Integration: Unifying Concept
Nevus sebaceus is the quintessential example of an organoid nevus, i.e., a hamartoma that, in the skin, consists of various elements indigenous to the organ. The components of nevus sebaceus are epidermal in the form of gentle mammillation, prominent papillation, or even digitations. They are sebaceous in the form of clusters of pyriform lobules of sebocytes, and follicles as primitive germs and a papilla situated just beneath surface epithelium (and absence of terminal follicles within the substance of the hamartoma itself). The apocrine component takes the form of numerous tubular structures that reside within the dermis and sometimes the subcutaneous fat.
A variety of cysts, cystic hamartomas, and benign and malignant neoplasms may develop within a nevus sebaceus. Even in a newborn, it is not uncommon for a benign neoplasm of apocrine character, namely, syringocystadenoma papilliferum, to be present in association with nevus sebaceus. An apocrine cyst, called apocrine hidrocystoma, is often present in a nevus sebaceus, as are cystic hamartomas like steatocystoma. The commonest neoplasm that occurs in nevus sebaceus is not basal-cell carcinoma, as has been asserted for almost three-quarters of a century, but trichoblastoma, the benign analogue of basal-cell carcinoma. Among the malignant neoplasms that sometimes appear in nevus sebaceus, in addition to basal-cell carcinoma, are sebaceous carcinoma and apocrine carcinoma.