Dermoid cysts are cysts that contain skin elements including squamous epithelium and dermal appendages (adnexa), such as sebaceous and sweat glands and hair.
They are discussed separately according to anatomic location:
Pathology
Dermoid cysts are ectodermal inclusion cysts containing skin and skin appendages; sebaceous glands, hair follicles, and sweat glands.
In contrast, epidermoid cysts, also ectodermal in origin, are lined solely by squamous epithelium 1. Teratomas, instead, contain tissues derived by at least two of the embryonic layers.
Sebaceous glands produce an oily liquid called sebum that helps protect the skin. Sebum is composed of various compounds, including triglycerides, cholesterol, squalene, free fatty acids, and wax ester 5. This liquid component appears "fatty" on imaging but should not be mistaken for adipose tissue, which is of mesodermal origin and therefore not found in dermoids.
Terminology
Although pathologically dermoid cysts have a clear and unambiguous definition, the term is often used loosely and variably depending on the body part and clinical specialty.
First, the term may refer to multiple histologies. For example, in the floor of the mouth, dermoid cysts may include all histologic types of dysontogenetic (developmental) cysts: epidermoid cysts, (true) dermoid cysts, and teratoid cysts 2. Since these share a similar embryonic development 1, they are often discussed together using "dermoid" or "dermoid/epidermoid" as umbrella terms (with teratoid cysts receiving little attention probably due to its rarity).
Second, the term may refer to different etiologies. For example, dermoid cyst has been used interchangeably with mature (benign) cystic teratomas, even though the latter connotes a neoplasm rather than a congenital sequestration-type lesion 3. This usage most commonly pertains to teratomas (germ cell neoplasms) in the ovaries and testes, but also can rarely apply in the head and neck 4.
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