The renaissance of human skin organ culture: A critical reappraisal

Highlights

• Human skin organ culture (hSOC) is an ancient but highly instructive and physiologically relevant skin research method.

• hSOCs have numerous applications for the study of the skin and its appendages.

• Recent advances in hSOC have further extended the range of methodological applications for the ex vivo study of the human skin.

• Compared with 3D skin “equivalent” assays, hSOC has its advantages and limitations.

Abstract

Human skin organ culture (hSOC) is a simple but highly instructive and clinically relevant skin research method. It has been used for decades to study the development, differentiation, and function as well as the response to wounding or test agents of intact human skin in the presence of its appendages and all resident cell populations. hSOC has also proven useful in toxicological and oncological studies and studies of skin aging (both chronological aging and photoaging), skin energy metabolism, skin immunology, pigmentation biology, and cutaneous (neuro-)endocrinology and neurobiology. The pathobiology and treatment of various dermatoses can also be assessed ex vivo by organ-culturing intact lesional human skin. In addition to morphological analyses by routine histochemistry, quantitative (immuno)histomorphometry has proven to be an excellent tool for quantitating and localizing protein expression patterns in defined skin compartments and distinct cell populations using a relatively small amount of precious human tissue. Finally, more recent technological advances, such as siRNA-mediated gene silencing and sensory reinnervation of hSOCs, have further extended the range of methodological applications for the ex vivo study of human skin; it has emerged as the ultimate preclinical assay system for investigative dermatology, including the testing of drugs, cosmeceuticals and nutraceuticals and more, and is just one step below human skin xenotransplant in vivo mouse models and clinical trials. Here, we critically review the renaissance and variety of hSOC assays, their applications and limitations, and we critically compare them with 3D skin “equivalent” assays. The review closes with perspectives on how this ancient but highly informative and physiologically relevant ex vivo skin research method may be further developed in the future.

Introduction

Given the complexity of human skin anatomy and physiology (Tan et al., 2017) and the hurdles encountered when studying the skin directly on patients or volunteers in vivo, it has long been a major challenge for skin biologists, investigative dermatologists, and the industries investigating the human body’s largest organ to develop appropriate assay systems for the study of human skin. Ideally, such systems should not only be pragmatic, accessible, affordable, and instructive but should be both physiologically and clinically relevant; that is, they should faithfully reflect at least some of the complexity of the human integument under well-defined, standardized and therefore reproducible preclinical research conditions.

hSOC successfully met this challenge many decades before it became possible to isolate and culture individual human skin cell populations, such as keratinocytes (Strudwick et al., 2015), fibroblasts (Lu et al., 2006) and melanocytes (Strudwick et al., 2015, Wilkins et al., 1985), or microdissected skin appendages, such as the hair follicle (HF) (Langan et al., 2015, Philpott et al., 1990), and before the development of 3D “skin equivalent” systems (Metcalfe and Ferguson, 2007).

Organ culture maintains most of the whole tissue structure, the tissue’s resident cell populations and their interactions with one another and the extracellular matrix (ECM); furthermore, it permits the study and manipulation of the behavior of animal cells, free of the systemic variations and neural influences that might arise in the animal both during normal homeostasis and under the stress of an experiment (Sorrell and Caplan, 2009, Stark et al., 2004). In 1898, Ljunggren et al. succeeded in culturing human skin fragments for long periods in ascites fluid and transplanting the tissue pieces back into the donors (Beaven and Cox, 1965). This approach dominated the field of ex vivo skin research for decades until the culture of dispersed, isolated cells in monolayer culture became popular during the 1950s (Pollak, 1969, Prunieras, 1975). For a timeline of key developments, see Fig. 1.

Given how easy and relatively economical hSOC is to set up, and in view of its impressive range of translationally highly relevant applications (see below), it is rather surprising how relatively infrequently hSOC is employed in skin research today. However, hSOC is at the verge of experiencing a long-overdue renaissance. Therefore, this review critically reappraises hSOC assays and delineates their wide range of applications as well as the limitations of hSOC.

The main rival technologies of hSOC are 3D “skin equivalent” (3D SE) assays, which are almost always missing far too many functionally important cell populations, ECM components and entire skin appendages to justify their claim of being a skin “equivalent” (see below and Table 4 for details). To avoid misinterpretations and misleading claims, we propose avoiding the rather misleading term “skin equivalent” (Reuter et al., 2009) and employing, whenever sensible and possible, hSOC as the most appropriate method for studying the human skin in a 3D context. The review closes with perspectives on how hSOC may be further developed in the future to defend its role as one of the most useful tools currently available in preclinical human skin research.