The skin is a complex organ that covers the entire surface of the body. From the anatomical viewpoint there are three distinct skin layers, from the exterior to the interior: an outer epithelium (epidermis), a layer of connective tissue (dermis) and subcutaneous adipose tissue (hypodermis). The dermis and hypodermis contain vessels, nerves and cutaneous appendages (hair follicles, sebaceous glands, sweat glands and nails).

The skin is the organ having the greatest surface area and weight of the human body. In a standard adult the average surface area is 1.5-2 square metres with a total weight of about 15 kg and considerable variability depending on sex, age, constitutional and nutritional factors.

The skin is a boundary organ which covers the entire body surface. It provides both protection against the external environment and an interface between the external world and the body interior. Integrity of the skin barrier is essential for survival; it prevents the passage of microorganisms and toxic substances, absorbs and blocks radiation, prevents water loss, contributes to body temperature regulation and plays an important role in immunological defence.

The outermost part of the skin, the horny layer, is the anatomical barrier that limits and regulates the penetration of the substances applied to it. The horny layer, in simple words, can be described as a wall of bricks in which the cells are cemented together by extracellular lipids. The skin barrier can be altered by various factors: genetic or constitutional (as in atopic dermatitis), environmental (temperature, humidity, solar irradiation), hygienic or cosmetic (excessive washing, use of irritant or sensitising products). In these conditions of modified skin permeability there will be excessive water loss and an increase in penetration of potentially toxic substances.

The skin is also a sensory organ having a dense network of nerve cells by which various sensory stimuli (heat, touch, pain, pressure, vibration) are received and transmitted to the central nervous system. The subcortical centres are involved in the skin sensory function, which is therefore related to the entire individual physical and psychic personality. Then there are efferent nerve endings in the skin, belonging to the autonomic nervous system, which innervate the smooth musculature of the vessels and appendages (sweat glands and hair erector muscles). These anatomical sub-layers underlie the involuntary reactions that occur in the skin in particular emotional situations (sweating, blushing or becoming pale).

The skin is the location of the sense of touch, the oldest sensory system, around which the first perceptive experiences of a newborn baby are organised. The skin has two intrinsic functions: the protective function against everything that is harmful in the external environment and the containment function of everything that must be kept and not lost to the environment. The skin, understood as a complex sensory structure, is the base on which the feeling of safety and identity of the human being is built and organised. Like the mouth, the skin represents a place and primary means of communication with the other people with whom significant relationships are established.

The skin presents different macroscopic characteristics depending on each subject and these biological differences make it unique. The biodiversity of the skin is connected with the interaction of genetic and environmental factors. The set of genes that constitute the DNA of an organism or a population is defined as the genotype; from the expression of these genes comes the phenotype which means the set of observable characteristics of a living organism. Genetic expression is influenced by the interaction of various genes, environmental factors and chance factors. The environmental factors have a fundamental role. The skin is exposed daily to external factors of a chemical and physical type (in particular UV radiation) and biotic agents (viruses, bacteria, mycetes). The skin is likewise influenced by endogenous factors of a hormonal, nutritional and pharmacological nature.

The colour of the skin - a feature that varies depending on race, age, sex and location on the body – is one of its commonly known and interesting macroscopic characteristics. It depends mainly on the presence of three different pigments in the skin: melanin, haemoglobin and carotene. Melanin is produced by melanocytes and is responsible for the brown colour of skin; it plays a fundamental role in protection from ultraviolet radiation. Six different phototypes are classified on the basis of the production and accumulation of melanin. Haemoglobin gives skin its red component; this colour is more evident in the areas of greatest blood circulation and varies depending on vasodilation and vasoconstriction. In the Negroid skin the red tone does not appear due to the presence of large quantity of melanin. Carotene is a liposoluble pigment that gives a yellow-orange tone. Regional differences in skin colour also depend on the thickness of the epidermis which determines the greater or lesser levels of transparency.

The skin differs according to age, sex and body location.

The development of the skin is an evolutionary process which starts in the first weeks of intrauterine life, ends at birth and progresses during the entire lifetime. The skin of a baby has idiosyncratic characteristics and greater susceptibility to external agents due to the thinness of the surface layers, a still immature structure and lower production of protective sebum. A baby’s skin is more vulnerable, especially to the action of UV radiation and toxic substances. The maturing process of the skin takes place over the years until the adult characteristics appear. Skin ageing is a physiological process caused by the interaction of genetic, external environmental and endogenous factors. Chronoaging usually implies genetically-programmed processes while photoaging indicates the changes due to environmental factors and lifestyles in particular reference to solar radiation. Senile skin appears drier due to the lower production of sebum and sweat and is more fragile due to slower healing processes and cell turnover, becoming thin and less elastic due to the reduction of the elastic tissue and collagen.

The skin differs by sex because it is a hormone-dependent organ due to the presence of receptors for androgens and estro-progestogens. Sex hormones regulate epithelial and mucous trophism, the activity of the fibroblasts and production of collagen and elastic tissue; they influence the qualitative and quantitative production of the sebaceous and sweat glands, the distribution and trophism of hair follicles. These differences are more marked after puberty and decrease during menopause when virilization phenomena appear in women due to the drop in estrogen and progestogen levels.

The skin presents different characteristics depending on the body location. The thickness of the epidermis varies from 0.5 mm in the eyelids to 3-6 mm in the palms and soles of the feet. In the dermis the collagen and elastic fibres are more abundant in the areas subject to greater tensile forces, such as the areas of the joints. The density of the appendage structures is also different. The sebaceous glands, which synthesize lipids that are an essential constituent of the cutaneous hydrolipid film, are distributed everywhere except for the palms and soles of the feet. The dimensions of the sebaceous glands vary depending on body location, being larger on the face and in the presternal region. The development and activation of the sebaceous glands is largely controlled by genital and adrenal androgens produced after puberty. The sweat glands are classified as apocrine and eccrine; apocrine glands are part of the follicle sebaceous unit and are present mainly in the areas of the armpits and genitals. Like the sebaceous glands, they are small and do not function until puberty. The eccrine sweat glands are distributed over the entire skin surface with higher density on the palms and soles of the feet, forehead and armpits. They produce sweat which evaporates on the skin surfaces and contribute to keeping the body temperature constant. The hair follicles are distributed over the entire skin surface with the exception of the palms and soles of the feet, lips, genital mucous membranes and ungual phalanges. In prepuberty there are short, thin and minimally- pigmented hairs, except for the hair and eyebrows where terminal hair is found. After puberty terminal hair also appears in the armpits and genital areas and, in males, on the trunk and area of the beard. Hormonal stimuli determine the density and characteristics of the hair. The morphology and number of hairs varies not only according to age and sex but also race, being more numerous in Caucasoids, less numerous and straight in Mongoloids and curly in Negroids.