Chapter 1

Skin biology and related science

Diagnosis of skin cancer relies almost entirely on clinical skills—either those of the dermatologist or those of the histopathologist. Both rely on being able to recognise and classify morphological patterns on the basis of training and experience (‘pattern recognition’). However, I believe an understanding of the ‘why and how’ of skin cancer is also important even if I cannot prove it is essential to making a correct diagnosis — in any case patients increasingly expect doctors to be able to answer the ‘why’ question.

In this chapter I describe the normal anatomy and physiology of skin, with an emphasis on ultraviolet radiation and how the skin responds to ultraviolet radiation. This in turn requires a rudimentary knowledge of pigmentation, DNA repair and one or two other areas of skin physiology. The focus throughout is on learning only what is necessary to understand and make sense of what comes later.

Epidermis

The epidermis sits on and is separated from the dermis by the acellular basement membrane. The epidermis is a stratified squamous epithelium comprising several different cell layers and cell types. Most (>90%) of the cells in the epidermis are keratinocytes, ectodermal derived cells, so named because they produce a range of proteins called keratins. Also found in the epidermis are melanocytes, neural crest derived cells that produce melanin, and Langerhans cells, bone marrow derived antigen presenting cells. Whilst both melanocytes and Langerhans cells are dendritic (star shaped) in shape they have different origins and functions [1].

The keratinocytes of the epidermis are organised into several cell layers one or more cells thick that can be distinguished morphologically when examined with light microscopy (Figure 1.1). The single cell layer immediately adjacent to the basement membrane is called the basal layer (in older texts stratum basale). The cells in this layer are more cuboidal than those closer to the surface, and comprise the proliferative compartment of the epidermis. It is not therefore unusual to see mitoses in this cell layer or in the cell layer immediately above the basal layer as cells divide and daughter cells move upwards and outwards towards the the skin surface.

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The cell compartment above the basal cell layer is called the prickle or spindle cell layer (stratum spinosum) and is comprised of multiple cell layers containing spindle shaped cells that have a characteristic prickle pattern on routine histopathology. This prickle shape is due to the adhesion contacts between cells called desmosomes, being accentuated when specimens are dehydrated for routine tissue processing for histopathology [2]. The layer above the spindle cell layer is called the granular cell layer (stratum granulosum) and is so named because of the abundance of intracellular granules. Outermost to the granular cell layer is the stratum corneum (horny cell layer). This is the outermost compartment of skin, the layer you can see and feel. It is made up of a variable number of dead layers of anucleate keratinocytes that have undergone a type of programmed cell death.

The changes in shape visible within the epidermis reflect the changes in a wide range of cellular components and behaviour as the cells from the basal cell layer divide and differentiate. This process of differentiation is characterised by changes in gene expression leading to changes in protein expression and lipid biosynthesis leading ultimately to the multiple layers of dead anucleate horny cells that line the outermost layer of skin interspersed with lipid [3].

Missing from the above account are details of the role played by the appendageal structures such as eccrine sweat glands (shown in Figure 1.1 to the right of the hair follicle) and the pilosebaceous unit (i.e. the hair and the associated sebaceous gland), all of which are epidermal in origin but are partially located at the level of the dermis (Figure 1.1). The potential importance of these appendageal structures for skin cancer is not obvious and is explained below.

Stem cells and the importance of the cutaneous appendages

If skin is traumatised such that only the epidermis is removed, then scarring does not occur. An example of such an injury would be the removal of the top of the blister that follows a long walk in a pair of unfamiliar shoes. Even though the outermost epidermis has been removed skin is still able to regenerate itself. If the level of damage is slightly below the epidermal-dermal interface but above a critical area roughly half way down the hair follicle at the bulge region then scarring is still unlikely, and regeneration can still take place. However if the damage is towards the bottom of the hair follicle then damage is likely to be followed by scarring. The implication is that the bulge area contains a pool of stem cells that are able to repopulate the damaged areas to reproduce normal skin. Stem cells for both keratinocytes and melanocytes are thought to reside in the bulge region.

The relevance of the above for skin cancer is that whilst the exact target cell for the various types of skin cancers are unknown, it is at least conceivable that some tumours arise from cells that normally reside close to the appendageal structures [4]. As we will see later, just because a cancer is called (for instance) a basal cell carcinoma does not mean it is derived from basal cells.

Pigmentation and the attraction of nakedness

Ultraviolet radiation (UVR) is harmful to living organisms because it is mutagenic, that is it damages the structure of DNA leading to inheritable mutations. Even single celled organisms such as bacteria have elaborate systems to repair UVR induced DNA damage. (The exact types of UVR and their various properties will be dealt with later, but for present purposes we will treat UVR as a homogenous entity that damages DNA). Even in our recent evolutionary past UVR has played a major role in shaping our health and development as a species.

Our primate ancestors were covered in dense body hair. Dense hair as anybody who has gone bald later in life realises (to their cost) affords excellent protection against UVR. You might have memories of excess sun exposure as a child and burning of your shoulders and cheeks, but it is unlikely unless you shaved your head that you burned on your scalp [1]. Irrespective of the colour of your skin, hair (even white hair) protects skin against UVR simply by blocking the path of UVR photons.

An earlier period of climate change meant however that our ancestors were forced to move from the African forest to the savannah, their traditional diet was no longer available and instead they had to become adapted to being able to hunt and chase other animals over long distances and time periods. This required changes in physiology particularly the ability to lose body heat efficiently during bouts of sustained exercise. Eccrine sweating is critical for this—humans are better able than any other mammal to lose body heat via eccrine sweating

[2]. Sweating however is most efficient if there is no overlying hair to limit evaporation, and therefore natural selection favoured hair loss for most body regions. The disadvantage of this evolutionary strategy should now be obvious to you. Once we lost dense body hair as a blocker of harmful UVR photons another defence mechanism was necessary. The solution evolution seized upon was to allow the pigment melanin to take on the role of providing photoprotection (that is protection against UVR).

Melanin: shifting from the visible to the invisible

Melanin pigments play widespread roles in nature acting as either camouflage (i.e. concealing oneself) or by contrast, allowing exhibition of biological fitness as in sexual display (i.e drawing attention to oneself) (Figure 3). The basis for both these contrasting roles is of course that melanins have particular physical properties within the visible part of the electromagnetic spectrum (i.e. humans perceive electromagnetic radiation between 400nm and 800nm as ‘light’ of varying colour).

Radiation of less than 400nm is referred to as ultraviolet radiation (UVR) and humans are not able to sense this radiation directly. Melanin pigments absorb well in this region and therefore can protect against cellular damage from UVR [3]. Melanin produced by melanocytes is passed into the surrounding keratinoctyes where it protects against the harmful photons of UVR. We will learn more about this in the next section.

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Melanin and melanocytes

Melanocytes are neural crest derived cells that migrate into skin at around 10 weeks of gestation. Melanocytes are found in the basal layer where they account for around 1 in 10 of the cells resting on the basement membrane (with the other 9 cells being keratinocytes). They are dendritic cells, that is they have lots of neurone like processes (‘dendrites’) that reach out and interdigitate with the surrounding cells.

The principal function of melanocytes in skin is to produce melanin . Although melanin is often referred to as though it were a single molecule it is a collection of chemically related entities all synthesised via a number of steps controlled by a range of enzymes from the amino acid tyrosine [5].

There are two broad classes of skin melanin, eumelanin which is brown or black, and pheomelanin which is red or yellow. The variation in human hair colour (and coat colour in other mammals such as cows and dogs) is entirely accounted for by variation in the amounts of eumelanins and pheomelanins. Similarly most of the variation we observe in human skin colour from people with different genetic ancestries is accounted for by variation in the amount and type of melanins produced [6].
Legend

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Melanin biosynthesis involves a number of chemical stages with products that could be toxic to the cell. It is therefore carried out within a lysosomal type vesicle called the melanosome. The mature melanosomes are then passed along the dendritic processes of the melanocyte and into the surrounding keratinocytes [7]. Each melanocyte via its numerous dendritic processes can communicates with around 30 keratinocytes. The colour of skin [8] is largely due to melanin in the keratinocytes rather than the melanocytes — it is just that the melanocytes synthesise the melanin before passing it to the surrounding keratinocytes.

Remember not only are there more keratinocytes than melanocytes in the basal layer but, as the keratinocytes divide and differentiate and move upwards, they will still contain the melanin that was passed to them when they were in the basal layer [9]. Whilst melanin is encapsulated within melanosomes, the position of melanosomes within the keratinocyte is not random. In the basal layer in particular melanosomes are clumped superiorly as protective crescents—little cellular ‘sun-hats’ that protect the DNA of the nucleus. As keratinocytes differentiate and move upwards this focussing is lost, with the melanin being more evenly distributed throughout the cytoplasm [9].

Melanin as a sunscreen

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Constitutive and facultative pigmentation

Observation of your own skin will reveal that the degree of pigmentation is not constant at all body sites. For instance the backs of the hands are darker than the palms, the buttock is usually lighter than the shoulders and the outer forearms darker than the inner forearms. In accounting for these difference we have consider two factors.

The first of these is constitutive pigmentation, which refers to the degree of pigmentation evident without any exposure to UVR. Different body sites have different constitutive skin colours just as different people have different constitutive skin colours. To some degree evolution has dictated that areas that receive more UVR (such as the outer forearm compared with the inner forearm) are darker; or that areas such as the palms and soles do not require much pigment because the thick stratum corneum as at these sites protects against UVR)[13]. Facultative pigmentation by contrast refers to the change in pigmentation as a result of exposure to UVR — this is commonly known as tanning.

[13]. Facultative pigmentation by contrast refers to the change in pigmentation as a result of exposure to UVR—commonly known as tanning.

Variation in constitutive pigmentation between people (and perhaps at different body sites) is a key determinant of susceptibility to most forms of skin cancer and we will return to it shortly. At this stage we should also note that there is some relation between constitutive pigmentation and the ability to tan. People with very pale skin constitutively—think of a red headed Scot for instance — are poor at acquiring a tan, whereas those with higher degrees of constitutive pigmentation are generally able to increase their amount of pigmentation (i.e. tan) to a much greater degree.