Review articleGender-linked differences in human skin
Introduction
Gender-linked physiological differences have been pointed out in body organs of healthy and diseased subjects, comprising the skin. The skin controls the gaseous exchanges with the exterior, regulates the thermal equilibrium of the body, contains sensory organs to interact with the environment, harbors saprofite micro-flora, provides a physical barrier against the penetration of inorganic matter, and a biochemical defense against viruses and other pathogens. It also provides a cushion against traumatisms and is equipped with dendritic cells to trigger immune responses.
This review summarizes anatomical, physiological and biochemical differences between male and female skin (summarized in Table 1) and discusses how this knowledge may contribute to the improvement of skin care interventions, particularly for men.
Section snippets
Hormones
Men and women skin differ in the metabolism of sex hormones, and in the response to them [1]. Testosterone is synthesized in the testes, in the ovaries and in the adrenal cortex. It is converted to de-hydro-testosterone by 5-α-reductase. Estrogen and progesterone are synthesized in the ovaries. Present at birth, their levels increase during adolescence and their quantitative difference determines the secondary sexual characteristics of the individual.
Skin is a steroidogenic tissue: it
Facial and body hair
Hair performs no vital function but its psychological functions are inestimable [5]. Hair follicles populate the entire cutaneous surface, with the exception of palms, soles, glans penis and muco-cutaneous junctions. They are formed before birth and no new follicles develop after birth. Hair is morphologically and biologically different in different parts of the body, and varies in structure, rate of growth and response to stimuli. Eyebrows and eyelashes do not respond to sex hormones, whereas
Sebum
Sebaceous glands are holocrine glands. Their vast majority is connected to hair follicles. On the scalp, forehead, cheeks and chin, their number varies between 400 and 900 per cm2, elsewhere there are less than 100 per cm2. No sebaceous glands are found on the palm or the sole. With androgenic hormonal stimulation, sebaceous glands increase in size and in secretory activity. Sebaceous glands consist of lobules of epithelial cells clustered at the extremities of the sebaceous duct. They
Sweat
Sweat can be produced by eccrine and apocrine sweat glands. These glands are located in the dermis and produce sweat that is secreted through tiny ducts onto the surface. Sweat from eccrine glands is an odorless, colorless hypotonic solution with pH between 4.5 and 5.5. It contains Sodium and Potassium Chloride, urea, lactate, bicarbonate, ammonia and oligo-elements such as calcium, phosphorous, magnesium, etc. [6]. Sweat is secreted by the three million or so eccrine sweat units, distributed
Surface pH
Sweat and sebum may influence the acidity of the skin. On women's forearms the pH was found to be about 5.6 ± 0.4 whereas in men's forearm, the pH was found to be 4.6 ± 0.4 [10], [16]. Other authors found that on women's forearm the average pH was 5.54 whereas in men it was 5.80 [17]. This difference could be the consequence of the fact that in Jacobi's experiment [10], the forearm was washed with water, and the stratum corneum was removed with Tesa film just before the measurements, whereas in
Skin thickness, collagen and water retention
The dermis contains water, ground substance and elastic fibers. These contribute to its thickness. At all ages, men's skin is thicker [18], [19], [20]. The extent of the difference varies with anatomical region. On the forearm it has been reported to be of the order of 20% [19]. After menopause, women's skin is 10% thinner than before menopause [21]. Skin thickness decreases in men and women, starting at the age of 45 [22]. Other authors report that skin thickness decreases linearly with age in
Skin tone
Skin color is modulated by melanin, blood and other pigments. Melanin is synthetized in melanocytes, dendritic cells located on the basal layer of the epidermis, each one injecting melanosomes into 24–36 keratinocytes. Melanocytes are differently distributed, varying from 17 ± 8 per mm of histology section on the shoulders, to 12 ± 7 per mm on legs and arms, to 3 ± 2 per mm on the anterior part of the trunk [29]. No gender-linked difference has been reported for melanocyte distribution. Yet, within
Immune systems and skin cancer
Men have a greater susceptibility to bacterial and viral infections [38]. Women are more prone to autoimmune and inflammatory diseases. Rheumatoid arthritis is four times more common in women [39], [40] and systemic lupus is nine times more common in women [41]. Men are more prone to skin cancer, with squamous cell carcinomas being twice more common in older men [42]. Men are prone to greater UV-induced immune-suppression [43]. The prevalence of melanoma is 1.72% in men and only 1.22% in women
Healing and other surface properties
Re-epithelialization is the covering by keratinocytes of an area devoid of epithelium because of mechanical or burn-induced ablation, and to achieve cellular multiplication and differentiation to generate a functional stratum corneum. Men have slower healing rates than women at all ages. Androgens are believed to be causally related to this phenomenon [47], [48].
The epidermis contains about 9.3 million cells per cm2, of which ∼7.5 million nucleated cells, ∼140,000 Langerhans cells, ∼200,000
Skin sensorial properties
The epidermis harbors sensory nerve endings which sense and transmit heat, pain, itch, etc. The skin contains Merkel cells and neural endings coated with Schwann cells. These cells detect stretch and shear occurring on the skin, and are equipped with temperature-sensitive receptors. Men and women have different tactile and sensorial perception. Men have less sensitivity to pain and temperature extremes [52], [53], [54] whereas women are less sensitive to cold [55].
Mast cells can release
Muscular mass and body fat
Men have larger lean body mass and less fat [59], [60]. In both genders the muscle mass decreases with age [59], [61], [62]. This contributes to the exterior aspect of the face and of body. The rate of muscular protein synthesis is identical in young women and in young men, and becomes 30% smaller in elderly women [63]. Larger muscle mass and higher rates of protein synthesis have long been attributed to higher levels of testosterone, yet, the reduction of muscular protein synthesis observed in
Cosmetic and dermatological implications based on gender differences in skin
Understanding gender-linked differences in skin physiology can help improving cosmetic treatments for anti-aging care via prevention, repair and protection, as well as to achieve skin smoothness, clarity and overall health. Some of the physiological differences between male and female skin can be ascribed to different levels of sex and stress hormones. This might lead for instance, to cosmetic interventions and dermatological treatments by modulating their binding to receptors in the skin and
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