Skin

Introduction

The skin is the largest organ in the body, both in weight and surface area. It shows significant regional variation, with the thickest skin being found on the soles of the feet while the thinnest is the delicate skin on the upper and lower eyelids; some of these variations are illustrated and discussed later in this chapter.

The skin is the external body surface and provides protection against a wide variety of external threats, including mechanical, water loss, biological, ultraviolet light and chemical. The most frequent mechanical insult is the frictional and shearing forces experienced by the soles and ventral aspect of the toes in walking and, to a lesser extent the palms and ventral aspects of the fingers during use of the hands. In these areas, skin structure is adapted to resist these shearing forces (see Fig. 9.20 ).

The skin provides moisture control , providing a barrier against both excessive water loss and wetting. It resists bacterial and fungal invasion; bacteria and fungi do live on the skin surface but cannot penetrate into underlying tissues unless the skin is breached. It is rich in antigen-presenting cells ( Langerhans cells ) and, when breached, an immune response against any foreign antigen is readily initiated.

Skin pigmentation from melanin provides protection against ultraviolet (UV) radiation, and exposure induces increased pigmentation (tanning). Skin has a metabolic function, namely the synthesis of vitamin D ₃ (cholecalciferol) by the action of UV light on the precursor, 7-dehydrocholesterol. Cholecalciferol is further processed in the liver and kidney to produce the active agent 1,25-dihydroxycholecalciferol, which is important in Ca ²⁺ metabolism and bone formation. Adequate levels of UV exposure are needed to ensure that enough vitamin D is synthesised. Individuals with darker skin (more melanin pigment) require more UV exposure to ensure adequate vitamin D levels.

Skin is important in thermoregulation . Adjustments to blood circulation through skin, particularly extremities (hands, feet and ears), provides for both heat conservation and heat loss. The production of a watery secretion by skin eccrine glands, known as sweat , and its subsequent evaporation, is a major heat-loss mechanism. In humans, body hair is scanty and so provides only minimal heat conservation; however, subcutaneous adipose tissue can provide some heat conservation.

The skin is the largest sensory organ in the body, containing a range of different receptors for touch, pressure, pain and temperature (see Ch. 7 ). As a regional variation in structure, sensory receptors are most numerous in skin which has the most physical contact with solid objects in the environment, such as soles and palms, fingers and toes (see Fig. 7.24 ), or with specialised sexual functions.

Hair and nails are specialised components of skin. In humans, hair mostly defines sexual differences, sexual maturity and age; in most other mammals, it provides protection against cold in the form of fur. Nails provide physical support for finger tips and toes and can serve as tools.

The overall appearance of the skin (condition, pigmentation, hair and nails) cannot be underestimated, both clinically as an important indicator of health and in human behaviour.

Rapid repair of injuries to the skin such as lacerations (cuts and tears) and other wounds is critical to minimise infection risks and, while this happens in all tissues, this can rightly be seen as a critical skin function.

Skin Structure

The skin has three main layers:

• The epidermis is a continuously proliferating stratified squamous epithelium which produces a non-living surface layer of the protein keratin , with associated lipid which is in direct contact with the external environment and is constantly shed.

• The dermis consists of fibrous and fibroadipose tissue which supports the epidermis, both physically and metabolically. It contains blood vessels, nerves and sensory receptors.

• The subcutis or hypodermis or panniculus is the layer beneath the dermis and usually consists of adipose tissue with supporting fibrous bands ( septa ). This layer contains the larger vessels which supply and drain the dermal blood vasculature.

In addition, there are the specialised skin structures and adnexae ( appendages ) such as nails , hair follicles , sebaceous glands , eccrine (sweat) glands and apocrine glands . The adnexae mainly occupy the dermis and variably the superficial subcutis. They arise as downgrowths from the epidermis into the dermis during embryological development.

Melanocytes

Melanocytes produce the pigment melanin which is responsible for skin and hair colour. The pigment exists in various forms from yellowish brown to black and has a protective function against ultraviolet light. Melanin is synthesised from the amino acid tyrosine by melanocytes within specific cytoplasmic organelles called melanosomes . These are transferred to the keratinocytes through a complex network of melanocyte cytoplasmic processes; these processes can be seen in electron micrographs of epidermis, running in the narrow spaces between keratinocytes (see Fig. 9.3 ). Within keratinocytes, melanosomes usually form a cap sitting over the nucleus.

Melanocytes are present as separated individual cells in the basal layer of the epidermis and are more numerous in areas which are more exposed to light. There is no great difference in numbers of melanocytes between white- and dark-skinned people, but they are considerably more synthetically active in darker-skinned people. Melanocytes can be stimulated into producing more melanin by increasing exposure to UV light. This may produce a socially desirable suntan, but forced stimulation of melanocytes has its draw-backs and can be associated with the development of malignant tumours (see textbox ‘Disorders of melanocytes’ ).

Innervation and Nerve Endings of the Skin

The skin has both an efferent and an afferent nerve supply. The efferent (outgoing from brain) supply consists of non-myelinated fibres from the sympathetic component of the autonomic nervous system. It supplies the blood vessels in the skin and is responsible for vessel diameter and hence blood flow. It also provides a supply to the skin appendages, particularly to arrector pili muscles and the eccrine sweat glands. The afferent nervous system (ingoing to brain) subserves sensation and comprises both myelinated and non-myelinated fibres. It is responsible for transmitting impulses from the various sensory nerve endings to the central nervous system, and with them cutaneous sensation. The sensory nerve endings in the skin are in the form of both free nerve endings and specialised encapsulated nerve endings, the ‘capsules’ being modifications of Schwann cells; these specialised nerve endings in the skin are Meissner, Pacinian and Ruffini corpuscles and are illustrated and discussed in more detail in Ch. 7 .

Free nerve endings (see Fig. 7.24 ) may be myelinated or non-myelinated and are mainly responsible for pain and itch sensations and detecting temperature. They occupy the papillary dermis and send twigs into the epidermis where some of them associate with Merkel cells ( Fig. 9.8 ); this combination acts as a slowly adapting mechanoreceptor. Free nerve endings also ramify around hair follicles.

Meissner corpuscles (see Fig. 7.24 ) are rapidly adapting mechanoreceptors responsible for touch sensation. They are particularly prominent in the papillary dermis of the pulps of the fingers and toes and soles and palms.

Pacinian corpuscles (see Fig. 7.25 ) are responsible for detection of deep pressure and vibration. In the skin, they are usually found deep in the subcutis, singly or in small clusters, being particularly numerous in the palms and soles.

Hair and Nails

Skin appendages first develop in the second trimester of intrauterine development as simple downgrowths of the surface epithelium (epidermis) into the developing subepithelial layers of mesoderm which will eventually become dermis and subcutis. The skin appendages include hair follicles, sebaceous glands, eccrine glands, apocrine glands and nails (fingers and toes).

Hair is produced in follicles in association with sebaceous glands and a smooth muscle bundle ( arrector pili ); these can be called pilosebaceous follicles or units . Hairs are long, thin, cylindrical shafts composed of keratin; hair shafts have a surface cuticle composed of a single layer of flattened keratin scales. This covers a cortex of keratin forming the bulk of the hair. Large hairs may have a central medulla.

In mammals, the function of hair and fur is thermoregulation, particularly heat conservation. Hair also serves a display function, providing colour and shape. The structure of the hair follicle is complex ( Fig. 9.10 ). Hair growth is cyclical, with three phases: a long phase of active growth ( anagen ), a short phase of involution ( catagen ) and a short inactive involuted phase ( telogen ). The growth cycle of hairs varies from site to site: scalp hair follicles have an anagen growth phase of more than 2 years and a short telogen resting phase of a few months; correspondingly, scalp hair can grow to a great length. Pubic hair, coarse trunk hair, eyelashes and eyebrows have a short growth phase (anagen) and a relatively long resting phase (telogen), thereby limiting hair length at these sites.

In infancy, childhood and in adult females, body hair is fine and soft and is known as vellus , in contrast to the coarser hair of the scalp, which is known as terminal hair . Male sex hormone production at puberty stimulates development of terminal pubic and axillary hair in both sexes and the replacement of vellus hair with terminal hair on the mature male body.

The structure of hair follicles depends on the type of hair being produced: follicles of the scalp and other terminal hairs tend to be long and straight, whereas those which produce fine, downy vellus hairs are relatively short. Curly hair may be produced by curved follicles or follicles in which the hair bulb lies at an angle to the hair shaft.

Contraction of the arrector pili smooth muscle makes the hair stand up, a response called ‘goose flesh’, in a thermoregulation-related response.

Skin Glands

The skin has a range of different glands including sebaceous, eccrine and apocrine glands. Sebaceous glands occur in two forms. The majority are associated with hair follicles and develop as lateral protrusions from the hair follicle near the junction between its upper third ( follicular infundibulum ) and lower two thirds. Sebaceous glands secrete a mixture of lipids called sebum which may provide some waterproofing of the skin surface and hair shafts; the sebum is secreted into the hair follicle ( Fig. 9.11 ). At some sites in the skin (areolae and nipples, labia minora of vulva, eyelids), the sebaceous glands are independent of hair follicles and open directly onto the skin or mucosal surface.

Each hair follicle has an arrector pili muscle consisting of a bundle of smooth muscle fibres. These insert at one end into the sheath of the follicle just below the sebaceous glands, and at the other end into the dermal papillary area beneath the epidermis. Each hair follicle and its associated arrector pili muscle and sebaceous glands is known as a pilosebaceous unit .

Eccrine glands are found throughout the skin and are essential for thermoregulation through the production of sweat, opening onto the skin surface. Apocrine glands , while similar in architecture to eccrine glands, have a very limited distribution and connect to the follicular infundibulum, the superficial part of pilosebaceous (hair) follicles, and have a possible function in producing odour.