The skin is a complex microenvironment. The normal structures of the epidermis, dermis, and cutaneous adnexa are morphologically and functionally complicated, and the histologic entities that occur in this tissue compartment are also numerous. Furthermore, cutaneous lesions may also be a part of systemic proliferations, or they may have exact morphologic counterparts in other sites. Principal examples of such disorders are hematolymphoid diseases and mesenchymal tumors. Because the diagnostic and immunohistochemical (IHC) issues pertaining to those conditions are covered in detail elsewhere in this text, comments on them are relatively limited in scope, mainly centering on lesions peculiar to the skin. Finally, as with melanocytic lesions of the skin (see Chapter 7 ), this chapter primarily focuses on antigenic profiles that can be obtained with routinely processed, formalin-fixed tissues and commercially available reagents.
Epithelial Tumors of the Skin
The many forms of differentiated epidermal and adnexal epithelia in the skin result in a potentially confusing nosologic categorization. Despite that diversity, most lesions can be grouped under five basic patterns of differentiation: (1) epidermal, (2) sweat glandular (eccrine and apocrine), (3) sebaceous, (4) follicular, and (5) endocrine. However, not all lesions can be easily classified in these five groups. As an example, a recently described entity, the clear-cell carcinoma with comedonecrosis, is a neoplasm characterized by an aggressive course (local recurrence or distant metastasis) that has features that resemble squamous cell carcinoma (SCC), sebaceous carcinoma, and eccrine carcinoma. Reflecting this morphologic diversity, this lesion expresses IHC features of those lesions, including cytokeratins 5 and 6 (CK5/6), the same as in SCC; epithelial membrane antigen (EMA) and CK17 in the clear cells, as in follicular structures or sebaceous carcinoma; and focal carcinoembryonic antigen (CEA) expression, as in eccrine carcinoma. Cases like this one are best classified as “adnexal,” and then the types of differentiation that they contain may be indicated.
Tumors with differentiation toward epidermal cells are the most common epithelial neoplasms of the skin. The most important of these are SCC and basal cell carcinoma (BCC). Furthermore, BCC is the most common malignancy in humans.
SCC is usually composed of polygonal cells with nuclear atypia and diverse degrees of keratinization, either in the epidermis (in situ) or invasive, and thus it is usually easy to diagnose. However, several microscopic subtypes of SCC may occasionally mimic both glandular and mesenchymal neoplasms in skin: these include adenoid (acantholytic), pleomorphic, small cell, and spindle cell forms. Because of the latter variants, an understanding of the immunohistologic attributes of squamous carcinoma is important to the differential diagnosis of cutaneous tumors in general. Fortunately, all forms of SCC show similar antigenic profiles.
SCC contains an abundance of cytokeratin intermediate filament proteins that range from 40 to 68 kD in molecular weight. The more well-differentiated cells synthesize high-molecular-weight (HMW) cytokeratin; in contrast, poorly differentiated tumors usually only express low-molecular-weight (LMW) keratin peptides. The concomitant expression of both cytokeratin and vimentin characterizes spindle cell, pleomorphic, and some acantholytic squamous carcinomas and metastatic renal cell carcinoma (RCC; Fig. 13.1 ).
Sarcomatoid carcinomas may sometimes express keratin only focally, and the best approach to their recognition is to use a broadly reactive mixture of monoclonal antikeratin as a screening reagent. CK5/6 could be included as a specific target therein, because it is more selective for squamous differentiation.
Another antigen displayed in SCC is EMA. Although variable amounts of this glycoprotein are encountered in most examples of SCC, diffuse expression of EMA is usually seen only in poorly differentiated lesions.
CEA staining has been described in SCC, but in our experience, diffuse reactivity is rare. Also, p63 protein is a nuclear determinant associated with both myoepithelial and epithelial differentiation, and it is seen in squamous, basal cell, and appendageal cutaneous carcinomas ( Fig. 13.2 ). It is a member of the p53 family, which includes the p53, p40, p63, and p73 polypeptides.
Squamous carcinomas of the skin lack S100 protein, chromogranin, synaptophysin, CD99, CD15, and CD57. Further, reactivity with human melanoma black 45 (HMB-45) or anti–melan-A is consistently absent, as are desmin and muscle-specific isoforms of actin in most spindle cell forms of these neoplasms.
There are no well-characterized proteins that exclusively define malignant epidermal differentiation. Molecules associated with epidermal keratinization, such as filaggrin and involucrin, are preferentially expressed in SCC but can also be seen in keratoacanthomas and a variety of benign keratinocytic proliferations. Although they may help distinguish SCC from BCC, those markers cannot separate SCC from adnexal tumors of the skin, particularly those of pilar differentiation.
Adenoid (acantholytic), pleomorphic, small cell, and spindle cell forms are variants of SCC that may mimic other lesions.
SCC is labeled with AE1/AE3, 34βE12, and CK5/6.
Poorly differentiated SCC is labeled with CAM5.2, AE1/AE3, p63, and p40.
In general, SCC is EMA, p63, and CD44 positive and BerEp4 negative.
In general, BCC is EMA negative and BerEP4 positive.
BCC, as with SCC, has several distinctive variants, all of which invoke dissimilar differential diagnoses. Among the better-recognized subtypes of BCC are the morpheaform, adenoid, clear cell, hamartomatous (also referred to as infundibulocystic ), and metatypical forms (i.e., with extensive squamous differentiation). Of all the subtypes, the ones associated with high rate of recurrence are those associated with an irregular, infiltrative border (sclerodermoid, morpheaform, infiltrative); small nest size (micronodular); extensive squamous differentiation; and eccrine differentiation.
In general, BCC lacks complex patterns of antigenic expression. As for SCC, this tumor displays reactivity for CK polypeptides; however, the molecular weight of these intermediate filaments are typically less than 50 kD. In contrast to SCC, EMA is not observed in any variant of pure BCC ( Fig. 13.3 ).
Occasionally, BCC (and SCC) can contain “passenger” melanocytes, and such cells will express melan-A and HMB-45 antigen. Similarly, a few lesions exhibit staining for endocrine-associated peptides including CD56, synaptophysin, and chromogranin A (CG). The apparent ability of BCC to express such specialized determinants has been used to support the premise that the cells of that lesion recapitulate the properties of epidermal “stem” cells. Nevertheless, the absence of vimentin, CEA, S100 protein, CD57, and CD15 suggests that there may be flaws in this “stem cell hypothesis” as applied to BCC. Indeed, BCC with additional patterns of differentiation such as “eccrine epithelioma,” “apocrine epithelioma,” and so-called basosebaceous epithelioma may show positivity for EMA, CEA, or CD15 in histologically divergent areas. Also interesting is the reduced smooth muscle actin (SMA) expression in some BCCs associated with an aggressive behavior.
Another useful glycoprotein marker present in most BCCs is recognized by the antibody BerEP4, directed at two epitopes, 34 kD and 39 kD, on human epithelial cells ( Fig. 13.4 ). In the skin, BerEP4 labels not only BCCs but also the cells of Paget disease, Merkel cell carcinoma (MCC), and other selected appendageal neoplasms. It is probably most useful in separating BCC with squamous differentiation from basaloid SCC, which is BerEP4 negative. Conversely, squamous tumors bind to the L-Fucose–specific lectin, Ulex europaeus I, which is usually negative in BCC. The distinction between basaloid SCC and BCC is diagnostically crucial in certain anatomic locations, such as the anal and perianal skin, because the former has a much worse prognosis.
Sweat Duct Tumors
The eccrine and apocrine glandular adnexa comprise the sudoriferous structures of skin. The neoplasms of these structures are histologically diverse but share certain immunohistologic features. All of these tumors demonstrate CK reactivity and the potential for expression of CEA, tumor-associated glycoprotein 72 (TAG-72, also known as CA72.4; Fig. 13.5 ), EMA, CD15, and p63. The last three of these substances are seen in varying proportions in both adenomas and carcinomas. EMA is more often observed in malignant sudoriferous neoplasms than in benign ones; indeed, spiradenoma and cylindroma typically lack this determinant. In contrast, CEA and CD15 are detected in roughly 70% to 80% of all eccrine and apocrine lesions regardless of their biologic potentials; TAG-72 appears to show a predilection for apocrine lesions and is usually not seen in eccrine tumors. This profile proves useful in the separation of sudoriferous neoplasms from other cutaneous glandular and epidermal neoplasms. It has also been instrumental in verifying the presence of sweat glandular differentiation in occasional examples of lymphoepithelioma-like carcinoma of the skin, a poorly differentiated tumor that shows histologic similarities to nasopharyngeal lymphoepithelioma.