5 curious facts about your skin

1. The skin is the biggest organ in the human body

When we think about human organs, the skin is rarely thought of. However, it is not only an organ but the biggest one. The skin can weigh up to 5 kgs and it has a surface area of approximately 2 m², which involves, roughly, a 10% of the body mass (1, 2).

As it happens with other organs, it is also possible to have skin transplants. When somebody suffers from a burn injury that affects a high percentage of the body, in vitro cultured epithelial sheets are routinely used to make autologous grafts, which can save the life of those patients (2).

2. The skin renews every 4-8 weeks

Cells of the epidermis require an almost constant renovation due to their continuous exposure to external physical (injuries, wind, fabric contact), chemical (cosmetics, pollution, disinfectants…) or biological (pathogen microorganisms, bacteria, insects…) agents that can be harmful. For this reason, the skin is a highly regenerative tissue. This also happens with epithelial cells from other tissues such as the intestine.

In order to fulfil this demanding cell renovation requirements, the epidermal progenitor cells are metabolically active and highly proliferative. That is to say, they need a lot of energy to be constantly dividing (3). Epithelial young cells move towards the outermost layers of the skin as they mature, until reaching the last layer of the epidermis, where they are in contact with the external environment.   

As time goes by, the dermis and epidermis get thinner and lose their regenerative ability, which is usually visible as dryness, wrinkles, or moles. This natural process could be accelerated by the action of external agents such as exposure to ultraviolet radiation, smoking or a poor moisturizing.

3. Until the 20th century the skin was thought to have no immune role.

The skin accomplishes many vital functions that help to keep the internal balance, or homeostasis. It seems logical to think that, being a cover of all our body, the main function of the skin is to protect the intern tissues. And so it is. One of its main functions is acting as a barrier that protects us physically from injuries. The adipocytes present in the inner layer of the skin, the hypodermis, cushions the impacts we suffer avoiding the damage of other tissues. However, the protective role of the skin is not limited just to a physical effect. It also has an important protecting role, discovered relatively recently, in which the immune system participates.

In the epidermis there is a wide variety of immune cells, such as Langerhans cells, which are dendritic cells responsible for interacting with the external environment and set an immune response if it is needed (4). Additionally, as we previously explained, sweat has a protective action against pathogen microorganisms since it has a low pH and contains antimicrobial peptides.

Other functions carried out by the skin and that contribute to the correct functioning of other tissues are (5):

• It avoids the loss of water, keeping cells and intern tissues moisturized.
• It regulates the body temperature keeping it constant.
• It has an endocrine function that coordinates the needed changes to strengthen the physical barrier and maintain the integrity in the dermis and the epidermis. For instance, synthesis of vitamin D is essential for the differentiation of keratinocytes.
• The skin has also an exocrine function. This could sound more familiar to us since this function is carried out by the sebaceous and sweat glands when they excrete sebum or sweat through the skin pores.
• Lastly, the skin is responsible for sensing. Touching, cold, heat or pain perceptions are possible due to the action of the nerve endings present in the skin.

4. White skin appeared as an adaptation to the environment after the first human migrations.

Skin color is one of the most variable and notable traits among humans. However, its genetic basis and evolution have not yet been fully disclosed. The data analyzed to date suggest that this enormous variability has been the result of natural selection: the skin has adapted to the different environmental conditions of the areas conquered by the first settlers. As a sign of this, it can be observed that, as a rule, the pigmentation of the skin is darker in areas near the equator and lighter as we approach the poles. What evolutionary advantage does skin color provide? The currently accepted theory is the dual theory of photoprotection/vitamin D. According to this theory, darker skin, typical of the equatorial regions, would serve as a defense mechanism against dermal lesions, skin cancer or immunosuppression caused by high ultraviolet radiation. In contrast, light skin would promote vitamin D synthesis in areas where solar radiation is low.

Numerous genetic studies conducted on different human populations and archaeological remains have identified about 15 genes involved in skin pigmentation. In each gene there are many possible variants [what in genetics is known as single nucleotide polymorphism (SNP)], which are combined in different ways in each person, being some combinations more frequent within the same population (6). However, it is suspected that there may still be some genetic regions involved in skin pigmentation to be discovered.

Research in skin genetics can be essential for the development of dermatological medicine. For example, having a complete genetic map of the DNA regions involved in skin pigmentation would allow the development of classification tools and prediction programs. Similar tools have been successfully designed for eye or hair color prediction (7).

5. The skin is different in each gender

The appearance of the skin is influenced by many factors. One of them is hormones. Androgens, among which the hormone testosterone is found, confer men’s skin some characteristics that differentiate it from women’s skin (8):

• Male skin is on average 20% thicker than female skin.
• It contains more collagen, giving it a firm look.
• Generally, men have more sebaceous glands than women. This makes their skin brighter and more likely to develop acne. However, at maturity, men’s skin is drier than women’s skin.
• Men usually sweat more than women in similar situations. This difference in sweating typically appears in adolescence.

In addition, frequent shaving makes the skin on men’s face and neck more sensitive and easily irritable than women’s skin.

REFERNCES

1. Chambers ES, Vukmanovic-Stejic M. Skin barrier immunity and ageing. Immunology. 2019 Nov 11.
2. Wong DJ, Chang HY. Skin tissue engineering. In: StemBook. Cambridge (MA): Harvard Stem Cell Institute; 2008.
3. Sreedhar A, Aguilera-Aguirre L, Singh KK. Mitochondria in skin health, aging, and disease. Cell Death Dis. 2020;11(6):444. Published 2020 Jun 9.
4. Clayton K, Vallejo AF, Davies J, Sirvent S, Polak ME. Langerhans Cells-Programmed by the Epidermis. Front Immunol. 2017;8:1676. Published 2017 Nov 29.
5. Yousef H, Alhajj M, Sharma S. Anatomy, Skin (Integument), Epidermis. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2020.
6. Del Bino S, Duval C, Bernerd F. Clinical and Biological Characterization of Skin Pigmentation Diversity and Its Consequences on UV Impact. Int J Mol Sci. 2018;19(9):2668. Published 2018 Sep 8.
7. Walsh S, Chaitanya L, Breslin K, et al. Global skin colour prediction from DNA [published correction appears in Hum Genet. 2017 Jul;136(7):865-866]. Hum Genet. 2017;136(7):847-863.
8. Zouboulis CC, Chen WC, Thornton MJ, Qin K, Rosenfield R. Sexual hormones in human skin. Horm Metab Res. 2007 Feb;39(2):85-95.