Merkel cell carcinoma (MCC) is usually found within the dermis but may extend into the subcutaneous tissue. The combination of vesicular nuclei with small nucleoli, abundant mitoses, and apoptosis is characteristic of this tumor when evaluated within the context of certain architectural features. Histologically, MCC has been classified into three distinct subtypes:[1-3]

Trabecular

• Cells are arranged in distinctly organoid clusters and trabeculae with occasional ribbons.
• Individual cells are round to polygonal in shape and are compactly arranged.
• Cell cytoplasm is comparatively abundant and often well defined.
• Mitoses are few to moderate in number.
• The tumor is usually found adjacent to adnexal structures, particularly hair follicles.
• Trabecular is the least frequent histologic pattern identified.

Intermediate

• The tumor exhibits a solid and diffuse growth pattern.
• Cells are less compactly arranged, and the cytoplasm is less abundant than in the trabecular type.
• Mitoses and focal areas of necrosis are frequently seen.
• Intermediate tumors arise adjacent to adnexa but may also invade the epidermis.
• Intermediate tumors may behave in a clinically more aggressive manner than trabecular tumors.
• Intermediate is the most frequent histologic subtype identified.

Small cell

• The tumor closely mimics small cell tumors of other sites.
• The tumors arise in the dermis and appear as solid sheets and clusters of cells.
• Areas of necrosis and crushing artifact are frequently seen.
• The clinical behavior of small cell tumors appears to be as aggressive as that of intermediate tumors.

References

1. Haag ML, Glass LF, Fenske NA: Merkel cell carcinoma. Diagnosis and treatment. Dermatol Surg 21 (8): 669-83, 1995.  [PUBMED Abstract]
   
   
2. Ratner D, Nelson BR, Brown MD, et al.: Merkel cell carcinoma. J Am Acad Dermatol 29 (2 Pt 1): 143-56, 1993.  [PUBMED Abstract]
   
   
3. Gould VE, Moll R, Moll I, et al.: Neuroendocrine (Merkel) cells of the skin: hyperplasias, dysplasias, and neoplasms. Lab Invest 52 (4): 334-53, 1985.  [PUBMED Abstract]
   
   

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