Therapeutic Improvement of Scarring: Mechanisms of Scarless and Scar-Forming Healing and Approaches to the Discovery of New Treatments

Table 1

In Vivo Models Used to Evaluate Scar Improvement Therapies.

Model	Endpoints studied	Utility	Limitations	References
Transgenic mice; (knockouts and overexpressors; incisions and punch biopsy wounds)	Regeneration, improvements in scarring measured using a range of parameters (gene, protein, histological analysis, tensile strength, macroscopic appearance, etc.). Endpoints typically studied at 14 to 28 days postwounding. Scars stable around day 70.	Initial target identification and validation. Gene modifications may elicit effects on the scarring response, inducing scarless healing or excessive scarring. Incisions most relevant for scarring, punch biopsies relevant for healing endpoints.	Gene deletions/additions can elicit lethal effects and may provide misleading data if compensatory mechanisms due to genetic alteration(s) occur within the animal (also see general comments on mice below).	[22, 23, 33–36]
Mouse; (incisions and punch biopsy wounds)	Improvements in scarring measured using a range of parameters (gene, protein, histological analysis, tensile strength, macroscopic appearance, etc.). Endpoints typically studied at 14 to 28 days postwounding. Scars stable around day 70 postwounding.	Outbred and inbred strains can be utilised. Gene expression data indicate that molecular processes have a relevant level of comparability to humans. Modulators of the scarring response can be evaluated in the absence of any potentially confounding effects seen in transgenic animals. Incisions most relevant for scarring, punch biopsies relevant for healing endpoints.	Degree of scarring in mice at macroscopic and microscopic levels is significantly less than in humans. Therefore relatively difficult to accurately quantitate improvements with treatments over the normal scarring response. Time points selected for assessment in published studies, for example, 14 to 28 days, are typically during the granulation tissue formation phase prior to formation of a stable scar. As such, these studies do not represent a suitable time point for evaluating the true scar reduction effects of therapies.	[29, 37, 38]
Rat; (incisions and punch biopsy wounds)	Improvements in scarring measured using a range of parameters (gene, protein, histological analysis, wound width, tensile strength, macroscopic appearance, etc.). Scars stable around 80 days post wounding.	Rats demonstrate comparability to scars in humans (volunteers) at the macroscopic, microscopic, and gene expression levels. Relatively easy to differentiate the effects of scar reducing agents.	Many scientific reagents are geared towards the study of mice and humans, and consequently there are some limitations in terms of reagents (e.g., antibodies for immunocytochemistry) to completely compare all mechanisms to those in man. Most studies use unsuitable time points of <70 days, when scars have not matured/stabilised.	[39–41]
Rabbit; (incisions and punch biopsy wounds)	Improvements in scarring measured using a range of parameters (gene, protein, histological analysis, macroscopic appearance, etc.). Endpoints typically studied at 20 to 40 days postwounding.	Ear wounds are often used as a model for chronic healing and excessive scarring.	Rarely used to assess normal skin wound healing on the back. Although some features of excessive scarring are modelled, the biological relevance of the ear wounds (involving cartilage) to cutaneous wounds in humans is not completely clear.	[42–44]
Pig; (minipigs, domestic swine and Red Duroc; incisions, excisions, and punch biopsy wounds)	Improvements in scarring measured using a range of parameters (gene, protein, histological analysis, wound width, tensile strength, macroscopic appearance, etc.). Endpoints typically studied up to 6–12 months postwounding when the scars are stable.	Structure of skin is reported to be most similar to humans. Gene expression data indicates that molecular processes have a relevant level of comparability to humans. Accepted species for wound healing studies. Large or multiple wounds possible due to size. Incisions most relevant for scarring, punch biopsies relevant for healing endpoints. Red Duroc pig is reported to model aspects of hypertrophic scarring in humans.	Degree of scarring in pigs at macroscopic and microscopic levels is significantly less than in humans. Lengthy and costly studies due to timing of relevant endpoints. Red Duroc pigs do not accurately model all relevant aspects of human hypertrophic scars and require significantly long studies and therefore have an associated potentially prohibitive cost. No robust evidence of translation of findings in models to effective therapeutics in prospective, double-blind and well-controlled trials in humans.	[9, 45–48]
Human volunteers; (incisions, excisions and punch biopsy wounds)	Improvements in scarring measured using a range of parameters (gene, protein, histological analysis, wound width, tensile strength, macroscopic appearance, etc.).	Suitable for demonstrating safety and efficacy. Model highly relevant and translates to patient-based studies. Easy to differentiate the effects of scar reducing agents on a range of clinically and scientifically relevant parameters.	Requirement of suitable infrastructure and expertise.	[13, 49–53]