1375 Cutaneous adnexal cysts revisited: A novel classification based on morphological and molecular features

Abstract

Cutaneous cysts have been classified by many dermatopathologists in different ways. We propose a novel classification of cutaneous adnexal cysts according to their origin in the folliculo-sebaceous unit and sweat glands. By examining the lining of the cystic structure, its origin can be easily identified. Epidermal cysts have an epithelial wall containing a granular layer and the keratinization through the lumen is lamellar, indicating an infundibular origin Tricholemmal cysts have an undulating epithelial wall with no granular layer and a compact keratinization, showing an isthmic origin. In steatocystoma, dermoid cyst and folliculosebaceous hamartoma, the epithelial lining shows a crenulated appearance which is seen in the sebaceous duct. Hidrocystoma shows the characteristic cuboidal epithelial lining of sweat glands with decapitation secretion in its apocrine forms. Metabolizing acquired dioxin-induced skin hamartoma (MADISH) is a cystic lesion with no sebaceous glands showing lamellar keratinization. Cytokeratin expression pattern is also consistent with the origin of the cysts. For example, the sebaceous duct keratin CK17 is expressed in steatocystoma, dermoid cyst and folliculo-sebaceous hamartoma. Epidermal cyst expresses CK10 as tricholemmal cyst, which also expresses CK17. Eccrine and apocrine hidrocystoma shows a broader expression of cytokeratins (CK1, CK5, CK7, CK8, CK10, CK14, CK18, CK19). CK16 is only expressed by folliculo-sebaceous hamartoma. In MADISH, as in folliculo-sebaceous hamartoma, CK15 is expressed. Follicular stem cell marker Lrig1 is expressed at the isthmic region of human folliculosebaceous and the activation of β-catenin in Lrig1-positive stem cells gives rise cystic proliferations reminiscent of MADISH in mice. Recently, cutaneous keratocyst and steatocystoma unified as sebaceous duct cyst has been proposed to be a hamartoma resembling the sebaceous duct. The classification proposed here aims to simplify the complexity of cutaneous adnexal cysts and to have a better understanding of the origin of the cystic lesions of the skin.