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Non Neoplastic Masses of Skin
There are many different masses of the skin that are not neoplastic. Neoplastic disease of skin is found at this site. Nonneoplastic masses are divided into three main groups. The cysts that arise in skin, hamartomas of skin and the other miscellaneous masses. In general, this group are expansile and usually incidental findings. Many are located in one site, some are multiple and some involve the entire skin. They are listed, in order, below .
A cyst is a structure with an epithelial lining and a centre filled with product. This could be keratin, glandular secretion or a combination of the two. Cysts are derived from the entire dermis, the epidermis, the follicles or the adnexal glands.
There are myriad cysts and their location and phenotype determines their name.
Dermoid cysts are composed of several or all elements of the skin and subcutis. They typically develop prior to birth, are found at birth or soon afterward. They indicate an embryological dysfunction and typically a failure of fusion of the ectoderm in the formation of the neuroectoderm. They are composed of all elements of skin and soft tissue including epidermis, dermis, adnexa, and subcutaneous tissues. A classical example is dermoid cysts of the back of Rhodesian ridgebacks dog
An epidermal cyst has an epithelial lining that mimics the epidermis. It has a centre filled with keratin that is some combination of keratin squames, compact orthokeratotic keratin or parakeratotic keratin. They typically occur where epidermis is implanted following trauma, usually penetrating injury.
Follicular cysts I derived from one or multiple parts of hair follicles including the infundibular, isthmus, and inferior regions. The naming convention is based on the phenotype of the epithelium and the type of keratin that is produced.
Follicular cysts can be primary (developmental) or secondary (acquired).
A cyst of the superficial part of the follicle that has a thick wall of epithelial cells including rete ridges and it is filled with compact keratin. It often has a broad opening to the surface.
An infundibular cyst is derived from the superficial part of a hair follicle - the part above the attachment of the erector pili muscle. It has a content of keratin squames and variable amounts of sebaceous secretion. The epithelial wall is the same as the infundibulum of the follicle. It usually has a stratum granulosum, along with the basal layer, the stratum spinosum and stratum corneum. This wall is of normal thickness
Isthmus (tricholemmal) cyst
An isthmus cyst is derived from the part of a hair follicle beneath the attachment of the erector pili muscle and above the entrance of the sebaceous gland in an anagen follicle and of the inferior region in a catagen or telogen follicle. It has a content of compact keratin homogenous keratin, and variable amounts of sebaceous secretion. The epithelial wall has no stratum granulosum but does have the basal layer, the stratum spinosum and stratum corneum. They are glassy in appearance.
Matricial (pilar) cyst
A matricial cyst is derived from the inferior portion of the hair follicle. The cyst contains solid keratin that has the ghostlike outlines of nuclei visible throughout. The epithelial wall is maturing basal cells and there is an abrupt transition from these basaloid cells to the keratin without a stratum spinosum. This keratinization is called matricial keratinization.
This is a hybrid cyst, containing elements with differentiation to the infundibular, isthmus and inferior portions of the hair follicle.The centre of the cyst contains keratin squames, compact keratin and matricial keratin.The epithelial wall has infundibular, isthmus and inferior follicular differentiation often dominated by matricial keratinization but which can also contain tricholemmal differentiation.
An apocrine cyst as a content that is apocrine secretion and a wall composed of a single layer of cuboidal epithelial cells resembling those of adjacent apocrine glands. Sometimes there is a bilayer resembling the apocrine duct. Apocrine secretion can contain lipids so sometimes there are macrophages beside the cyst that contain lipofuscin or lipochrome pigments. Inspissated apocrine secretion is usually brightly eosinophilic.
Sebaceous (duct) cyst
A sebaceous cyst is a structure that contains sebum, which has a fibrillar appearance, and the wall has sebaceous epithelial differentiation with epithelial cells a highly vacuolated cytoplasm. There may be differentiation to the sebaceous duct which is a cuboidal and bilayered structure sometimes with keratin production resembling the Isthmus portion of the follicle.
Eccrine cysts have a content that is fluidic and watery and the wall of cuboidal epithelial cells that are pale staining. They are only found where eccrine glands are located, which in dogs and cats is the nasal planum and the footpads.
A milium cyst is similar to and eccrine cyst. Multiple examples in one area a called milia. These are cystic structures of the eccrine duct and they contain keratin or are partially keratinized. The keratin is usually basket weave or keratin squames. They are located where eccrine glands are found.
A hamartoma is a mass that is an excessive amount of tissue in a location where it is expected to occur. Many people consider these congenital and are typically recognized in young animals. Delayed or tardive development is the explanation provided for those that arise in older animals. Others have a much broader view and consider these to be both developmental and induced lesions.
There are myriad hamartomas and their location and phenotype determines their name.
Collagenoma (collagenous hamartoma, collagen nevus)
A collagenoma is a proliferation of dermal collagen within the dermis. These are well circumscribed masses within the dermis that causes the epidermis to be raised slightly but they are not pedunculated and they have a very low cellularity, similar to adjacent normal dermis. It is typically composed of bundles of dermal collagen in a similar arrangement to normal dermis but the bundles are larger than normal. They usually displace resident structures.
These are very rare well demarcated thickenings of the epidermis with an abrupt change from normal to thickened epidermis. The affected epidermis has all layers and orthokeratotic keratin on the surface. Melanocytes many be present.
There are many different types of masses that are listed as fibroadnexal in type. They have varying degrees of collagen, adipose tissue, sebaceous glands and sebaceous ducts. Some may have large hair follicles but many have no hair at all.
Follicular hamartoma (Giant hair type hamartoma)
Follicular hamatomas are clusters of large hair follicles. Each follicle has a normal structure and usually a hair shaft in each but they are larger than surrounding hair follicles and they form an expansile lesion in the dermis.
A sebaceous hamartoma are composed almost exclusively of sebaceous glands. It is an expansile and well circumscribed nodule or mass, has the normal differentiation of sebaceous glands, is located within the dermis, and does not have other adnexal structures.
Pilar neurocristic hamartoma
This hamartoma derives its name from a hair follicle (pilar) and mesenchymal cells of the neural crest, usually with spindle cells, Schwann cells, pigmented dendritic cells and or melanocytes.They are often pigmented in appearance. They are multifocal/multinodular.
Smooth Muscle Hamartoma
Well demarcated and expansile nodules of smooth muscle that form a mass in the dermis is a smooth muscle hamartoma. These are derived from the arrector (erector) pili muscle.
Vascular hamartoma, angiomatosis, vascular neoplasia and arteriovenous shunts have many overlapping features and require great patience and dedication to separate. Vascular hamartoma is typically seen in young animals and are nodules that are well circumscribed and expansile in their growth habit. They are composed of blood vessels that include arterioles and veins thus they have smooth muscle in their wall, they usually have a feeder vessel/arteriole. They are frequently located on the scrotum of males but they can occur anywhere.
A seroma is a lesion induced by trauma where there was a space created in the subcutis, often by shear force or surgery. This space fills with edema fluid initially and subsequently develops a surrounding layer of granulation tissue. It is common for them to appear to expand, or to remain static for a very long time. Some eventually scar.