Pilar cyst

Pilar cyst
Diagnosis in short
Pilar cyst. H&E stain.
LM	cyst lining by a stratified squamous epithelium without a granular layer - contains keratin; no significant nuclear atypia; +/-granulomatous inflammation (due to rupture)
LM DDx	epidermal inclusion cyst
Gross	nodule +/-yellow colour
Site	skin - usu. scalp
Prevalence	very common
Prognosis	benign
Clin. DDx	other skin cysts

Pilar cyst, also known as a trichilemmal cyst, is a common benign skin cyst.

General

• Very common.
• Benign.‡
• The clinical history for these typically says sebaceous cyst.

Note:

• ‡A super rare malignant counter part is described; approximately 40 cases are reported in the english literature.

Gross

• Classic location: head ~90%.

Microscopic

Features:

• Keratin.
• Cyst lining:
  • Has no granular layer - key feature.
    • Keratohyaline granules (as seen in the granular layer) may be seen focally.
  • Inner most cyst lining cells are large cells with abundant eosinophilic cytoplasm.

DDx:

• Epidermal cyst - has a granular layer.
• Squamous cell carcinoma arising from a pilar cyst.

Images

• 

  Trichilemmal cyst - intermed. mag. (WC)

• 

  Trichilemmal cyst - high mag. (WC)

• 

  Trichilemmal cyst - very high mag. (WC)

www:

• Trichilemmal cyst (dermatlas.med.jhmi.edu).
• Trichilemmal cyst (flickr.com).

Sign out

Scalp (Cyst), Excision:
- Trichilemmal cyst (pilar cyst).

Block letters

SCALP (CYST), EXCISION:
- TRICHILEMMAL CYST (PILAR CYST).

SKIN CYST, LEFT FLANK, EXCISION:
- TRICHILEMMAL CYST (PILAR CYST).

SCALP (CYST), ANTERIOR, EXCISION:
- TRICHILEMMAL CYST (PILAR CYST), RUPTURED.

Micro

The sections show a cyst that is lined by squamous epithelium without a granular layer. Focally, keratohyaline granules are seen in the cyst lining cells. The innermost cyst lining cells are large and have abundant eosinophilic cytoplasm. The cyst contains keratin.

See also

• Dermal cysts.