This article was last reviewed and updated on May 9, 2019
by Pavandeep Gill, MD and Allison Osmond, MD FRCPC
Seborrheic keratosis is a non-cancerous disease that starts in the skin.
It is a very common condition that is seen more often as people age.
The abnormal tissue is sent to a pathologist to confirm the diagnosis and to exclude other conditions that can look like seborrheic keratosis.
The outside of our body is covered in skin. The skin is made up of different layers of tissue including the epidermis, dermis, and subcutaneous fat. The epidermis is the layer on the outer surface and it is what we see when we look at our skin.
The epidermis is made up of many kinds of cells including keratinocytes, melanocytes, Merkel cells, and cells of the immune system. The keratinocytes in the epidermis produce a material call keratin which makes the skin waterproof and strong and protects us from toxins and injuries.
What is seborrheic keratosis?
Seborrheic keratosis is a non-cancerous (benign) disease that involves the keratinocytes in the epidermis. When viewed through a microscope, the normal epidermis is made up of several layers of keratinocytes stacked one of top of another, like the bricks in a house. In seborrheic keratosis, there is an increased number of small, round, and flattened keratinocytes.
Small spaces called cysts filled with keratin are also seen in the epidermis. Pathologists call these spaces "keratin pearls". Your pathologist will examine your tissue sample carefully to make sure there is no sign of cancer or pre-cancerous disease.
Seborrheic keratosis is a very common condition and seen more often as people age.
Without a microscope, seborrheic keratosis may look like a raised or bumpy area of skin with a light tan or black colour. Seborrheic keratosis is often described as looking as if it was “stuck on” to skin. The size of a seborrheic keratosis can range from a few millimeters to a couple of centimeters and many patients have more than one. It may look similar to a wart or liver spot.
Variants of seborrheic keratosis
Pathologists divide seborrheic keratosis in groups called variants based on how the cells look under the microscope. Variants of seborrheic keratosis include:
Acanthotic – When examined under the microscope, the epidermis in this type of seborrheic keratosis is much thicker than in a regular seborrheic keratosis.,
Papillomatous - In this type of seborrheic keratosis, the cells at the surface grow in tall projections that pathologists call papillary.
Adenoid/reticulated - Thin groups of keratinocytes are seen extending into the deeper layers of the skin.
Clonal - All of the keratinocytes in this type of seborrheic keratosis have a very similar look as if they were all ‘cloned’ from a single cell.
Inverted follicular keratosis - Instead of growing up from the surface, the abnormal cells in this type of seborrheic keratosis grow downwards into the skin.
Inflammatory – In addition to the abnormal keratinocytes, lots of immune cells are all seen in this type of seborrheic keratosis.
Lichenoid - Many immune cell called lymphocytes are seen at the bottom of the epidermis, below the abnormal keratinocytes.
Desmoplastic – This type of seborrheic keratosis can look very similar to a cancer called squamous cell carcinoma. Although they look similar, desmoplastic seborrheic keratosis is still non-cancerous (benign).
Treatment is not usually necessary for seborrheic keratosis. They can be removed for cosmetic purposes or when the diagnosis is not clear without the help of a microscope.