Research Article

Fundamental in vitro 3D human skin equivalent tool development for assessing biological safety and biocompatibility – towards alternative for animal experiments

¹ Department of Mechanical and Aerospace Engineering (DIMEAS), Politecnico di Torino, 10129 Turin, Italy
² Universitätsklinikum Knappschaftskrankenhaus Bochum, Department of Surgery, Hospital of the RUHR-University, 44892 Bochum, Germany
³ Department of Experimental Surgery, Centre for Clinical Research, RUHR-University, 44801 Bochum, Germany
⁴ Institute of Pathology, RUHR University, 44789 Bochum, Germany
⁵ Interuniversity Centre for the Promotion of the 3Rs Principles in Teaching and Research, Centro 3R, 56122 Pisa, Italy

^* Corresponding authors: valeria.chiono@polito.it; jochen.salber@rub.de

Received: 17 December 2020
Accepted: 3 February 2021

Abstract

Nowadays, human skin constructs (HSCs) are required for biomaterials, pharmaceuticals and cosmetics in vitro testing and for the development of complex skin wound therapeutics. In vitro three-dimensional (3D) dermal-epidermal based interfollicular, full-thickness, human skin equivalent (HSE) was here developed, recapitulating skin morphogenesis, epidermal differentiation, ultra-structure, tissue architecture, and barrier function properties of human skin. Different 3D cell culture conditions were tested to optimize HSE maturation, using various commercially available serum/animal component-free and/or fully defined media, and air-liquid interface (ALI) culture. Optimized culture conditions allowed the production of HSE by culturing normal human dermal fibroblasts (NHDFs) for 5–7 days in CELLnTEC-Prime Fibroblast (CnT-PR-F) medium and then culturing normal human epidermal keratinocytes (NHEKs) for 3 days in CELLnTEC-Prime Epithelial culture (CnT-PR) medium on them. Co-culture was then submerged overnight in CELLnTEC-Prime-3D barrier (CnT-PR-3D) medium to stimulate cell-cell contact formation and finally placed at ALI for 15–20 days using CnT-PR-3D medium. Histological analysis revealed uniform distribution of NHDFs in the dermal layer and their typical elongated morphology with filopodia. Epidermal compartment showed a multi-layered structure, consisting of stratum basale, spinosum, granulosum, and corneum. NHDFs and keratinocytes of basal layer were positive for the proliferation marker Kiel 67 (Ki-67) demonstrating their active state of proliferation. The presence of typical epidermal tissue proteins (keratins, laminins, filaggrin, loricin, involucrin, and β-tubulin) at their correct anatomical position was verified by immunohistochemistry (IHC). Moreover, transmission electron microscopy (TEM) analyses revealed basement membrane with lamina lucida, lamina densa, hemidesmosomes and anchoring fibers. The epidermal layers showed abundant intracellular keratin filaments, desmosomes, and tight junction between keratinocytes. Scanning electron microscopy (SEM) analyses showed the interwoven network of collagen fibers with embedded NHDFs and adjacent stratified epidermis up to the stratum corneum similar to native human skin. HSE physiological static contact angle confirmed the barrier function. The developed HSE represents a fundamental in vitro tool to assess biocompatibility of biomaterials, pharmacotoxicity, safety and effectiveness of cosmetics, as well as to investigate skin biology, skin disease pathogenesis, wound healing, and skin infection.