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• cystic or papillary lesions:
  • hair follicular infundibular origin (granular layer = infundibular keratinization):
    • follicular infundibular cyst: the common epidermal inclusion cyst (EIC; "milia" or "milium cyst" when small) lined by granular squamous & having squames contents
    • dilated pore of Winer: conical/vertical infundibular acanthoma with minimal lateral outgrowths &  goes deep rather than wide; squamous with granular layer.
    • pilar sheath acanthoma: broader-based & shallow proliferative "pore"...even with formation of lateral squamous sinuses; usually less than 1.0CM & tend on the face of middle-aged to elderly; squamous with granular layer. [S-04-14658B]
  • hair follicular isthmic origin (no granular layer = trichilemmal  keratinization):
    • trichilemmal cyst: non-granular squamous lining and cheesy, rancid homogeneous eosinophilic matter contents
    • proliferating pilar cyst: scalp lesion in women; squamoid atypia and mitoses up to 10 per 10 hpfs [S08-13180].
    • steatocystoma: thin epithelial lining with sebaceous cell clusters & corrugated keratin & oil contents. [S-05-7423]
  • hair follicular bulb origin:
    • hair matrix cyst: lots of basaloid cell component in cyst wall (fore-runner of pilomatricoma [calcifying epithelioma of Malherbe])
  • hair follicular apocrine gland origin:
    • apocrine hidrocystoma:contains homogeneous eosinophilic dark fluid & lining cells with decapitation secretion
    • hidradenoma papilliferum: only in genital skin²; two-cell lining & stroma not filled with plasma cells & papillary stalks which can extend through a surface pore "like a hermit crab"
    • syringocystadenoma papilliferum (SCAP)²: scalp & face; two-cell-layered lining papillations (like hidradenoma pap.) and stroma filled with plasma cells & situated in a skin dell or depression.
    • tubular apocrine adenoma (TAA): proliferation remindful of papillary eccrine adenoma but with apocrine cellularity.
  • sweat duct eccrine origin:
    • eccrine hidrocystoma: cuboidal to flattened eccrine-like lining cells and dark fluid contents...(could be partly adenoma cellularity & even have coincidental surface AK/PMK). [S-04-14534] 
    • eccrine syringofibroadenoma: like syringoma but with fibroadenoamtous quantity of tumor stroma & some microcystic spaces.
    • papillary eccrine adenoma (PEA): tumor borders often not sharply circumscribed; remindful of syringoma except microcystic spces not teardrop-shaped and lining cells a little papillated. Analogy with intraductal breast duct hyperplasia.
    • solid & cystic hidradenoma: see eccrine acrospiroma, below.
  • mixed epithelia:
    • mixed-epithelial adnexal cystoma: ("benign adnexal tumor with multi-directional differentiation" )
• lesions of small nests, ducts, or cords:
  • hair follicular origin:
    • epithelial:
      • outer hair sheath: tip-off = clear cells & slight peripheral palisading.
        • tumor of follicular infundibulum: sunexposed H&N of females; subepidermal plate of BCC type cells with the plate being small-calibre fenestrated with some papillary dermal like elements². Cells have more cytoplasm than BCC, with some PAS positivity.
        • pilar sheath acanthoma: see above, cystic
        • trichoadenoma: face or trunk solitary nodule; well-defined but poorly circumscribed dermal nodule of micronodules with trichilemmal keratin horn cysts and minimal small-cell sqamobasaloid periphery around the epithelial micronodules. Like a trichoepithelioma but with horn cysts predominating¹. [S-05-13931].
        • trichilemmoma: H&N location; keratotic papule or plaque remindful of inverted keratosis or "acanthoma"; paler basaloid cells with deep margin palisading.
      • germinative epithelium:
        • trichoepithelioma: to be diagnosed (especially) as a solitary lesion, this must be basaloid (cellularity predominates) with some squamous "horn cysts" plus abortive hair papillae and hair shafts and a very minority of lesion that looks like BCC³. Has CD34 + stroma & a few Merkle cells. [S-04-14935; S-05-2325]
        • desmoplastic trichoepithelioma (DT): nodule on faces of young women; cords and nests of basaloid (squamoid-like) cells remindful of keratotic infiltrative-type or morpheaform [S-05-7788; S-05-7730] BCC. DT absent  in mitoses and individually necrotic cells¹. Rule out MAC. See DDX IHC. Excision is curative. [S-05-6716]
        • trichofolliculoma: trichoepithelioma-like with central dilated primary follicle and having about all components of pilosebaceous complex¹.
    • mixed epithelial & mesenchymal:
      • basaloid follicular hamartoma:
    • pilar mesenchyme:
  • sweat ductular origin:
    • syringoma: eccrine; eyelids, forehead, face & neck; rounded to tadpole/teardrop-shaped basaloid cell clusters with predominant lumens. Rule out MAC.
    • cylindroma: scalp & face; eccrine (some say apocrine origin); somewhat like spiradenoma but with smaller cell clusters...bimorphism...fitted like jigsaw puzzle pieces; hyaline droplets & sheaths. Negative for stromal vascular dilation & serum collections.
    • eccrine syringometaplasia: acanthotic epidermal lesion with underlying irregular & elongated & branched, proliferated sweat ducts.
  • mixed tumor (chondroid syringoma):

• lesions of expansile nodules & sheets of tumor cells:
  • poroma: cellularity may remind you of an SK...eccrine; palms & soles...nevus vs. verruca; slightly paler basaloid thickening of deeper epidermis and even duct-like thickenings deeper, variants being:
    • hidroacanthoma simplex: totally intra-epidermal poroma (may remind you of an SK).
    • dermal duct tumor: (may remind you of an SK), totally intradermal poroma, but one might luckily see attachment to small intra-epidermal poroma patch.[S-05-2469]
    • syringoacanthoma: (may remind you of an SK), poroma with surface keratinization & surrounding acanthosis.
    • poroid hidradenoma: see below.
  • trichilemmoma: see above.
  • proliferating pilar tumor: see proliferating pilar cyst, above.
  • nodular hidradenoma:
    • eccrine acrospiroma (clear-cell hidradenoma; solid & cystic hidradenoma; myoepithelioma of skin): 
  • poroid hidradenoma: if hidradenoma features predominate (see poroma).
  • spiradenoma:
    • eccrine spiradenoma: tender to palpation; basaloid, two-cell population, /w basaloid surrounding larger, pale cell (s)...some tubularity; basement membrane material cuffing. Positive for stromal vascular dilation & serum collections
• lesions of lipidized (sebaceous gland) cells:
  • sebaceous hyperplasia: enlarged seb. gland (s).
  • nevus sebaceous: nevoid, circumscribed plaque (clinically of nevoid size & character) of benign sebaceous gland tissue mostly of seb. cells.
  • sebaceous adenoma: cytologically benign & greater than 50% of cells look sebaceous. Watch out for Torre syndrome.
  • sebaceoma (sebaceous epithelioma): cytologically benign & less than 50% of cells look sebaceous. Watch out for Torre syndrome.
• malignant tumors:
  • sebaceous carcinoma
  • eccrine porocarcinoma
  • microcystic adnexal carcinoma
  • apocrine carcinoma
  • others

1. Murphy GF, Dermatopathology, 1995, 505 pages...outline p.229 (EBS's office).
2. Cutaneous Adnexal Tumors..., Wick MR & Swanson PE, 1991, 238 pages (BWD's office).
3. Lever WF, Histopathology of The Skin, 4th Ed.
4. eMedicine site, Preito VG, as of 22 August 2005.

(posted 30 November 2004; latest additions 24 March 2007)