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Tissue Structure and Function
Published in Joseph W. Freeman, Debabrata Banerjee, Building Tissues, 2018
Joseph W. Freeman, Debabrata Banerjee
In skin, the epidermis can be further subdivided into the following layers: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale (Figure 4.17). The cells in the epidermis travel from the bottom to the surface, which means cells are formed at the basal layer. The cells move up the strata, changing shape and composition as they die because of isolation from their blood source. As they die, the cytoplasm is released, and the protein keratin is inserted. Cells eventually reach the corneum and slough off (desquamation). This process is called keratinization and takes place within about 30 days. This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and pathogens out. It makes skin a natural barrier to infection.
Bioprinting of human skin
Published in Ali Khademhosseini, Gulden Camci-Unal, 3D Bioprinting in Regenerative Engineering, 2018
Tania Baltazar, Carolina Catarino, Pankaj Karande
The human skin contains multiple cell populations beyond keratinocytes, fibroblasts, and the cells from the appendages, and whose functions are fundamental for maintaining proper skin homeostasis. The Langerhans cells, from the stratum spinosum, and the dermal dendritic cells are among the cells responsible for initiating the immune response against external pathogens (Metcalfe and Ferguson 2007; Bechetoille et al. 2007; Zhang and Michniak-Kohn 2012). Perception of touch, temperature, and pressure in the skin can be attributed to Merkel–Ranvier cells within the stratum basale (Metcalfe and Ferguson 2007; Bechetoille et al. 2007; Zhang and Michniak-Kohn 2012). Melanocytes, another important component of the epidermis, are also evenly distributed among the proliferative keratinocytes of the stratum basale (Velasquillo et al. 2013). The melanin produced by these cells is transferred to the surrounding keratinocytes, enhancing skin protection against UV light (melanin is a known broadband UV absorbent) among other beneficial effects (Brenner and Hearing 2008; Yu 2002). Functionalization of skin models using nerve cells could lead to the recovery of tactile perception at grafted sites. It has been shown that Schwann cells encapsulated in the dermal compartment of a reconstructed skin model can promote neurite migration in vitro and functional recovery of nerves in the graft in vivo (Blais et al. 2009). The inclusion of immune cells in the different compartments of the reconstructed skin creates what is known as immunocompetent skin models. These models are an important tool in the industry for assessing skin sensitization potential of new drugs and chemicals (Chau et al. 2013).
Skin Tissue Engineering: Past, Present, and Perspectives
Published in Rajesh K. Kesharwani, Raj K. Keservani, Anil K. Sharma, Tissue Engineering, 2022
Sinem Selvin Selvi, Merve Erginer Hasköylü, Ebru Toksoy Öner
Skin is evolved to act as a protective barrier against penetration of pathogens, toxins, and any other external damage together with perceptual, neural, cosmetic, and regulatory properties (Figure 5.2). It maintains body hydration and electrolyte balance. Self-renewal activity of skin results in new skin layers forming every 2–3 weeks. Skin mainly consists of three layers called epidermis, dermis, and hypodermis. Epidermis contains squamous epithelium, melanocyte cells (pigment containing), Langerhans cells and Merkel cells (pressure-sensing), and keratinocytes, which act as a protective barrier. Stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum are five different layers of epidermis. Cells in the basement layer (stratum basale) proliferate, differentiate, and migrate to upper layers, reach stratum corneum, the outermost layer that comes in contact with cosmetics, toxins, pathogens, textiles, and many other surfaces. Corneocytes in the stratum corneum covered with thin hydrophobic lipid layer that protect water loss from skin. Dermis, beneath epidermis, is mainly responsible for mechanical properties of skin and consists of fibroblasts, ECM, fibronectin, proteoglycans, elastin, collagen, blood vessels, lymphatic vessels, sebaceous glands, sweat glands, hair follicles, and nerve endings. It is separated from epidermis with basement layer and collagen, integrins, and laminins in its structure are responsible for epithelial-mesenchymal crosstalk. Hypodermis separates dermis from muscles (muscular fascia) and mainly consists of adipose tissue (Bhushan, 2017; Vig et al., 2017; Wong and Chang, 2009; Yildirimer et al., 2012). Collagen and elastin are structural proteins that support this tissue. Collagen binds cells together and is responsible for shape and function of animal body while elastin maintains extensibility to those collagenous tissues and is crucial for characteristic functions (Bailey, 2001). Hyaluronic acid (HA) is a high-molecular weight linear glycosaminoglycan synthesized by fibroblasts and keratinocytes and plays important roles in skin hydration and barrier function (Adrien et al., 2017).
Polymeric biomaterials for wound healing applications: a comprehensive review
Published in Journal of Biomaterials Science, Polymer Edition, 2022
Ahmed Olanrewaju Ijaola, Damilola O. Akamo, Fouad Damiri, Cletus John Akisin, Emmanuel Anuoluwa Bamidele, Emmanuel Gboyega Ajiboye, Mohammed Berrada, Victor Onyebuchukwu Onyenokwe, Shang-You Yang, Eylem Asmatulu
Human skin is the largest organ in the body, consisting of roughly 8% of its mass and enclosing its external surface. The outer area of the skin depends on the body’s weight and height, with skin thickness ranging from 1.5 to 4.0 mm [1]. Human skin performs several functions including fluid homeostasis, sensory detection, a barrier against microbial invasion, self-repair, self-renewal, selective permeability, and protection against external forces such as chemical, osmotic, thermal, and mechanical damages [2, 3]. Anatomically, skin is made up of three vital layers—epidermis, dermis, and hypodermis. The epidermis is the superficial layer of the skin that has a thin and highly cellular structure; it is the peripheral layer with high impermeability that controls water loss and protects against external harmful stimuli. From deep to superficial, the epidermis consists mainly of four layers as follows: stratum basale, stratum spinosum, stratum granulosum, and stratum corneum. On the palms and soles, a pale clear to pink layer, the stratum lucidium, is noted just above the granular layer. Stratum basale or stratum germinativum is the deepest layer of the epidermis (closest to the dermis) and the only layer where cells divide and move to the upper layers [4]. The adult epidermis is made up of mainly three cellular components the keratinocytes, melanocytes and Langerhans cells amidst other cells like basal and merkel cells and some non-cellular components. The keratinocytes produce keratin critical for normal functioning of the epidermis, the melanocytes produce melanosomes the pigment that determines the racial difference in skin color while the Langerhans cells are mainly for immune responses. The non-cellular components including keratin, desmosomes, hemidesmosomes, collagens, laminin granules, tonofibrils and vitamin D3, interacts with the cellular component to provide epidermis structural integrity, prevent water loss, up-regulate antimicrobial peptide synthesis for immune system and binds the dermis to the epidermis. Below the epidermis is the dermis, which consists of elastin, glycosaminoglycan (GAG), fibroblasts, and the collagen (COL)-rich extracellular matrix (ECM) [5]. The dermis performs the following functions: physical strength to the skin, support for nerve bundles, extensive vasculature, lymphatic system, and inflammatory and immune response [2]. The hypodermis, the layer underneath the dermis, is composed of a very large amount of vascularized adipose tissue, and is associated with the skin’s mechanical and thermoregulatory properties [6].