In vitro skin three-dimensional models and their applications

Abstract

Abstract. Skin fulfils a plethora of eminent physiological functions ranging from physical barrier over immunity shield to the interface mediating social interaction. Prone to several acquired and inherited diseases, skin is therefore a major target of pharmaceutical and cosmetic research. The lack of similarity between human and animal skin and rising ethical concerns in the use of animal models have driven the search for novel realistic three-dimensional skin models. This review provides a survey of contemporary skin models and compares them in terms of applicability, reliability, cost and complexity.Keywords: Skin, phenotypic screening, 3D models, pharmaceutical research Skin -composition and functionHuman skin covers an area of almost 2 m 2 in the adult and consists of the three major layers, subcutis, dermis, and epidermis. The subcutis is composed of adipose and epithelial cells. It harbours blood vessels, neurites of peripheral neurons, Vater-Pacini mechanosensors, and, partially, also sweat glands and hair follicles. It connects the skin to periosteum and fascia, absorbs forces, and mediates thermal insulation. The dermis supplies the epidermis with mechanical support and nutrients. It is stratified into an inner, reticular, and an outer, papillary, zone. The dermis houses most sebaceous glands, sweat glands, hair follicles, smooth muscle cells, and capillary beds and, thus, regulates skin moisture, body temperature, and performs the secretory function of skin. The papillary layer is characterized by relatively loose connective tissue, where Meissner corpuscles sense touch. Immune cells, particularly mast cells and dendritic cells, are patrolling in the papillary layer and mediate local inflammatory reactions and immune surveillance. Finally, dermal fibroblasts secrete extracellular matrix (ECM) and basement membrane components. These are primarily collagens I and III, and a proteoglycan-rich ground substance [1]. The resulting ECM mediates tensile strength of the dermis. The dermo-epidermal junction is centered around a special basement membrane. This is composed of a laminin/collagen IV scaffold and further typical basement membrane components such as perlecans and nidogens [1].The epidermis is a squamous epithelium of 50-100 m thickness. It is devoid of blood vessels but contains keratinocytes, Merkel cell mechanosensors, Langerhans immune cells, and melanocytes. The 1 These authors contributed equally.

The importance of the neuro‐immuno‐cutaneous system on human skin equivalent design

“…• Afferents are further classified into Aβ, Aδ and C nerve fibres with defined roles in the skin related to their action-potential propagation speed, a function of their degree of myelination 20,27,65 • Secrete neuropeptides, neurotrophins, neurohormones 8,39,40 • Sensation, touch, response to mechanical, chemical or thermal stimuli, 'nociception' 9,40,65 • Pain, neurogenic-inflammation 6,28,40 • Vascular regulation, vasodilation via sensory nerves, vasoconstriction via neuropeptide signalling 8 Adipocytes Hypodermis • Absorbs mechanical loads, insulates 10 • Mediates fibroblast recruitment during wound healing 89 • Energy source responsible for triglyceride production 90 • May function as endocrine organ through secretion of growth factors, hormones and cytokines to communicate with the rest of the NIC/NICE systems, associated with lipid metabolism and other metabolic processes 89 17 UV-induced immune events 25 and sensitization. 26 These models contributed useful information towards the understanding of skin pathologies including atopic dermatitis, cancers or drug development.…”

“…The proper differentiation of cells. 10,24,60 In summary, the most important considerations for the design of an optimal skin biomaterial would be the following: mechanically robust and/or flexible to allow for skin movement, biodegradable at controlled rate to optimize integration with cells or the local tissue (for implants), bio-inertness of the scaffold, conductivity to aid neural integration and/or reintegration, and additional factors to promote healing to enhance neural regrowth and/or wound healing. 24,61 A crucial avenue of skin research, and of relevance to the innervation field, is wound healing.…”

“…Beyond clinical application, HSEs are also used for commercial applications (in vitro testing/diagnostics) for testing permeability, sensitization or toxicity studies (Figure ) . Typically, these HSEs only contain 2 or 3 cell types, keratinocytes and fibroblasts, and sometimes melanocytes .…”

2D vs. 3D Cell Culture Models for In Vitro Topical (Dermatological) Medication Testing

“…Typically, however, it is difficult to ensure wounds that are equal in size using this method [63], and thus for this reason, testing of pharmaceutical products on these types of cultures are not ideal. 2D skin cell cultures also lack essential functions that could mimic in vivo processes, such as immune functionality and blood perfusion [65]. 3D wound healing models have thus attempted to more closely mimic in vivo wound healing processes and are available as histocultures or HSEs.…”

Assessing the Photoprotective Effects of Fluorescent Sphingomyelin Against UVB Induced DNA Damage in Human Keratinocytes

“…Given the relatively time-efficient protocols, reduced use of lab resources, and control over culture parameters, spheroids prove useful for investigation into skin applications. Due to the historical success of sheet-based skin models ( Figure 16), few research groups have developed skin spheroids [79], [80]. Some research groups have found success combining KRTs with dermal fibroblasts in a dual culture system [79].…”