The skin is the outermost layer of our body. It can reach a surface of up to 1.8 m2(1) and, it is mostly covered with hair. For a long time, the skin was seen as a static part of the body that was responsible for the physic protection of our intern tissues. However, nowadays we know that the skin is not only a simple body-cover, it is indeed a very complex and dynamic organ able to receive information from our surrounding environment and adapt to it. For instance, the skin is the tissue responsible for regulating the body temperature; it has also a potent antimicrobial action thanks to the peptides present in the sweat or the excreted fat (2).
Structurally, the skin has three layers that vary in functions and thickness. The epidermis is the thinnest and outermost skin layer. It is roughly 0.1 mm thick (1) and it is formed by thinner layers of overlapped cells, most of them Keratinocytes (3). These cells act as an external barrier: they avoid water loss and protect our inner organs from physical damage or pathogens. As we move deeper in the epidermis, we can find layers with different kind of cells such as Langerhans cells and melanocytes. The mission of the melanocytes is to avoid that the UV radiation penetrates and damages deeper cells and, to that end, they synthesize melanin (4). On the other hand, Langerhans cells, which can also be found deeper in the skin, are immune cells able to detect and react against microorganisms that have overcome the keratinocytes barrier (5).
Right below the epidermis, is the dermis. This layer is thicker than the previous one (3-4mm) and has got a completely different structure. Cells in the dermis are not disposed in layers, in contrast, cells are embedded in a matrix of collagen and elastic fibers filled with amorphous extracellular “ground substance” containing glycosaminoglycans, such as hyaluronic acid, proteoglycans, and glycoproteins. Moreover, the dermis is also formed by blood vessels, nerve endings, hair follicles, immune cells, sebaceous glands and other cells such as the ones synthesizing collagen: fibroblast. The nerve endings located in the dermis are able to sense hair movements and act as mechanoreceptors allowing sensation to extend beyond the skin surface. Thus, we can sense cold or heat. The flexibility of the skin relies mainly on the structure and composition of the dermis which also helps to regulate temperature (6).
Lastly, the deepest layer of the skin is the hypodermis. It is also known as subcutaneous layer and it is composed of elastic and collagen fibers displayed like in the dermis. Despite blood vessels, immune cells and nerve endings can also be found in the hypodermis, the main role of this part of the skin is to accumulate fat in cells named adipocytes and to produce vitamin D. The number of adipocytes accumulated in the hypodermis depends on the part of the body and the person (7).
As you may have seen, the skin is a very complex and structured tissue that receives a lot of information from our surrounding environment. Currently we know that the skin plays a pivotal role in the immune response. However, it was not until late XXth century when the importance of this organ in the regulation of our defense system was discovered (8). Interestingly, something similar occurs with the nervous system. As science makes progresses, it is clearer that some skin problems can be a consequence of nerve endings damage. Therefore, it is crucial to take care of our skin and to keep it moisturized.
References:
1. Chambers ES, Vukmanovic-Stejic M. Skin barrier immunity and ageing. Immunology. 2019 Nov 11.
2. Schittek B, Hipfel R, Sauer B, Bauer J, Kalbacher H, Stevanovic S, et al. Dermcidin: a novel human antibiotic peptide secreted by sweat glands. Nat Immunol. 2001 Dec;2(12):1133-7.
4. Lambert MW, Maddukuri S, Karanfilian KM, Elias ML, Lambert WC. The physiology of melanin deposition in health and disease. Clin Dermatol. 2019 Sep -Oct;37(5):402-417.
3. Matejuk A. Skin Immunity. Arch Immunol Ther Exp (Warsz). 2018 Feb;66(1):45-54.
5. West HC, Bennett CL. Redefining the Role of Langerhans Cells As Immune Regulators within the Skin. Front Immunol. 2018;8:1941.
6. B Brown TM, Krishnamurthy K. Histology, Dermis. [Updated 2018 Dec 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK535346/
7. Agarwal S, Krishnamurthy K. Histology, Skin. [Updated 2019 Jan 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK537325/
8. Streilein JW. Skin-associated lymphoid tissues (SALT): origins and functions. J Invest Dermatol. 1983 Jun;80 Suppl:12s-16s
