Skin cells for toxicity testing

Dermal toxicity is an important consideration for the cosmetic, personal care, pharmaceutical and chemical industries. In vitro skin models are used in the assessment absorption, sensitization, irritation, and corrosion, and genotoxicity. To understand these in vitro assays, we will first discuss skin biology, followed by a discussion of models for dermal toxicity testing, and an overview of Lonza’s solutions and building blocks for establishing dermal toxicity testing in your lab.

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Introduction to skin biology

The skin provides a vitally important protective separation between the internal and the external environments. There are three structural layers to the skin: the epidermis, the dermis and subcutis. The epidermis is the outer layer, serving as the physical and chemical barrier between the interior body and the exterior environment. The dermis is a deeper layer providing the structural support of the skin. Subcutis is made up of loose connective tissue and fat, which can be up to 3 cm thick on the abdomen.

Epidermis is a stratified squamous epithelium consisting of several cell types. The most abundant cell type of epithelial layer of the skin is the keratinocytes which synthesize the protein keratin. Protein bridges called desmosomes connect the keratinocytes, which are in a constant state of transition from the deeper layers to the superficial. The epidermis varies in thickness based on the tissue of origin. The four separate layers of the epidermis are stratum basale (basal or germinativum cell layer), stratum spinosum (spinous or prickle cell layer), stratum granulosum (granular cell layer) and stratum corneum (Fig. 1). These layers are formed by the differing stages of keratinocyte maturation. In addition to their structural and barrier function, keratinocytes also play a crucial role in influencing immune response by secreting inhibitory cytokines and activating Langerhans cells, in response to injury.

Dermal fibroblasts are cells within the dermis layer of skin which are responsible for generating connective tissue and allowing the skin to recover from injury. Dermal fibroblasts generate and maintain the connective tissue which unites separate cell layers. Furthermore, these dermal fibroblasts produce the protein molecules including laminin and fibronectin consisting of the extracellular matrix. By creating the extracellular matrix between the dermis and epidermis, fibroblasts allow the epithelial cells of the epidermis to affix the matrix, thereby allowing the epidermal cells to effectively join together to form the top layer of the skin.

Skin cells release inflammatory mediators (cytokines and chemokines) in response to chemically-induced tissue and cell damage. These cytokine and chemokines dilate and increase permeability of blood vessels and attract immune cells to the injury site for antigen clearance and tissue repair. Inflammatory mediators also stimulate nerve endings leading to itching and stinging sensations.

Layers of Skin Epidermis

Figure 1. The epidermis is a self-renewing tissue composed mainly of keratinocytes in various stages of terminal differentiation. Keratinocytes are produced in the stratum basale (basal layer), and undergo a programmed series of differentiation. Keratinocytes also receive melanin from melanocytes in the form of pre-packaged organelles termed melanosomes. Image Courtesy of D'Orazio J, Jarrett S, Amaro-Ortiz A, Scott T - Int J Mol Sci (2013)

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Skin cell models for toxicity testing

While animal models remain one of the most important tools in preclinical Tox studies, in vitro systems such as culturing primary human cells are becoming increasingly important for decision making in the drug development pipeline. Epithelial cells like HeLa, CHO and fibroblast like 3T3, 293, COS immortalized cell lines were historically used for developing in vitro models. While these cell lines are easier to grow than primary cells, cell lines have indefinite an life span triggering mutations and most of them originate from cancer tissues.

The use of primary cells can potentially reduce the number of animal studies, and their associated costs, if targets are preliminarily screened in an in vitro model comparable to human in vivo environment. The approach also helps realize differences and variances seen in cell performance as research transitions from an in vitro human model into in vivo animal studies. This approach also alleviates many of the ethical concerns associated with animal use expressed across many regions of the world.

In vitro skin models usually assess cell viability as the main end point but also focus on evaluating the release of the key inflammatory mediators that trigger tissue repair. Increasing experimental evidence shows that three-dimensional (3D) culture is a more comparable format to human in vivo because it more closely allows for the reconstruction of human skin tissue. Various studies have shown that in vitro skin micro-tissues comprising of a fibroblast layer containing dermis topped by a stratified layer of keratinocytes can be formed. The total duration to obtain the full thickness skin models can vary from 28–30 days from seeding of the cells to differentiation.

Keratinocyte and Fibroblast Co-Culture Donor 1

Types of skin cell toxicity tests that are carried out to assess substrate safety:

Skin irritation/corrosion tests induce varying concentrations of chemical exposure to assess damage to the skin either as reversible (irritation) or irreversible (corrosion).
Skin sensitization tests induce measure contact dermatitis following exposure to chemical agent.
Dermal penetration: The extent and rate at which a chemical is enters the body through skin (also known as skin absorption or percutaneous absorption).
Acute systemic toxicity: Adverse effects occurring within a short time after administration of a single (usually extremely high) dose of a substance via skin (dermal) absorption.

Tools for developing your skin cell models

We offer a comprehensive range of normal and diseased human primary dermal cells along with 3D culture tools, protocols and support team to ease your transition into generating biologically relevant in vitro skin models. For diseased cells, detailed donor information can also be requested by contacting our scientific support team. If you have custom requests, please contact our CellBio Services team.

To access a complete range of our dermal portfolio, visit our products below:

Matched donor sets available – contact our scientific support team

Co-culturing Lonza Matched Donor Keratinocytes and Fibroblasts

A High-throughput, 2D Skin Model Download Document

Mechanistic toxicity

	Cell type	Recommended BulletKit^® Media
Keratinocyte culture	Human Epidermal Keratinocytes, Adult, Single Donor Human Epidermal Keratinocyte, Neonatal, Single Donor Human Epidermal Keratinocyte, Neonatal, Pooled	KGM^®-Gold Keratinocyte Growth Medium or KGM^®-Gold Keratinocyte Medium, Calcium Free
Melanocyte culture	Human Epidermal Melanocytes, Adult	MGM^®-4 Melanocyte Growth Medium
	Human Epidermal Melanocytes, Neonatal	MGM^®-4 Melanocyte Growth Medium
Fibroblast culture	Human Dermal Fibroblasts, Adult	FGM^®-2 Fibroblast Growth Medium
	Human Dermal Fibroblasts, Neonatal	FGM^®-2 Fibroblast Growth Medium
Endothelial cell culture	Human Dermal Microvascular Endothelial Cells, Adult	EGM^®-2MV Endothelial Cell Growth Medium
	Human Dermal Microvascular Endothelial Cells, Neonatal	EGM^®-2MV Endothelial Cell Growth Medium