What is skin?

New medical students are frequently surprised to learn that, excluding the digestive tract, skin represents the largest organ of the human body and is responsible for such a diverse range of functions. Most individuals take their skin for granted, rarely considering its importance, unless forced to, perhaps by consequence of health or cosmetic reasons. In healthy individuals, it is responsible for securing the internal environment. It provides a barrier of protection against many external elements including heat, cold, pollution, and disease. While constantly maintaining its essential barrier function, skin also plays a vital role in the production of vitamin D and regulates the activity of several hormones. It is also important for determining physical appearances, and for providing a means to investigate the world and to facilitate communication with other beings and objects.

Understanding the function and structure of skin provides an insight into why many of today’s leading dermatologists are now recommending an organic approach to skin care.

About skin

The skin consists of two main layers: the epidermis on the outside, and the dermis underneath. The dermis is the main structure of the skin and while varying in thickness throughout the body, it performs the basic role of supplying nutrients and moisture to the epidermis at each location. The dermis contains a watery ground substance and a mixture of two important skin proteins called collagen and elastin. The word collagen is derived from the Greek word kolla, meaning glue. The type of collagen found in the dermis provides a structural adhesive for the skin and is produced by groups of cells known as fibroblasts. Fibers of the pliable protein, elastin, are distributed between layers of collagen to form a strong yet flexible matrix.

Specialist cells

Mixed throughout the dermis are the specialist cells which serve to guard against internal disruptions as well as those caused by external irritation. Mast cells produce chemical messengers like histamine that is released as part of the body’s defence reaction whenever allergens like pollen, smoke, dust, or chemical toxins are encountered. The macrophages are scavenger cells which facilitate the removal of foreign substances with the assistance of the leucocytes or white blood cells which contribute to the body’s immune response against bacterial infections.

Sebaceous and sweat glands

The dermis also contains the sebaceous and sweat glands. Sebaceous glands produce an oily substance known as sebum which serves to moisturise and protect the external layers of skin and hair. They are influenced by a hormone which causes the amount of sebum to increase during puberty, often resulting in excessively oily skin and the familiar acne caused by inflammation and blockage of the glands. With advancing age, the production of sebum slows, often leading to a drier skin.

The eccrine sweat glands play an important role in temperature regulation. When body temperature rises, they release fluid that cools the skin as it evaporates. During exercise in hot conditions, the body may release several litres of sweat every hour. At the same time blood is circulated through capillaries near the surface. The resulting heat exchange initiates a further cooling effect. If the body is exposed to extreme cold, blood vessels in the dermis will contract, diverting their normal blood supply to thermally protect the brain and vital organs.

Closely related to sweat glands are the apocrine glands which are located mainly in the armpits, groin, and chest. Along with fluids and electrolyte, they release hormone like substances known as pheromones which are likely to play some role in sexual attraction, bonding, and possibly the timing of female menstruation.

Receptors

From an anatomical perspective, a large section of the cerebral cortex is devoted to processing the sensory signals that are delivered by nerves via receptors located in the skin. There are special types of receptor called Meissner’s corpuscles. Lying at the dermal and epidermal junction, they are located in great numbers on the palms and soles of the feet. Vater-Pacini corpuscles lie deeper in the dermis and are used to detect sudden changes in pressure while the Merkel’s disks occur at the epidermis and can relay sensations of continuous pressure. In combination with nerve receptors attached to surface body hairs, these enable the range of touch sensations that are important for basic human activities such as emotional comforting, detecting danger and locating objects in space.

Outer layer

The epidermis begins with a layer of basal cells located at the dermal and epidermal junction. Initially, basal cells rest on a membrane where they grow and divide. After undergoing several divisions each basal cell transforms into a keratinocyte which undertakes a migration toward the outer surface. Requiring approximately twenty eight days to reach its destination, the keratinocyte begins to progressively synthesise filaments of a waterproof protein known as keratin. When the keratinocyte reaches the outer layer of skin, there is no further role than to provide a tough exterior coating. This outer, most visible layer of skin consists entirely of dead keratin cells that are woven together. The process continues indefinitely since the surface of skin is constantly worn away and must be replaced by the next generation of lifeless keratin.

Immune system

The immune system is also represented by cells within the epidermis. Langerhans cells provide a defence against certain types of environmental invaders. In combination with other scavenger and antigen presenting cells they limit the damage that may otherwise be caused by noxious substances, bacteria, viruses, and parasites.

Skin color

Skin pigmentation is determined by cells at the lowest level of the epidermis. Known as melanocytes, they produce melanin which influences the normal colour of each individual’s skin, eyes, and hair. The amount of melanin produced within the epidermis also affects the ability to tan darker in response to sunlight. With advancing age and cumulative exposure to sunlight there may be an increase in the size, number and darkness of blemishes that are caused by excessively active melanocytes clumped together in patches.

Physical barrier

The outermost section of the epidermis is called the stratum corneum. In direct contact with the external world, it is perhaps the most significant of all the physical barriers in the body. It is composed entirely of dead keratin cells that are arranged in interlocking columns. Its thickness varies across the body. Areas of high wear like the soles of the feet can be up to thirty times thicker than softer skin around the lips and eyes.

Despite its barrier function, there is constant exchange between the skin and the external environment. Medicinal ointments and other topical treatments will enter the body through sweat or sebaceous glands. It is understood, for example, that most steroid creams will transit via the sebaceous glands. This is determined by their need to be absorbed in a region with increased fat solubility. As well as medicines, the skin will absorb pollutant chemicals present in the atmosphere as well as many of the synthetic substances within cosmetic and personal hygiene products.

While healthy skin can inactivate or lessen the impact of some dangerous substances, this ability is compromised whenever the stratum corneum is excessively worn, dried out, or stripped of its protective coating of oily sebum. This may occur as a consequence of illness, excessive grooming, or prolonged stress.

Hair

Originating from follicles in the dermis, hair is a specialised appendage which, like the outermost layer of skin, consists of a keratin protein. When visible, it provides some thermal insulation and additional physical protection for the epidermis. Each hair follicle has a minute muscle that contracts with cold and, occasionally, physical stress. This causes the hairs to stand on end. The effect is to retard the flow of air across the surface, conserving body warmth by reducing the surface exchange of heat. In dogs and other animals the erect hairs will exaggerate physical size and may decrease the chances of them being attacked by a rival. Anthropologists often describe modern humans as the naked apes. Dermatologists and others familiar with the skins magnified appearance might question the lack of accuracy, since even visibly smooth areas of the body are covered with fine unpigmented velus hairs. Despite their diminutive scale, these hairs function as extremely sensitive touch sensors. Their input can enable detection of minute events like the disturbance of air which occurs whenever someone approaches quietly in a still room.