The skin is a remarkable structure upon which we are completely dependent to protect the internal organs from the external environment. Dermatology is the study of all the skin and its adnexal structures.
- Skin disease is common, accounting for 25% of patient visits to a family doctor
- There are more than 2000 specific skin diseases, and many more subtypes are known
- Skin disease is not commonly associated with mortality, but there is a high association with morbidity
- Skin disease can have serious psychosocial effects
- Skin lesions may be the presenting feature of an underlying systemic illness
Sound knowledge of the normal function and anatomy of the skin enables us to understand how symptoms and physical signs relate to disease. The clinical signs seen in the skin are a consequence of the underlying pathological process, so being able to interpret what you see will lead to the diagnosis.
1.1 Functions of the skin
The anatomy of the skin reflects the many functions that it has to perform; at certain sites the skin is anatomically subtly different to allow for a specific role. In general the skin allows for a stable internal environment because it:
- acts as a physical and immunological barrier, protecting the body from chemical, antimicrobial, heat and radiation damage
- regulates body temperature by evaporation of sweat in warm weather and constricting blood vessels and contracting arrector pili (goose bumps) muscles in cold weather
- maintains fluid balance by excreting water and salts (sweat) and preventing water loss from the body due to close contact of keratinocytes in the epidermis
- facilitates awareness of temperature, pain, touch and vibration
- acts as a means of communication in terms of our appearance, odour (apocrine gland secretion) and skin colour
- is responsible for synthesis of vitamin D and storage of fat in the subcutaneous tissue
- forms nails, which protect the digits, enable scratching and add to dexterity
1.2 The biology of normal skin
The skin is the largest organ in the body and is composed of three layers, each contributing to the special functions of the skin (Figure 1.1):
- epidermis
- dermis
- subcutis
The epidermis (Figure 1.2) is the outermost part of the skin and is in a constant state of regeneration. The thickness of this layer is site-specific and can range from 0.03 mm on the eyelids to 1.5 mm on the soles of the feet.
The main cell types of the epidermis are:
- keratinocytes
- melanocytes
- Langerhans' cells
- Merkel's cells
Keratinocytes
These are organised into layers, based on distinct structural features. The basal layer consists of a single layer of keratinocytes, which are the active stem cells. These cells proliferate and commit daughter cells to terminal differentiation as they move to the skin surface. The average transit time for a cell to travel from the stratum germinativum to the stratum corneum is 40–56 days. The ‘brick wall' structure of the epidermis is provided by desmosomes, which hold the cells together.
The distinct layers of the epidermis (Figure 1.2) are as follows. Stratum germinativum (basal cell layer) This is composed of columnar epithelium cells arranged on their short axis. The cells are the active stem cells, where cell division starts; the cells then undergo terminal differentiation.
Figure 1.2: ‘Brick wall' structure of the epidermis: the keratinocytes are like the bricks and the lipid mixture surrounding them is like the mortar.
Stratum spinosum (prickle layer) Column-shaped keratinocytes move into the stratum spinosum from the stratum basale. They become polygon-shaped and start to synthesise keratin. The spiny appearance of the cells is due to desmosomes.
Stratum granulosum (granular layer) Here the cells have a granular appearance due to keratohyalin granules. Lipids are produced at this layer to form a water barrier. In the transition to the next layer the cells lose their nuclei and organelles.
Stratum corneum This is formed of flattened non-viable corneocytes, usually several layers thick. On the palms and soles this layer is thicker, whereas on mucous membranes it is absent.
Melanocytes (cells producing pigment)
These are dendritic cells derived from the neural crest that are also found in the basal layer at a ratio of 1:10 to basal keratinocytes. Melanocytes synthesise melanin, which is packed into melanosomes and transported to basal keratinocytes (Figure 1.3). The melanosomes form a ‘melanin cap', which protects the basal keratinocyte from UV-induced DNA damage.
Colour of the skin
This is determined by a number of factors. Fitzpatrick's skin type (Table 1.1) is a numerical classification of the colour and the skin's response to UV light. Skin colour is due to the amount of melanin produced by the melanocytes and distributed to the keratinocytes.5
The most common form of biological melanin is eumelanin, a brown–black polymer derived from the amino acid tyrosine; it is found in the brown colour in skin and hair. Pheomelanin, which has a pink-to-red hue, is found in particularly large quantities in red hair, the lips, nipples and glans penis.
Langerhans' cells (antigen-presenting immune cells)
These bone marrow-derived cells have a role in skin immunity: recognition, uptake and presentation of antigens to sensitised T cells. They are dendritic cells found in the mid-epidermis.
Merkel's cells (touch receptors)
These are scarce, small, round cells which transmit sensory information in the skin to the sensory nerves.6
Figure 1.4: Filaggrin mutation note the dry scaling of the skin due to loss of an effective barrier.
Dermoepidermal junction
This is the meeting point of the dermal and epidermal layers of the skin by means of the basement membrane zone (BMZ) (Figure 1.5). This zone comprises a network of molecules linking the keratin filaments of the basal keratinocytes to the collagen fibres in the superficial dermis. The main functions of the BMZ are to provide adhesion between the epidermis and the dermis, and to provide a means of communication between cell types.
The basal layer of the epidermis is held and anchored by hemidesmosomes and keratin intermediate filaments. The basal keratinocytes rest on the basal lamina. This serves as a permeable barrier for communication between cells, structural support and a template for wound healing.
Figure 1.5: Basement membrane zone, with the connecting fibres from the basal keratinocytes to the dermis.
The dermis provides the nutrition and support to the epidermis. It is between 1 and 4 mm thick, depending on age and body site. It is divided into two layers: the papillary dermis, which is in contact with the BMZ, and the reticular dermis beneath it. The dermis consists of collagen (90%), elastin fibres (10%) and ground substance (glycoproteins and proteoglycans), and a cellular component of fibroblasts, mast cells, plasma cells and histiocytes. Collagen is a triple-stranded helical molecule, coiled and cross-linked to form microfibrils. The microfibrils are arranged into bundles, which are further organised into collagen fibres. Elastin is generally confined to the lower part of the dermis, where the fibres are arranged in parallel. Blood vessels, lymphatic channels, Meissner's corpuscles (responsible for pressure sensation) and pacinian corpuscles (for sensing vibration) are also found within the dermis.
Figure 1.6: Blistering of the skin in a child with epidermolysis bullosa, with separation of the epidermis and dermis.
These are the predominant cell type in the dermis; they are responsible for the synthesis and degradation of the connective tissue, and are the key cells in wound healing. Figure 1.7 shows abnormal overactive wound healing, which can lead to the formation of keloid scars.
Mast cells
These are histamine-containing cells located near dermal blood vessels. They are responsible for immediate-type hypersensitivity reactions.
Glomus bodies
These consist of an arteriovenous shunt surrounded by a capsule of connective tissue. They are found in large numbers in the fingers and toes. Their role is to shunt blood away from the skin surface when exposed to cold temperatures, to prevent heat loss.
Adnexal structures
The pilosebaceous follicles (hair and sebum) and the eccrine (sweat) glands are structures embedded within the dermis, but are derived from and are continuous with the epidermis.
The pilosebaceous unit (Figure 1.8) consists of the hair follicle and the sebaceous glands, responsible for the production and secretion of sebum.
- The eccrine glands are not connected to the hair follicle (see Figure 1.1) and open directly on to the skin surface. They function to regulate body temperature by excreting sweat to allow cooling.
- Apocrine glands are larger than eccrine glands. They release region-specific secretions that bacteria act on and are commonly found in the axilla and genital skin.
Structure of hair
Humans have up to five million hairs over the surface of the skin. Most of this is vellus hair, which is a fine short hair distributed over most of the body. Terminal hair is the longer and coarser hair that is typically found on the scalp, and in the axillae and the pubic area.
The structure of the hair follicle is shown in Figure 1.8. Hair grows from a highly active matrix within the hair bulb, moving along the inner root sheath. Each follicle is made up of three parts: the cortex and medulla, which have pigment cells and are responsible for the colour of the hair, and the cuticle, which is keratinised and provides the strength of the hair.
The hair's course passes the sebaceous gland, and the lipid-rich sebum lubricates it before it exits the skin. The arrector pili muscle inserts into the hair bulb and is responsible for contraction in the cold (‘goose bumps'). An important structure within the hair follicle is the hair bulge, where the epithelial and melanocytic stem cell populations reside. This is important in generating hair follicles and sebaceous glands, and re-epithelisation during wound healing (Figure 1.9).
Figure 1.9: A wound bed that is allowed to heal by granulation will show within it islands of re-epithelialisation originating around the hair follicles. ① Island of re-epithelialisation in ulcerated skin.
Figure 1.10: The hair growth cycle, showing the changes from anagen (active growing hair), catagen (rest phase of the hair) and telogen (shedding of the hair).
- Anagen: hair is actively growing (4–7 years).
- Telogen: there is no growth of hair or any activity in the hair bulb (a few months). This is a resting phase until the cycle is repeated.
As the new anagen hair germinates within the same follicle, it pushes out the telogen hair. Each hair follicle is independent and goes through the growth phases at different times.
Subcutis
This layer is immediately below the dermis and consists of predominantly adipocytes (fat cells) organised into lobules separated by septa, used mainly for fat storage. It also consists of fibrous bands anchoring the deep fascia, and elastin and collagen fibres attaching it to the dermis. It contains blood vessels, lymphatics and nerves en route to the dermis.
Regional skin variation
Thickness of the epidermis varies; at glabrous sites (non-hair-bearing skin) the stratum corneum is up to 10 times thicker than at non-glabrous sites.
Within non-glabrous skin the hair follicle type and density can vary between different body sites and through different stages of life, e.g. terminal hair follicles may give way to vellus hair follicles in the scalp. In areas such as the axillae, in addition to eccrine glands, apocrine glands are present.
Structure of nail
The nail consists of the nail plate, bed, matrix, proximal and lateral folds, and hyponychium (Figure 1.11). The nail matrix is a wedge-shaped structure that contains highly specialised epithelium which produces the cornified cells of the nail plate. The nail plate is visible proximal to the nail fold and grows along the nail bed to the distal free edge of the plate. The nail bed is tightly connected to the nail plate and is continually keratinised to maintain that adhesion. Fingernails grow 0.1 mm/day and toenails 0.03 mm/day.12
The nail contributes to tactile sensation and acts as protection for the nail tip. There are glomus bodies in the nail bed and matrix; these are temperature-sensitive organs involved with skin thermoregulation.
1.3 Dermatological terminology
It is essential that the formal language of dermatology is used to describe, record and communicate examination findings accurately (see Chapter 2). A guide to the names of different types of skin lesions is shown in Table 1.2.
The underlying pathology correlates well with the physical signs seen in the skin, although it is imperative to take the pathology diagnosis in context with the clinical case, e.g. lichen planus and lichenoid keratosis would be histologically identical, yet the former presents as a widespread rash and the latter as an individual lesion.13
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Figure 1.12: Freckles are macular lesions. They are entirely flat and flush with the surrounding skin.
Figure 1.13: Papules are small and palpable. In this example the papules are clustered together on the chest and are itchy; the diagnosis is an insect bite reaction.
Figure 1.14: A plaque of Bowen's disease. Intraepidermal squamous cell carcinoma is often confused with psoriasis.
Figure 1.17: This patient has eczema herpeticum. Herpes simplex virus complicating eczema. Note the vesicle [1] and multiple punched out erosions coalescing together [2].
