What is Skin Pathology?
Skin lesions that are removed are sent to the pathologist and are reported a day or two later. The speciality of skin cancer pathology is a branch of pathology and skin pathologists are doctors who have specialised in that specific branch of pathology.
The epidermis is the thin uppermost layer of the skin (<1mm thick). The epidermis replaces itself every 4-6 weeks and is subdivided into 5 layers from uppermost to deep:
- Stratum Corneum – the uppermost layer that the skin sheds continually.
- Stratum Lucidum
- Stratum Granulosum
- Stratum Spinosum
- Stratum Basale – the deepest layer of the epidermis where the main action takes place
Melanocytes: Cells that are situated in the Stratum Basale (bottom most layer of the epidermis). There are around 15 keratinocytes to one melanocytes. Melanocytes produce melanin that is, in turn, taken up by keratinocytes. So one melanocytes supplies around 15 Keratinocytes with the melanin pigment. Abnormal melanocytes are the hallmark of melanoma.
Keratinocytes: Cells that form the main part of the stratum Basale. These migrate up the epidermis, and become more mature during this migration, becoming gradually thinner and resulting in different appearances of the five layers of the Epidermis. New keratinocytes replace the old ones. Abnormal Keratinocytes are the hallmark of Solar Keratosis, Intraepithelial Carcinoma & Squamous Cell Carcinoma.
Rete Ridges: The downward projections of the epidermis into the dermis.
Stratum Basale: Deepest part of the epidermis where cells (keratinocytes) replicate and produce more keratinocytes that migraine upwards. Also contain melanocytes.
Stratum Granulosum: The middle of the five layers of the epidermis, consisting of Keratinocytes that are flat and have no nuclei. The Breslow Thickness of a melanoma is measured from this layer.
Stratum Corneum: This is the uppermost layer of the Epidermis and consists of dead keratinocytes (cells) which are filled with keratin. The function of the stratum corneum is as a barrier.
Dermis: The thickest part of the skin and made up of the papillary dermis at the top and the reticular dermis underneath. The dermis is very thick on the back eg 7-8mm, and thin on the eyebrow eg 1mm.
Dermal Papillae: finger-like projections upwards of the dermis into the epidermis that also contains superficial dermal blood vessels
Merkel Cells: Found in groups in the Basal Layer of the epidermis. They connect with nerves from the dermis and help transmit touch sensation. Rarely develop into Merkel Cell Cancer.
Langerhans Cells: Found mainly in the Epidermis (also the upper part of the dermis) and are cells involved in immune function. They detect foreign cells such as bacteria and other antigens.
Pigment is normally caused by melanin.
Increased melanin in the keratinocytes & in the melanocytes: lentigo (age spot or simple freckle), freckle on the lip, pigmented solar keratosis, cancers (pigmented bowens, pigmented BCC), ink spot lentigo, Seborrhoeic Keratosis
Increased melanin in the dermis taken up by melanophages: Lichen planus like keratosis (LPLK) – these lesions look like flat pink or grey lesions.
Some other Terms used in skin cancer pathology reports
Acanthosis = Thickened Epidermis eg. Clear Cell Acanthoma / Large Cell Acanthoma
Full thickness atypia = abnormal keratinocytes that fill the whole epidermis (eg. Intraepithelial carcinoma)
Parakeratosis = nuclei are retained in the keratinocytes of the upper epidermal layer called the stratum corneum
Melanocytic Proliferation: More Melanocytes than normal – eg. naevi & melanoma