The human body corresponds to a group of organs assembled together. Every organ performs specific and essential roles needed to create the balanced physiology necessary for a healthy human being. Each organ is a complex of many tissues designed for its specific tasks. The skin, made up of 3 layers (epidermis, dermis and hypodermis). In fact, the skin covers the entire body and isolates it from the outer environment. Besides, acting as a protective shield against external factors (such as ultraviolet radiations) and pollutants (such as chemical toxins and smoke), skin plays other important physiological roles including regulation of body temperature and vitamin production such as vitamin D. Many diseases could harm the skin, including auto-immune ones such as psoriasis, scleroderma, dermatomyositis, epidermolysis bullosa, and many others.
Human’s skin, with a total area of about 2 square meters and a weight reaching about 9 Kg (20 pounds), represents the human’s body largest organ (1). In human species, there are different types of skin that are classified under the Baumman skin typing system that takes into consideration the oiliness, sensitivity, pigmentation, and wrinkles. In general, the skin structure is divided into three layers: Epidermis; Dermis; and Hypodermis (Figure1).
The epidermis is the most superficial layer of the skin and the most important one from a cosmeceutical point of view because it is responsible for the texture and moisture of the skin and contributes to its color. The epidermis is formed out of 4 layers: Stratum corneum; Granular layer; Spinous layer; and Basal layer (Figure 2).
Keratinocytes are the cells that comprise the majority of the epidermis. These cells contain mostly keratin filaments providing structural support. Keratinocytes are produced out of stem cells present at the basal layer next to the dermal-epidermal junction. Next, to keratinocytes in the basal layer, we can find melanocytes that are responsible for producing melanin. After the stem cells duplicate, the cells migrate towards the surface of the epidermis this process is called keratinization. While migrating, the cells differentiate into different properties depending on the location of the cells. Spinous cells are called so because they are linked by structures called desmosomes. Desmosomes are the adhesion structures that create bonds between cells. The next layer is called granular because it contains visible granules. Finally, as seen in figure 2 stratum corneum contains cells that lost their nuclei and granules(2).
The dermis is positioned between the epidermis and the hypodermis (Figure 3 ). It is the thickest layer of the skin and therefore is a very important target for the cosmeceutical industry. Its thickness varies in different parts of the body. The dermis contains fibroblasts, nerves, blood vessels, sweat glands, and molecules such as Glycosaminoglycans (GAG), Glycoproteins (GP), elastin but mostly collagen. The junction between the epidermis and the dermis is known as the dermal-epidermal junction (DEJ) (2,3).
Subcutaneous tissue or the hypodermis is the deepest layer of the skin (2). In general, it is composed of adipocytes, fibrous tissue, and blood vessels. It represents about 9 to18% of a male’s weight and 14 to 20% of a females weight. In severe obese people, it can reach up to 70% of the total body weight. Adipocytes, the cells responsible for carrying fat are divided into two types: white adipocytes and brown adipocytes. White adipocytes are cells that have a large droplet of lipid that fills most of the cell. Brown adipocytes are fat cells that have multiple lipid droplets and play a major role in non-shivering thermogenesis. The subcutaneous tissue (also known as the superficial fascia), is divided into three layers: apical, mantle, and the deeper layer.
- Melina R. How Much Does Your Skin Weigh? [Internet]. Live Science. [cited 2019 May 23]. Available from: https://www.livescience.com/32939-how-much-does-skin-weigh.html
- Baumann L, Baumann L. Cosmetic dermatology and medicine: principles and practice. 2nd ed. New York: McGraw-Hill; 2009. 366 p.
- Agarwal S, Krishnamurthy K. Histology, Skin. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 [cited 2019 Jul 3]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK537325/