Background
Sebaceous glands are oil-producing glands present in the dermis of mammalian skin. Sebaceous glands are usually attached to hair follicles and are part of a complex skin-adnexal unit known as the folliculoapocrine (sweat) apparatus. The glands are present over the entire body, with the exception of the palms of the hands and the soles of the feet; they are most abundant on the scalp and central face.
Skin adnexal tumors with sebaceous differentiation are uncommon, difficult to classify, and may be controversial. The main controversy concerns the microscopic features, which vary from well-to-poorly differentiated and sometimes undifferentiated varieties. A spectrum of morphologic features can be encountered within the same neoplasm. Most reports include few cases. Large series of cases for comparison and follow-up have not been published.
Because of the intimate association of sebaceous glands with other adnexal structures in the folliculosebaceous-apocrine unit, many sebaceous neoplasms show complex histopathologic features with mixed sebaceous, pilar (hair), and apocrine (sweat) gland differentiations. Therefore, the term sebaceous neoplasm is necessary to include these complex skin adnexal tumors with varying degrees of sebaceous differentiation. The term sebaceous neoplasm includes benign and malignant tumors with different degrees of sebaceous differentiation. The following terms are recognized within this category: sebaceous adenoma, sebaceous epithelioma (sebaceoma), sebaceous carcinoma, basal cell carcinoma with sebaceous differentiation, sebocrine adenoma, and sebomatricoma.
Skin lesions containing benign sebaceous gland proliferation (eg, sebaceous hyperplasia), congenital hamartomas (eg, nevus sebaceus), and lesions with ectopic sebaceous structures (eg, Fordyce spot, Montgomery tubercles) are generally not considered to be true sebaceous neoplasms.[1]
When patients with numerous SAs and/or other neoplasms with sebaceous differentiation have an associated internal malignancy, the clinical condition is known as Muir-Torre syndrome.
Pathophysiology
The sebaceous gland is a secretory structure consisting of sebaceous lobules and ducts. Although the sebaceous gland is a glandular adnexal structure, it is topographically and ontogenetically related to the hair sheath. Therefore, antigenic similarities exist between the isthmus of the outer hair sheath and the germinative basaloid cells of the sebaceous gland. Sebaceous glands are most abundant in the skin of the head and neck regions, particularly in the central face, but they are also present throughout the hair-bearing areas of the body and the mucocutaneous junction, including the labium minus. The eyelids have modified sebaceous glands (ie, Zeis glands of cilia associated with eyelashes, meibomian glands within the tarsal plates of the upper and lower eyelids).
Using transmission electron microscopy, a small number of melanocytes can be identified that synthesize melanin-containing organelles (melanin granules) in the ducts and acini of human sebaceous glands.[2]
Ectopic sebaceous glands, known as the Fordyce condition, commonly occur on the vermilion border of the lips and on the buccal mucosa of adults; these structures are not considered sebaceous neoplasias.
As one of the skin's adnexal structures, the sebaceous gland is intimately associated with hair (pilar) and arrector pili muscle. Because of the intimate association of sebaceous glands with hair and apocrine ducts, many sebaceous neoplasms show complex histopathologic features with various elements of pilar and sweat gland differentiation.
Pilar and sebaceous neoplasms share a common expression for both high and low molecular weight cytokeratin that is not seen in most eccrine or apocrine tumors, pointing to similar cellular differentiation patterns of hair and sebaceous structures. Furthermore, the architectural features of sebaceous tumors resemble those of pilar neoplasms, and not those of sweat gland tumors. Many antigens associated with sweat gland neoplasms, including S-100 protein, CA72.4, gross cystic disease fluid protein 15 (GCDFP-15), and carcinoembryonic antigen (CEA), are absent in sebaceous tumors.[3]
Mature sebocytes express antigenicity for epithelial membrane antigen (EMA) and related substances. Some studies have reported selective labeling of sebocytes and their neoplasms for nuclear androgen receptor protein (ARP), but with some contradictory results. Reactivity for immunoglobulin A, lipase, milk fat globule–associated (ovarian carcinoma–associated) sebaceous antigen OV-2, and OKM5 (CD36) antigen may be selective for sebaceous lesions.[4, 5, 6]
All sebaceous glands are derived from the embryonal stratum germinativum (epidermal basal layer), which gives rise to the epidermis and epidermal appendages.
During the 13th to 15th weeks of intrauterine life, part of the primary epithelial germ develops into pilosebaceous pegs; later, their outgrowths form hair and sebaceous glands. Sebaceous glands are found next to hair follicles and arrector pili muscles. Arising in the 15th to 20th weeks of gestation, the most superior bud on the primitive pilosebaceous germ develops into the apocrine gland, which is a specialized sweat structure. These integral structures are known as folliculosebaceous-apocrine units. Development of sebaceous glands begins in the scalp and the face at the beginning of the second trimester (80-mm stage) and progresses in a cephalo-caudal direction.
Any sebaceous gland in the body has the potential to develop into sebaceous neoplasms. These tumors are cutaneous adnexal tumors that show varying degrees of sebaceous differentiation.
Sebaceous adenoma is defined as a benign epithelial neoplasm composed of sebaceous gland–like structures or tumors with well-recognized sebaceous differentiation by microscopic examination.
Epidemiology
Frequency
United States
The exact incidence of sebaceous neoplasms is unknown. Prior to 1967, sebaceous neoplasms were regarded as rare solitary tumors, with only a few published reports of them. However, in 1967, Muir and his colleagues[7] first described the association of multiple sebaceous tumors and carcinoma of the larynx and the intestine. Torre[8] noted that a patient with multiple skin neoplasms showing sebaceous differentiation later developed carcinoma of the ampulla of Vater in the duodenum and in the colon. Multiple sebaceous neoplasms in a patient may be associated with the clinical Muir-Torre syndrome.
International
No reported increased incidence of sebaceous adenoma has occurred in any particular geographical location.
Mortality/Morbidity
Sebaceous adenomas are benign cutaneous adnexal neoplasms that do not have a potential for aggressive growth or metastasis that causes death. Local recurrences are occasionally encountered following incomplete removal of the tumor. When multiple sebaceous adenomas are associated with Muir-Torre syndrome, visceral carcinomas, including adenocarcinomas of the colon, stomach, duodenum, hematologic system, genitourinary tract, endometrium, and larynx (in decreasing order of frequency) may also be present. The most significant feature of Muir-Torre syndrome is the favorable prognosis of each of the associated carcinomas. However, some of these malignancies have metastatic potential; deaths due to internal malignancies have been reported.
Race
No reported predisposition is reported for any particular race.
Sex
Sebaceous adenomas affect men and women equally.
Age
Sebaceous adenomas frequently appear on the face or scalp of middle-aged and older individuals, after age 50 years. The mean age at onset is 60 years.
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