Cross sections at the level of the bulb would show a central island of Þbrous tissue corresponding to the dermal papilla, which makes it very easy to identify. If the dermal papilla is not seen due to tangential sectioning, one can identify it by the presence of the hair matrix cells, and of course the surrounding perifollicular Þbrous sheath. At the level of the suprabulbar zone, one can identify the presence of inner and outer root sheaths and decreased or absent matrix cells and the surrounding perifollicular Þbrous tissue sheath (Fig. 13.15). At the isthmus level, one can see the hair shaft surrounded by the trichilemmal, abruptly keratinized cuticle and the inner root sheath, the outer root sheath, and the perifollicular Þbrous tissue sheath (Fig. 13.16). At the infundibulum level, the epidermal keratinization with granular layer can be seen. Finally, follicular germs and papillae seen in embryonic development and the very early anagen stage can be recapitualized by BCC and some other follicular adnexal neoplasms such as trichoblastomas.

13.3.1.5 The Bulge

The bulge is the follicular protrusion at the attachment of the arrector pili muscle (Fig. 13.17). Horizontal sectioning of the bulge shows rounded or oval basaloid structure with a blend of suprabulbar and isthmic cellular features with surrounding perifollicular Þbrous tissue sheath (Fig. 13.18). Again, the latter differentiates the bulge from BCC, which is usually surrounded by immature mucinous stroma. The latter is not usually seen in inÞltrating and morpheaform BCC, which are identiÞed by the unique strands/ribbons of basaloid cells with characteristic stroma, rather than groups of basaloid cells with peripheral palisading (Fig. 13.19a, b).

The ÒblebÓ of the isthmus [18] is probably a misshapen cystic bulge lined by isthmic epithelium with corneocytes arrayed in concentric, lamellar fashion. They are surrounded by a prominent basement membrane. Bulges and blebs can be encountered in frozen sections and should be differentiated from BCC.

13.3.1.6 The Mantle and Sebaceous Glands

Mantles are symmetrically distributed cords of basaloid cells extending from the upper portion of the hair follicles, (lower infundibulum) and they represent the origin of, or Òthe resting stageÓ of mature sebaceous gland lobules (Fig. 13.20). They should be differentiated from BCC by the variable degree of early sebaceous differentiation and their characteristic Òskirt-likeÓ appearance with elongated ribbons of basaloid cells (Fig. 13.21).

Sebaceous gland lobules pour their holocrine secretions in the infundibulum via the sebaceous ducts. The sebaceous duct is a crenulated channel connecting the sebaceous gland lobules with the infundibulum and consequently is lined by thin, stratiÞed keratinocytic epithelium. Its lumen contains Þne particles representing the holocrine sebaceous secretions.

Horizontal sectioning of sebaceous ducts should be differentiated from epithelial pearls of SCC by lacking: cytologic atypia and atypical mitoses and by the presence of a granular layer and crenulated lining. Sebaceous gland lobules are surrounded by a layer or two of immature basaloid germinative cells. When the latter is cut tangentially, they may produce a group of basaloid cells that mimic BCC. The presence of foam cells can help resolve that mimicry. BCC with sebaceous differentiation is a debatable entity, however, in the authorÕs experience, it does exist [19, 20]. Looking at the original biopsy is priceless in this situation, so that the Mohs surgeon would be familiar with the histologic and cytologic nature of the original neoplasm. There is an increasing number of MMS performed on sebaceous carcinoma (SC). The presence of doubtful basaloid cells with mantle-like

Fig. 13.19 (a) Intermediate-power view of a permanent section showing a hair follicle at the isthmic level above the bulge with some telogen features showing crenulated, irregular border with surrounding prominent perifollicular Þbrous tissue

sheeth. Note the arrector pili muscle at the left. (b) Intermediate power view of a Mohs section showing residual BCC. Note the basaloid tumor formation is surrounded by immature mucinous stroma

changes can pose a problem in interpreting Mohs slides for sebaceous carcinoma. In this case, the Mohs surgeon should consider these groups as a positive part of sebaceous carcinoma and perform an extra stage. These authors classiÞed SC into: (1) SC in situ,

(2) SC, inÞltrating, low-grade with or without Pagetoid spread, (3) SC, inÞltrating, high-grade, with or without Pagetoid spread, and (4) SC with extra

ocular and extra cutaneous involvement, including metastases. The dermatopathology report should mention the grade and the presence or absence of Pagetoid spread [20]. The lipid stain oil red O is not routinely used in the Mohs laboratory. It can be very helpful though, especially in cases of sebaceous carcinoma of the eyelid. Goblet cells, normally present at the eyelid, are oil red O-negative [21].