bars in MASTER BORDER to left or CLICK.
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