A ermoid cyst is a cystic teratoma that contains developmentally mature skin complete with hair follicles and sweat glands, sometimes luxuriant clumps of long hair, and often pockets of sebum, blood, fat, bone, nails, teeth, eyes, cartilage, and thyroid tissue. Because it contains mature tissue, a dermoid cyst is almost almost always benign. The rare malignant dermoid cyst usually develops squamous cell carcinoma in adults; in babies and children it usually develops endodermal sinus tumor.