Trichilemmal Cyst
A trichilemmal cyst, also called a pilar cyst or wen, is a benign cyst that arises from the isthmus of the hair follicle — the segment between the insertion of the arrector pili muscle and the follicular infundibulum. Although it can look much like an epidermoid cyst, its biology and histology are entirely distinct.
Characteristic Trichilemmal Keratinization
- Keratinization without a granular layer: in normal epidermis, spinous cells accumulate keratohyalin granules and form a granular layer before becoming the horny layer. In a trichilemmal cyst, the lining cells change directly from plump spinous cells into compact keratin, with no granular layer (stratum granulosum) at all.
- Nature of the cyst contents: the keratin inside is dense, sticky and homogeneous, unlike the loose, laminated, onion-skin keratin of an epidermoid cyst.
Genetics and Family History
- Familial pilar cysts: about 30–50% of cases run in families with an autosomal dominant pattern of inheritance. These patients often have several cysts on the scalp at the same time.
- Genetic linkage: molecular studies point to loss of heterozygosity or mutations in the chromosome 3p14.3–p21.2 region, which affect differentiation of the follicle's outer root sheath.
At Haute-Derma, our management of trichilemmal cysts is built around their particular biology: the wall is thick and tough and does not stick to the surrounding tissue. High-frequency skin ultrasound is used to assess wall thickness, the degree of calcification and the boundary with the scalp aponeurosis (galea aponeurotica). Because the wall is so robust, we favor en bloc intact micro-extirpation through a very small incision. The cyst is removed without rupturing, the procedure is shorter, and the entire cyst is taken out without trimming any hair or leaving a bald scar on the scalp.
Histology, Dermoscopy and Ultrasound Diagnostic Criteria
Characteristic Microscopic Features
The histology of a trichilemmal cyst shows a set of hallmark features:
- Cyst wall: lined by stratified squamous epithelium without folds. The basal cells show clear palisading, and above them lie enlarged, swollen pale cells. There is no granular layer.
- Abrupt keratinization: the junction between the plump epithelial cells and the keratin in the lumen is sharp, with no transitional keratohyalin stage.
- Cyst lumen: filled with dense, homogeneous, compact keratin. In about 25% of cases there is calcification, seen as gray specks or hard bands of calcium within the cyst.
Dermoscopy and High-Frequency Ultrasound Criteria
Advanced imaging at Haute-Derma lets us reliably distinguish trichilemmal cysts from other scalp lumps:
- Dermoscopy:
- Smooth, taut scalp with no punctum: unlike an epidermoid cyst, a trichilemmal cyst has no central punctum. The overlying skin is smooth and shiny, with hair follicles pushed aside.
- Peripherally branched vessels: fine capillaries encircle the base of the nodule as a result of mechanical compression.
- High-frequency ultrasound (20–50 MHz, the gold standard):
- Thick-walled hypoechoic mass: round or oval, with a very well-defined border.
- Internal calcification foci: a very typical sign — bright hyperechoic foci with acoustic shadowing inside the cyst.
- Distinct fibrous capsule: the thick capsule shows up clearly on ultrasound and explains why these cysts shell out so easily in one piece.
Clinical Features
- Usual location: more than 90% of trichilemmal cysts occur on the scalp. The rest appear on the face, neck or scrotum.
- Appearance: a smooth, dome-shaped nodule under the skin that feels firm and rubbery, or hard if heavily calcified. Because it is not attached to the epidermis (no punctum), it moves freely under the skin.
- Number: people with a family history often have multiple cysts on the scalp at once.
Clinical and Histologic Comparison
| Trichilemmal Cyst | Epidermoid Cyst | Trichoblastoma |
|---|---|---|
| No granular layer (trichilemmal keratinization) | Well-developed granular layer | No true cyst structure |
| No central punctum | Central punctum often present | No central punctum |
| Very thick, tough wall; not attached to the skin | Thin wall that tears easily when grasped | Solid tumor |
| Calcification common (25%) | Calcification rare | No calcification |
| Follicular isthmus | Follicular infundibulum | Embryonic hair germ |
Intact Micro-Excision and Scalp Recovery at Haute-Derma
Because a trichilemmal cyst has a thick, tough wall that is not attached to the epidermis, Haute-Derma uses a microsurgical technique that preserves the scalp as much as possible, with no need to cut any hair:
High-Frequency Ultrasound Assessment and Vessel Mapping
- Depth and calcification: 20–50 MHz ultrasound measures the exact size of the cyst, the degree of internal calcification and its boundary with the scalp aponeurosis.
- Following the hair direction: the incision is marked along the natural direction of hair growth so that the scar is fully hidden once healed.
Intact Micro-Extirpation of the Tough-Walled Cyst
- Micro-incision between the hairs: a microsurgical blade makes a small (2–4 mm) incision parallel to the follicles without cutting through the surrounding hair shafts.
- Blunt dissection: curved microsurgical scissors gently free the connective tissue around the cyst wall. Because the wall is thick and tough, the cyst separates easily from its surroundings.
- En bloc extraction: the cyst is gently grasped and drawn out whole, without rupturing the wall, keeping the keratin and calcium contents sealed inside.
- Cosmetic intradermal closure: the cavity is closed with fine 6-0 absorbable monofilament sutures, avoiding tension on the scalp and preventing secondary hair loss (alopecia).
Tissue Repair and Follicle Support
- Pincer Biomodulation: a molecular biotherapy applied directly to the base of the cavity to speed connective tissue regeneration and protect the surrounding hair follicles.
- Vbeam 595 nm laser: calms vascular redness around the incision so the scalp settles and recovers fully within 3–5 days.
