| Abstract|| |
A hybrid cyst is a rare condition which includes any type of cyst arising from the various parts of the pilosebaceous unit. We report a case of hybrid cyst with infundibular, isthmic, and pilomatrical differentiation. A 12-year-old boy presented with a mass on the right cheek. The physical examination revealed an oval-shaped, dark-gray protruding mass with dimensions of 0.9 cm × 0.6 cm that was non-tender to palpation. The mass was totally excised. The diagnosis of a hybrid cyst was confirmed by histological evaluation. It was a well-encapsulated mass and showed combined histologic features of infundibular, isthmus, and pilomatrical differentiation. The hybrid cyst is a follicular cyst that includes two or more components of a cystic lesion arising from the pilosebaceous unit. Occasionally, there have been reports of hybrid cysts consisting of two components in differentiation, but those showing more than two components of differentiation have been extremely rare. The pathogenesis of this unusual disease is not yet known.
Keywords: Epidermal cyst, hybrid cyst, pilomatricoma, trichilemmal cyst
|How to cite this article:|
Choi YS, Min SK, Shim HS, Kim MC, Lim JS, Han KT. Follicular hybrid cyst on the cheek: A combination of infundibular, isthmic, and pilomatrical differentiation. Indian J Pathol Microbiol 2012;55:563-5
|How to cite this URL:|
Choi YS, Min SK, Shim HS, Kim MC, Lim JS, Han KT. Follicular hybrid cyst on the cheek: A combination of infundibular, isthmic, and pilomatrical differentiation. Indian J Pathol Microbiol [serial online] 2012 [cited 2020 Dec 5];55:563-5. Available from: https://www.ijpmonline.org/text.asp?2012/55/4/563/107823
| Introduction|| |
Various benign cutaneous cysts such as epidermal cysts, trichilemmal cysts, vellus hair cysts, steatocystoma, or pilomatricoma can arise from a different part of the pilosebaceous unit. In rare instances, a cyst sometimes shows multiple differentiation of the other parts of a pilosebaceous unit. This combination was termed hybrid cysts. , Herein, we report the case of a hybrid cyst showing combined differentiation of infundibulum, isthmus, and pilomatrix.
| Case Report|| |
A 12-year-old boy was brought to our outpatient clinic with a complaint of a small nodule on the right cheek of 3 months duration [Figure 1]. There was no family history of cutaneous disorders. He had no remarkable past medical history. The physical examination revealed a dark-gray, oval-shaped, 0.9 × 0.6 cm 2 protruding mass that was non-tender to palpation. The lesion was totally excised under local anesthesia, and no recurrence had been reported for 12 months. Histopathologically, the nodule was well-circumscribed and located within the subcutaneous tissue. The microscopic finding indicated the diagnosis of a hybrid cyst as a composition of infundibular, isthmus, and pilomatrical differentiation. It showed a small unilocular cystic mass connected to overlying epidermis with a prominent opening. At the upper portion of the cyst, the superficial layer was composed of stratified squamous epithelium with keratohyaline granule and laminated keratin consistent with derivation from the follicular infundibulum. At the lower portion of the cyst, bright red trichohyaline granules, the sign of inner root sheath differentiation, and basophilic small basaloid cells, sign of matrical differentiation, were shown. The cystic contents had shadow cells with the absence of nuclei, indicating component of matrical differentiation. Moreover, another area of the cyst, the squamous epithelial lining, became slightly larger resembling trichilemmal keratinization with none or scarce granular layer, which indicated trichilemal differentiation. Therefore, it has been diagnosed as follicular hybrid cyst that suggests differentiation toward many parts of a hair follicle, including the infundibulum, isthmus, and pilomatix [Figure 2].
|Figure 1: Gross clinical photo of right cheek, small mass (less than 1×1 cm2)|
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|Figure 2: Histopathologic findings. (hematoxylin and eosin stain, ×200) (Above, left) Infundibular differentiation, laminated keratin (arrow), and keratohyaline granule (arrow head), (Above, right) Isthmic differentiation, trichilemmal differentiation with no or scarce keratohyaline granular layer (arrow), (Below, left) Pilomatrix differentiation, inner sheath differentiation with red trichohyaline granule (eosinophilic globule, arrow), matrical differentiation with basophilic small basaloid cell (arrow head), and melanocyte (thin black arrow) (Below, right) Pilomatrix differentiation, shadow cells (arrow) with sign of matrical differentiation|
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| Discussion|| |
Cutaneous follicular cysts are common cutaneous neoplasms. There are several varieties of follicular cysts arising from a different part of the pilosebaceous unit, namely infundibulum, isthmus, and the inferior portion. Follicular cysts include epidermal cysts, trichilemmal cysts, vellus hair cysts, steatocystoma, and pilomatricoma. The epidermal cyst, the most common benign cutaneous neoplasm, arises from the infundibular portion. The trichilemmal cyst originates from the isthmic portion, and the pilomatricoma shows differentiation toward the follicular matrix, inner root sheath, and hair.  Rarely, follicular cysts can show combined histologic structure including more than two of the pilosebaceous unit. This concept is a hybrid cyst.
In 1966, McGavran et al.,  first described hybrid cyst as a cystic tumor that was a combination of infundibular and trichilemmal cyst. After that, in 1983, Brownstein et al.,  reported seven cases of follicular hybrid cyst, in which the upper portion near the epidermis was a infundibular cyst and the deep portion was a trichilemmal cyst. Recently, the concept of hybrid cyst was changed that would not be restricted to those combination of infundibular and trichilemmal cyst. Because any cyst arising from the various parts of the pilosebaceous unit can combine with another to form a large series of follicular hybrid cyst. Under this expanded spectrum concept, various combination of follicular cysts were included in hybrid cyst such as infundibular and trichilemmal cyst, infundibular and pilomatricoma cyst, trichilemmal and pilomatricoma cyst, eruptive vellus hair cyst and steatocystoma, and eruptive vellus hair cyst and trichilemmal cyst. Takeda et al., reported 15 cases of hybrid cysts in Japan. The most frequent histological type of hybrid cyst was infundibular and trichilemmal cyst (9 of 15 cases, 60.0%). 
Barbara et al., investigated the patients with Gardner's syndrome I in order to determine the type of cutaneous cystic neoplasm which is one of the manifestations of this syndrome. A cutaneous cystic lesion was found in 53% of patients and the most common type was epidermoid cyst.  Narisawa et al.,  reported 1 case of Gardner's cyst, in which multiple follicular cysts were present from one patient with Gardner's syndrome showing infundibular, isthmic portion of outer root sheath, sebaceous gland, bulge area, vellus hair differentiation.  Nishie et al.,  reported the follicular hybrid cyst in a patient with myotonic dystrophy. The cyst from the scalp was differentiated toward infundibulum, isthmus, and inner root sheath differentiation. They hypothesized that this hybrid cyst might be derived from follicular stem cells under the genetic influence of the myotonic dystrophy gene.  It is known that Gardner's syndrome accompanies the follicular cyst that shows various forms of differentiation of the hair follicle such as infundibular and trichilemmal keratinization, and inner root sheath differentiation. It has been proposed that the cyst is derived from pluripotent follicular stem cells. Yet, the pathogenesis of this unusual disease is not clear.  Some studies hypothesized that the follicular germinative cell often differentiate from various parts of the pilosebaceous unit.  However, in our case, the germinative cell was not shown.
AAs mentioned above, in most cases, the hybrid cysts consist of two components differentiated from the pilosebaceous unit. However, the cysts differentiating from more than three components have been rarely reported. May et al., reported two cases of follicular hybrid cyst showing infundibular, isthmic, pilomatrical differentiation from healthy patients. 
Therefore, more case report and study for pathogenesis might be necessary. Herein, we report the case of the follicular hybrid cyst that is differentiated into many parts of hair follicle, including the infundibulum, isthmus, and pilomatix.
| References|| |
|1.||McGavran MH, Binnington B. Keratinous cysts of the skin. Identification and differentiation of pilar cysts from epidermal cysts. Arch Dermatol 1966;94:499-508. |
|2.||Brownstein MH. Hybrid cyst: A combined epidermoid and trichilemmal cyst. J Am Acad Dermatol 1983;9:872-5. |
|3.||Narisawa Y, Kohda H. Cutaneous cysts of Gardner's syndrome are similar to follicular stem cells. J Cutan Pathol 1995;22:115-21. |
|4.||Takeda H, Miura A, Katagata Y, Mitsuhashi Y, Kondo S. Hybrid cyst: Case reports and review of 15 cases in Japan. J Eur Acad Dermatol Venereol 2003;17:83-6. |
|5.||Leppard B, Bussey HJ. Epidermoid cysts, polyposis coli and Gardner's syndrome. Br J Surg 1975;62:387-93. |
|6.||Nishie W, Iitoyo M, Miyazawa H. Follicular cyst in a patient with myotonic dystrophy: A case of cyst with differentiation toward follicular infundibulum, isthmus, inner root sheath, and hair. Am J Dermatopathol 2001;23:521-4. |
|7.||Nishie W, Kimura T. Follicular germinative cells in pilomatricoma. Am J Dermatopathol 2006;28:510-3. |
|8.||May SA, Quirey R, Cockerell CJ. Follicular hybrid cysts with infundibular, isthmic-catagen, and pilomatrical differentiation: A report of 2 patients. Ann Diagn Pathol 2006;10:110-3. |
Department of Plastic Surgery, St. Vincent Hospital, 93-6, Ji Dong, Paldal-gu, Suwon, Gyeonggi
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]