The epidermis is the thin uppermost layer of the skin (<1mm thick). Keratinocytes are the predominant cell-type of the stratum Basale. These cells migrate up the epidermis and become more thinner & more mature during this migration. The degree of maturity of the keratinocytes define each of the 5 levels of the epidermis. The epidermis replaces itself every 4-6 weeks.
The 5 layers of the epidermis, from top to bottom, are:
- 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
- Stratum Lucidum
- Stratum Granulosum – The middle of the five layers of the epidermis, consisting of Keratinocytes that are flat and have no nuclei. The Breslow thickness, measured from The Stratum Granulosum, defines the depth of penetration of a melanoma.
- Stratum Spinosum
- Stratum Basale – Deepest part of the epidermis where cells (keratinocytes) replicate and produce more keratinocytes that migraine upwards. This layer also contain melanocytes.
Melanocytes are the pigment-producing cells that found in the Stratum Basale. 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.
Rete Ridges are the downward projections of the epidermis into the dermis.
The Dermis is the thickest part of the skin, in two sections:
- The upper Papillary dermis
- The lower Reticular Dermis.
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 is found in:
- lentigo (age spot or simple freckle)
- freckle on the lip
- pigmented solar keratosis,
- cancers (pigmented bowens, pigmented BCC)
- Ink spot lentigo
- Seborrhoeic Keratosis
Melanin taken up by melanophages in the dermis is found in Lichen planus like keratosis (LPLK) – these lesions look like flat pink or grey lesions.
Some other Terms used in skin cancer pathology reports
Acanthosis describes thickened Epidermis eg. Clear Cell Acanthoma / Large Cell Acanthoma.
Full thickness atypia = abnormal keratinocytes that fill the whole epidermis (eg. Intraepithelial carcinoma).
Parakeratosis describes nuclei that are retained in the keratinocytes of the Stratum Corneum.
Melanocytic Proliferation describes the situation where there are more Melanocytes than normal – for example, moles & melanoma.