Epidermoid cyst

Author: Mikael Häggström ^{[note 1]}
Generally presents as a skin cyst. Also known as epidermal inclusion cyst (EIC) and sebaceous cyst.

Typical findings:^[1]

• Stratified squamous epithelium with a granular layer
• Cyst wall does not contain eccrine glands, sebaceous glands or hair follicles
• Abundant keratin flakes in cyst lumen
• Often giant cell granuloma in ruptured cysts, similar to foreign body granuloma

Mainly:

• 
  Trichilemmal cyst, containing, from external (top) to internal (bottom):^{[image 1][2]}
  - Fibrous capsule
  - Small, cuboidal, dark-staining basal epithelial cells in a palisade arrangement, with no distinct intercellular bridging
  - Swollen pale keratinocytes, which increase in height closer to the interior
  - Solid eosinophilic-staining keratin
  There is no granular cell layer (in contrast to an epidermoid cyst).
• 
  Pilomatricoma: Stroma surrounding irregularly shaped islands containing basaloid cells (darkly stained, round or elongated), which abruptly or gradually transitions into ghost cells (having pale cytoplasm and a central clear area), which in turn may transition into necrosis.^{[image 1][3]}

Further information: Skin cyst

Look for signs of cyst rupture, which may manifest as inflammation, including granulomas and microabscesses.

Example:

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