In this blog Senior Bioassay Scientist Tsemay Tse and Bioassay Study Manager Alex Timmis discuss the latest innovations in skin research, the rise in in vitro alternatives and how the models created at Cellomatics are helping therapeutic discovery.
Current health issues
Recognised as the largest organ of the body, the skin’s primary function is to act as a physical barrier against the continuous assault from the external environment. Consequently, the World Health Organisation (WHO) has acknowledged skin disease as a major public health issue [5].
While many skin conditions are addressed using ingestible therapeutics, topical applications remain a popular method for direct therapeutic delivery [6]. These topical agents are subject to strict regulation by both the European Commission [13] and the Organisation for Economic Co-operation and Development (OECD) [14]. Despite these stringent measures, the safety and efficacy of some commercial products are later found to be of concern—either through emerging scientific research or reports of adverse health effects that only surface after prolonged use [15][16].
Given these concerns, the most intuitive approach to product validation and safety evaluation would be the use of model organisms with strong physiological relevance. However, rising public concern and the growing “cruelty-free” movement have led to animal testing bans in many countries [8].
Since 2004, the European Union and the United Kingdom have enforced a ban on animal testing for cosmetics and personal care products [17]. This shift did not reduce the demand for product testing but instead redirected it towards in vitro assays. As a result, the last two decades have witnessed a global surge in the investment and development of in vitro cosmetic models. A notable outcome of this effort was the development and standardisation of 3D skin models, including the reconstructed human epidermis (RHE) and human skin equivalents (HSE), published in 2004 [23] and 2008 [25], respectively.
Impact and existing field of research
Although regulatory changes have accelerated the adoption of in vitro models, the foundational work in this field dates back several decades.
The concept of reconstructed skin originated in 1975, following the isolation and successful culture of primary human keratinocytes by James Rheinwald [22]. Since the 1980s, L’Oréal has reported using in vitro “reconstructed skin” or “skin equivalent” models for cosmetic testing [20].
RHE and HSE models have since become widely adopted in both basic research and cosmetic development. While these models are relatively easy to culture, transport, and handle, they also involve complex optimisation processes and stringent culture timelines. This created a niche business opportunity for artificial skin companies to provide ready-to-use models for routine formula testing. As a result, these models have gained traction as industry standards, now regulated by the OECD [24].
How our models help therapeutic discovery
Currently, the market is dominated by manufacturers and service providers offering standardised artificial skin models primarily for routine testing. However, these models offer limited customisation and often fail to replicate disease-specific physiology.
At Cellomatics, we aim to address this gap by leveraging the significant “maturation window” of artificial skin models to develop customised models for a wide range of skin conditions and diseases. By combining our disease biology expertise with our proven capabilities in assay development, we offer bespoke artificial skin-based preclinical research services.
Our approach enables clients to test proprietary therapeutics in models that more closely mimic physiological conditions—offering a significant improvement over traditional 2D cultures and even basic 3D systems.
Our innovation and results
The first milestone in our artificial skin R&D programme was the establishment of foundational 2D assays using Primary Normal Human Epidermal Keratinocytes (NHEKs). As NHEKs constitute approximately 90% of the epidermis, they are widely recognised as a standard model for pre-clinical research and early-phase cosmetic testing.
While monoculture systems such as these are invaluable tools, it is important to acknowledge that keratinocytes do not function in isolation in vivo. In the native skin environment, they interact continuously with pigment-producing melanocytes, immune cells, and other dermal components, responding to both internal and external stimuli. The absence of these cellular interactions in 2D monoculture assays can limit the physiological relevance and translational potential of the resulting data.
A central objective of our R&D has therefore been to compare the differential responses of 2D and 3D mono- and co-culture models when exposed to identical stimuli. This approach has allowed us to determine the optimal level of model complexity required to generate robust, physiologically relevant data across a range of functional assays.
For our initial experiments using the 2D keratinocyte monoculture model, we selected ultraviolet B (UVB) radiation as a classical external stressor known to induce ageing and photo-damage. UVB radiation is a common environmental factor with well-documented effects on skin cells, including the induction of senescence [2], inflammation [3], and carcinogenesis [4].
Using a series of independent assays, we successfully established UVB exposure parameters capable of eliciting clear and reproducible markers of inflammation, cytotoxicity, apoptosis, and cellular senescence.
In brief, undifferentiated keratinocytes were cultured to approximately 90% confluence to mimic the viable, proliferative layers of the skin. At this stage, cells were subjected to controlled doses of UVB radiation and then incubated for 4 to 24 hours, depending on the endpoint of the assay.
To assess inflammatory responses, we quantified the release of key cytokines—TNFα, IL-2, IL-4, IL-6, IL-8, and IFNγ—24 hours post-UVB exposure (Figure 1). Both low and high UVB doses triggered a statistically significant increase in the secretion of all six cytokines compared to the unexposed control group, indicating a strong pro-inflammatory response to UVB stimulation in this model.
Figure 1. Induction of inflammation in primary human keratinocytes by UV-B irradiation. Primary healthy human keratinocytes were exposed to different doses of of UV-B radiation. After 24 hours post-insult, the supernatants were analysed for the release of inflammatory cytokines, TNF-α, IL-6, IL-8, IFNγ, IL-4 , and IL-2. Using internal assay standards, the level of each inflamatiory cytokine was quantified. To determine significance, all conditions exposed to UV-B were compared to the basal control (***p<0.001; **p<0.01; *p<0.05; n=2±SEM).
In conjunction with our analysis of inflammatory responses, we also sought to semi-quantify the extent of irreversible cellular damage induced by varying doses of UVB irradiation in the keratinocyte monoculture. Using an LDH (lactate dehydrogenase) assay, we measured the percentage of cell death relative to a total cell lysis control treated with 2% Triton X-100 (Figure 2).
This experiment was critical in determining the maximum UVB exposure threshold that did not result in significant cytotoxicity. Establishing this upper limit enabled us to tailor UVB dosing for subsequent functional assays—applying lower doses for more sensitive readouts such as reactive oxygen species (ROS) production and Caspase-3/7 activation, and higher doses for endpoints involving DNA damage and gene expression profiling. This strategy ensures that the lowest effective UVB dose is applied in an assay-specific manner, optimising both the physiological relevance and reproducibility of our results.
Figure 2. Percentage cytotoxicity induced by UVB irradiation of primary human keratinocytes. Primary healthy human keratinocytes were first stimulated with two doses of UVB radiation and incubated for a further 24 hours prior to analysis by LDH assay. Data represents percentage cytotoxicity normalised against a 100% killing control of 2% Triton X-100. Statistical significance was determined by comparison of all conditions against the no UVB control (***p<0.001; **p<0.01; *p<0.05; n=2±SEM).
Two well-established cellular responses to UVB-induced photo-ageing are apoptosis and senescence. Existing studies have shown that UVB exposure induces apoptosis in keratinocytes, in part via upregulation of p21, a cyclin-dependent kinase inhibitor also known as CDKN1A [12].
Using a Caspase-3/7 activation assay, we confirmed that UVB exposure triggers apoptosis in a dose-dependent manner (Figure 3). To further investigate the molecular drivers of this response, we analysed the expression of key senescence-associated genes by quantitative PCR (qPCR). Our results revealed a significant upregulation of p21, but no significant change in p16 expression, following UVB exposure (Figure 4).
These findings support the hypothesis that UVB-induced apoptosis in keratinocytes is mediated, at least in part, by p21-driven pathways, and suggest a lesser role for p16 in this context.
Figure 3. UVB exposure drives apoptosis in primary human keratinocytes. Quantification of apoptosis, by way of cas3/7 activity levels was determined by Caspase-Glo® 3/7 Assay. Primary healthy human keratinocytes were stimulated with 5 UVB doses and returned to incubate for 6 hours. Statistical significance was determined by comparison of irradiated conditions to the basal unirradiated control. (***p<0.001; **p<0.01; *p<0.05; n=2±SEM).
Figure 4. UVB exposure induces senescence in primary human keratinocytes. Differential expression of senescence gene markers confirmed that UVB induced keratinocytes senescence triggered through p21, but not p16, activated Caspase 3/7 cascade [12]. Primary healthy human keratinocytes were stimulated with 2 doses of UVB and returned to incubate for 24 hours. qPCR data was analysed using the ΔΔCT method, with statistical significance determined using the average fold-change in gene expression normalised to the basal unirradiated control. (***p<0.001; **p<0.01; *p<0.05; n=2±SEM).
To further explore the induction of senescence by UVB exposure, we assessed the gene expression of β-galactosidase (GLB1) in keratinocytes with and without UVB exposure. GLB1 is a well-recognised marker of senescence and is reported to initiate senescence activity through the hydrolysis of β-galactose from glycoconjugates [26].
Using qPCR analysis, we observed a significant increase in GLB1 expression following exposure to a lower UVB dose of 1,200 mJ/cm², while a significant decrease was noted at the higher dose of 2,400 mJ/cm². This pattern suggests that GLB1-driven senescence may represent an early cellular response to sub-lethal levels of UVB irradiation. In contrast, higher UVB doses—those sufficient to cause pronounced DNA damage and cytotoxicity—appear to favour apoptotic pathways over senescence.
These findings highlight the dose-dependent nature of UVB-induced cellular fate decisions, with lower doses promoting a senescence-like state and higher doses driving apoptotic mechanisms.
Figure 5. UVB exposure effects the gene expression of beta-Galactosidase, a marker of senescence. Primary healthy human keratinocytes were stimulated with 2 doses of UVB and returned to incubate for 24 hours.. Gene expression of the senescence marker, GLB1 was assessed by their gene expression in the cell pellet. Statistical significance was determined relative to the basal unirradiated control (***p<0.001; **p<0.01; *p<0.05; n=2±SEM).
To investigate more immediate cellular responses to UVB exposure, we examined the induction of oxidative stress by quantifying the production of reactive oxygen species (ROS) in keratinocytes before and after irradiation.
In this assay, antimycin A (a known ROS inducer) and N-acetyl cysteine (NAC) (a ROS scavenger) were included as positive and negative controls, respectively, to validate the assay conditions. Our results demonstrated a significant increase in ROS levels in primary human keratinocytes following exposure to 240 mJ/cm² of UVB radiation (Figure 6).
These findings confirm that UVB irradiation triggers a rapid oxidative stress response, which is likely to contribute to downstream cellular damage pathways, including inflammation, apoptosis, and senescence.
Figure 6. UVB exposure induces oxidative stress in Primary healthy human keratinocytes. Primary human keratinocytes were stimulated with 240 mJ/cm2 UVB and returned to incubate for 4 hours. Reactive oxygen species (ROS) was used as a quantifiable measure of oxidative stress. Antimycin and NAC served as ROS positive and negative controls respectively. Statistical significance was determined by comparison against the basal unirradiated control. (***p<0.001; **p<0.01; *p<0.05; n=2±SEM).
Cellomatics’ commitments and ongoing efforts
The skin serves as the body’s first line of defence against environmental insults. Understanding the damage caused by common external stimuli—and developing reliable, animal-free models to test potential treatments or protective agents—could benefit the nearly 1.8 billion people worldwide who suffer from chronic skin conditions [5].
At Cellomatics, we are committed to being at the forefront of designing and validating physiologically and clinically relevant in vitro skin models to support both drug development and cosmetic formulation.
We are actively expanding the range of scalable, high-fidelity skin models with enhanced physiological relevance, bridging the gap between in vitro experimentation and clinical outcomes. By leveraging our expertise in primary keratinocyte culture and immunocompetent cell systems, we are developing next-generation models—such as 3D human skin equivalents with immune competence—that offer customisable outputs, including imaging-based data, and provide superior insights into drug efficacy and cosmetic safety.
This article highlights our ongoing efforts and ambitions to support innovation in skin research. Our mission is to deliver the most current, customisable, and reliable in vitro skin models, tailored to meet the specific needs of our clients in both the pharmaceutical and cosmetic industries.
To learn more about our innovations in skin research and the services we offer, please do not hesitate to get in contact with the team today.
References
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC7271707/
[2] https://pubmed.ncbi.nlm.nih.gov/36528129/
[3] https://pmc.ncbi.nlm.nih.gov/articles/PMC8069861/
[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC5295856/
[6] https://pmc.ncbi.nlm.nih.gov/articles/PMC10249123
[7] https://www.ncbi.nlm.nih.gov/books/NBK24645/
[8] https://single-market-economy.ec.europa.eu/sectors/cosmetics/ban-animal-testing_en
[9] https://pubmed.ncbi.nlm.nih.gov/27343554/
[10] https://pmc.ncbi.nlm.nih.gov/articles/PMC4842382/
[11] https://pmc.ncbi.nlm.nih.gov/articles/PMC9671911/
[12] https://pubmed.ncbi.nlm.nih.gov/16155000/
[13] https://single-market-economy.ec.europa.eu/sectors/cosmetics/legislation_en
[14] https://legalinstruments.oecd.org/en/instruments/OECD-LEGAL-0146
[15] https://www.theguardian.com/world/2015/aug/14/ivory-coast-skin-whitening-ban-effects
[16] https://www.theguardian.com/us-news/2025/feb/25/black-women-beauty-products-hazardous-ingredients
[17] https://single-market-economy.ec.europa.eu/sectors/cosmetics/ban-animal-testing_en
[18] https://pmc.ncbi.nlm.nih.gov/articles/PMC7292860/
[19] https://ijdvl.com/adverse-reactions-to-cosmetics/
[20] https://www.loreal.com/en/articles/research-innovation/the-incredible-destiny-of-reconstructed-skin/
[21] https://pmc.ncbi.nlm.nih.gov/articles/PMC10836957/
[22] https://pubmed.ncbi.nlm.nih.gov/1052770/
[23] https://pubmed.ncbi.nlm.nih.gov/15349789/
