Sebaceous Cyst versus
Intracutaneous Cornifying Epithelioma

The skin is a very complex organ. It is the largest organ of the body, accounting for about 12% of the total body weight of an adult dog, 24% of a newborn puppy. In addition to acting as the flexible, protective cover for all other body parts, the skin aids in temperature regulation, sensory perception, blood pressure control, pigmentation, excretion, vitamin D production, storage (fat, water, electrolytes, vitamins), as an indicator of internal disease, and is the first lines of defense for infectious agents entering the body. In addition, it contains adnexal tissues that produce hair, claws, and several different types of glandular secretions.

The skin has two basic layers, the epidermis and dermis. The epidermis is the thinner protective outer layer, that itself is divided into five distinct levels. The most important function of the epidermis is to produce the protective horny layer that we see. The deeper and much thicker dermis is there to support and nourish the epidermis. While the secretory glands lie within the dermis, they are actually deep extensions of the epidermis. These include apocrine and eccrine sweat glands and sebaceous glands. There is one apocrine sweat gland associated with each hair follicle. They produce a white, proteinaceous, odorless milky fluid. Skin odor is often the result of bacteria growing in this secretion. Milk is produced by specialized apocrine sweat glands. Eccrine sweat glands, that are vital to cooling for thermal regulation in man, only occur in the footpads of dogs and appear to have no thermoregulatory function in dogs.

The sebaceous glands empty through short ducts into the hair follicles. There may be one to many in any given hair follicle. They produce an oily secretion composed mostly of cholesterol, waxes and fatty acids. This secretion keeps the horny layer of the epidermis soft and pliable, helps retain moisture in the skin and provides the glossy sheen to the hair. The production can be increased by testosterone and decreased by estrogen.

What we have commonly called sebaceous duct cysts are more correctly follicular cysts. True sebaceous duct cysts are extremely rare in the dog and rarely exceed 1 cm diameter. The true sebaceous cyst forms when the duct emptying the individual cyst becomes obstructed. This can occur when there is a degeneration of the hair follicle, trauma or by cystic changes in the duct or cells of the individual gland. The secretory lining continues to produce the sebum. Since it has nowhere to go, it accumulates in the gland, resulting in the palpable enlarging cyst.

The follicular cyst forms when the hair follicle becomes plugged. The sebum from the sebaceous glands accumulates in the hair follicle. Depending on the depth of the individual follicle in the skin, the cyst may be firmly attached to the dermis and move with it or be under the skin and move independently. They can range from a few millimeters to two inches in diameter. When incised, a grayish white cheesy material resembling toothpaste can be expressed. While clinical appearance is highly suggestive of the presence of a follicular cyst, microscopic examination of the wall is needed to differentiate these from other cysts and tumors. The cyst lining is comprised of a specific cell type, stratified squamous epithelium. If the cyst is sectioned intact, it will contain concentric rings of keratin, along with the cholesterol and waxy matrix. If ruptured, there will be a specific type of inflammatory response in the surrounding tissues from the highly irritating debris. The best treatment is surgical excision. If surgery is not practical, incising the cyst wall and expression of the contents will give temporary relief.

The intracutaneous cornifying epithelioma, more correctly called keratoachanthoma or infundibular keratinizing acanthoma, is a rare benign tumor of dogs, accounting for 2-3 percent of canine skin tumors. They occur most frequently on the back, the neck, the thorax and limbs of males under five years of age. They tend to be solitary or in small numbers. While their exact cause is unknown, purebred dogs seem to be most likely to get this type of tumor and occur as a generalized form most commonly in the Norwegian Elkhound and Keeshond, occasionally in the German Shepherd Dog and Old English Sheepdog. The solitary form has been seen in the Collie, Lhasa Apso and Yorkshire Terrier. Most of these tumors are 0.5 to 4 cm diameter and commonly have a pore opening to the skin surface. They may be firm to fluctuant and vary in depth in the dermis or subcutaneous tissues. The deeper tumors may not communicate with the skin surface and may be confused with a cyst or other type of tumor. While they are not invasive or metastatic, the generalized form may produce up to 50 tumors a year throughout the life of the dog.

The best way to diagnose this tumor is by examination under a microscope by a trained pathologist. They must be differentiated from other types of tumors (such as melanomas and mast cell tumors), cysts, papillomas, etc. The pathologist will see a laminated keratin filled crypt that has an opening to the skin surface. The wall is thick, complex and folded with distinctive cells that differentiate it from other tumors and cysts.

The most successful treatment is surgical removal. Oral administration of retinoids (isotretinoin {Accutane}) have been helpful in reducing the numbers of new tumors in some dogs with the generalized form. The drug may have to be given for three to four months before benefit is seen and must be continued, at least intermittently, for the life of the dog. Other forms of treatment, such as anticancer drugs and autologous vaccines, have been ineffective in the dog. Some of these tumors will resolve spontaneously. Bathing with a sulfur/salicylic shampoo (such as Sebolux) may help keep the skin opening from clogging with the keratin debris so the tumors don't enlarge as quickly, but this has not been scientifically proven.