Page 59 - The Netter Collection of Medical Illustrations - Integumentary System_ Volume 4 ( PDFDrive )

Plate 2-32 Benign Growths

PILAR CYST
(TRICHILEMMAL CYST)

Pilar cysts are relatively common benign growths that
occur most frequently on the scalp. They go by many
names, including wen, trichilemmal cyst, and isthmus- Isthmus
catagen cyst. Most are solitary, but it is not uncommon
to see multiple pilar cysts in a single individual. Their
appearance is similar to that of epidermal inclusion
cysts, but the pathogenesis is completely different. Pilar cysts are
There is a malignant counterpart called a metastasizing frequently found Pilar cysts develop from
proliferating trichilemmal cyst. The malignant trans- on the scalp. within the isthmus of the
formation of a pilar cyst is exceedingly rare. Subsets of Dome-shaped, hair follicle apparatus.
these growths are inherited. firm dermal
Clinical Findings: Pilar cysts occur most frequently nodules
on the scalp. They can be mistaken for epidermal inclu-
sion cysts. The main clinical differentiating points
are that pilar cysts do not have an overlying central
punctum, and they tend to be a bit firmer to touch.
These cysts occur more commonly in adults, and they
have a tendency to affect women more often than men.
They typically manifest as slowly growing, firm dermal
nodules with no overlying epidermal changes and no
central punctum. These cysts do not drain, as epidermal
cysts sometimes do. They also rarely get inflamed.
Almost exclusively found in the scalp, they are for
the most part asymptomatic. Patients present to the
clinician because of an enlarging nodule. As opposed
to the epidermal inclusion cyst, which essentially has
no malignant potential, the pilar cyst does have a
small proliferating and malignant potential. This risk is
very low.
Some families show an autosomal dominant inheri-
tance pattern. The exact gene defect has yet to be deter-
mined, but a possible gene has been mapped to
chromosome 3. Most patients with the hereditary
version of this condition have solitary lesions. Numer-
ous lesions are infrequently encountered in the inher-
ited form.
Pathogenesis: Pilar cysts are also called trichilemmal
cysts, because they are derived from the outer root
sheath of the hair follicle, which undergoes trichilem- Low power. Dermal cyst with compacted central High power. The epithelial lining does not contain
mal keratinization. This form of keratinization is keratin. The epidermis is unaffected. a granular cell layer and is composed of stratified
unique in that there is no granular layer. The hereditary squamous epithelium.
version of this disease was originally thought to be
caused by a defect in the gene encoding β-catenin. This
has been disproven, and the familial gene has been
mapped to the short arm of chromosome 3, although the overlying epidermis is unaffected. These cysts show removed very easily after excision through the overly-
the exact genetic defect has yet to be elucidated. These an absence of intercellular adhesion molecules. The ing skin into the cyst wall. The cyst almost always
cysts are believed to be derived from the isthmus of cysts can become calcified or ossified. The cysts have a “pops” out with slight lateral pressure, and only a small
anagen-type hairs. They are formed from deeper ele- unique peripheral rim of keratinocyte nuclei, which is incision is needed. After removal, care needs to be taken
ments of the hair shaft apparatus than the epidermal very helpful in classifying them. The central aspect of to decrease the amount of dead space left, to avoid
inclusion cyst are. the cyst contains homogenous pale, eosinophilic, com- seroma formation. This can be prevented by removing
Histology: Pilar cysts are composed of compact layers pressed keratin. some of the redundant overlying epidermis and sutur-
of stratified squamous epithelium without a granular Treatment: Simple surgical excision is curative. The ing the deeper tissues together to close the space left by
cell layer. The cysts are found within the dermis, and recurrence rate is minimal. These cysts typically are the removed cyst.

THE NETTER COLLECTION OF MEDICAL ILLUSTRATIONS 45