Small Animal Section: No.
9
BENIGN SKIN CYSTS, HAMARTOMAS AND TUMOUR-LIKE
LESIONS
CYSTS : Cystic lesions develop from hair follicles and pre-existing glands
and are sub-classified as:
-
Infundibular cyst:
This is a simple cyst lined by stratified squamous epithelium with all layers
present. Lesions are solitary or multiple with cornified cells loosely packed
in the cyst lumen – also referred to as epidermal cyst or epidermal inclusion
cyst
-
Dilated pore:
A dilated infundibular cyst with hyperplastic epithelium and communicates with
the overlying epidermis via a pore that is quite wide. The wall consists of
hyperplastic epithelium
-
Isthmus cyst:
This simple cystic lesion is lined by cells that resemble the middle segment
of the hair follicle and is lined by a stratified squamous epithelium without
a granular layer
-
Trichoepitheliomatous cyst (panfollicular cyst):
These cysts may contain 2 or 3 layers of the follicular epithelium and show
abrupt keratinisation similar to that found in a benign pilomatricoma. It is
believed that the trichoepitheliomatous cysts may progress into benign
pilomatrixomas or trichoepitheliomas
-
Dermoid cyst:
This congenital dermal or subcutaneous cyst is usually lined by a normal
appearing epidermis and all appendages. It may communicate via a small pore
-
Apocrine cystomatosis (cyst):
These intradermal apocrine cysts are lined by an atrophic single layer of
apocrine secretory epithelium and are filled with clear secretion. They are
common around the neck in dogs.
HAMARTOMAS
: This refers to a
mass of disorganised but mature tissue indigenous to the particular site.
Hamartomas are regarded as congenital but many are clinically inapparent and
recognized only later in life.
-
Epidermal hamartoma (pigmented epidermal nevus):
Focal discreet plaques with hyperpigmentation, hyperkeratosis, acanthosis and
papillomatosis. Papilloma virus infection has been noted in the dog
-
Follicular hamartoma:
Localized aggregate of enlarged primary hair follicles surrounded by a fibrous
root sheath and variable amount of dermal collagen
-
Sebaceous hamartoma:
These are relative common in the deep dermis where excessive proliferation of
mature well-differentiated sebaceous glands are observed
-
Apocrine hamartoma:
A presumed congenital proliferation of apocrine sweat glands within the dermis
and/or panniculus adiposus. The overlying epidermis is hyperplastic
-
Fibroadnexal hamartoma:
This is a focal proliferation of the pilosebaceous units as well as increased
dermal collagenous fibrous tissue. Some of these lesions may not be
congenital and may represent a fibroadnexal transformation associated with
trauma
-
Collagenous hamartoma :
nodular poorly
circumscribed focus of redundant collagen in the superficial dermis
-
Vascular hamartoma :
rare mass of
redundant blood vessels on scrotal skin of pigmented dogs
TUMOUR-LIKE LESIONS
-
Squamous papilloma (non-neoplastic):
A non-neoplastic squamous papillomatous mass composed of epithelium and
supportive dermal stroma
-
Callus and pressure point comedones:
Multiple cystic lesions with hyperplastic stratified squamous epithelium found
on pressure points. Some are secondarily inflamed.
-
Fibroepithelial polyp (cutaneous tag):
Focal or multifocal redundant dermal fibrous tissue covered by hyperplastic
epidermis where the stroma is more prominent than in a squamous papilloma
-
Fibropruritic nodule:
Multifocal hyperplastic chronic dermal nodules associated with chronic
self-trauma and secondary granulomatous inflammation
-
Calcinosis circumscripta :
Apocrine cystic calcinosis on
footpads, pressure points and rarely on the tongue
Reference
Goldschmidt et al. (1998) WHO
International Histological Classification of Tumours
of Domestic Animals :
second series Vol III
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