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Introduction
The skin, acting as a barrier between the human body and the external environment, protects the body from mechanical injuries and the effects of external factors, such as physical [solar radiation, including ultraviolet (UV), temperature, and wind] and chemical (e.g., disinfectants, detergents) factors, as well as pathogens (bacteria, viruses, etc.) [1]. Additionally, it performs secretory and excretory functions [2]. Skin is also responsible for the biosynthesis of many hormones and hormone-like substances, including corticosteroids, androgens, and estrogens [3, 4], e.g., the biosynthesis of cholecalciferol (vitamin D3) and its active metabolites [5]. Furthermore, skin is involved in the metabolism of glucose, proteins, and lipids [6, 7].
The skin has a multilayered structure consisting of the epidermis, dermis, and subcutaneous tissue, where keratinocytes are the dominant epidermal cells. In the basal layer of the epidermis there are melanocytes that produce pigment and perform protective functions [8] and Langerhans cells, which, among others, process microbial antigens, playing a key role in immune mechanisms of the skin [9]. However, in the dermis, fibroblasts are surrounded by an extracellular matrix composed of glycosaminoglycans, proteoglycans, and structural proteins, such as collagen and elastin, as well as macromolecules, fibrin, and hyaluronic acid, which ensure skin elasticity and mechanical resistance [10]. Blood and lymphatic vessels are also located in this part of the skin, supplying oxygen and performing nutritional functions, as well as providing a transport route for immune cells [11].
The integrity and, consequently, the effective metabolic activity of individual skin cells is ensured by the cell membrane, which separates the cell’s interior from its environment and regulates transport and signaling between skin cells and their surroundings [7, 12]. The cell membranes of epidermal and dermal cells contain various types of lipids, including sterols, phospholipids, ceramides, and glycosphingolipids [13]. Among the structural components of the cell membrane, a metabolically important group are phospholipids [12, 13], which are found in the largest amounts in the cells of the basal layer, including keratinocytes, where they constitute about 70% of all cellular lipids [12]. The stability of the lipid bilayer structures of membranes depends on the composition of phospholipids as well as lipid-lipid and lipid-integral membrane proteins interactions [14, 15]. The functions of proteins and other membrane biomolecules are influenced by cholesterol,...