**1. Introduction**

Despite the implementation of prevention strategies, the incidence and prevalence of pressure ulcers (PU) in today's society is high. Because of population aging and medical advances, this disease has become a health problem, which also entails social and economic impact. [1] Research on the pathogenesis and mechanisms involved in PU and wound healing is crucial. However, research in this field has been hindered by a lack of experimental animal models [2].

Regarding the etiology of PUs, external pressure is viewed as the main factor. Other patient-specific factors leading to derangement in tissue perfusion may account for an observed development of a

> 33

pressure ulcer. It is well known that ischemia–reperfusion injury contributes to the pathophysiology of PUs more significantly than a single, prolonged ischemic insult [3,4]. Previously published PU models have been developed using animal skin, which imposes limitations when studying the wound healing process and the extrapolation of results and the e ffect of treatments on humans.

Once a pressure ulcer has developed, its course becomes insidious and its evolution towards healing is prolonged. The managemen<sup>t</sup> of these patients needs a comprehensive and multidisciplinary team approach, involving plastic surgeons, nurses, nutritionists, rehabilitation therapists, etc. The goal in these chronic wounds is prompt tissue recovery. [1,5] Treatments that accelerate wound closure are of vital importance and interest for its potential benefits for these patients. The available evidence supports the use of di fferent therapeutic options based on patient and wound characteristics, but none preponderates as the gold standard. Tissue engineering applied to wound healing, and specifically to PU, and approaches incorporating cellular therapy and growth factors are an expanding field in this issue. According to the American and European Pressure Ulcer Advisory Panel guidelines, Platelet-Derived Growth Factor (PDGF) can be used for Category/Stage III and IV pressure ulcers that have delayed healing. However, the evidence is not su fficient to recommend or refute the routine use of other growth factors in the treatment of pressure ulcers. [6] Rees et al. [7] published a randomized double blind, placebo-controlled study in which the e fficacy of becaplermin gel (recombinant human PDFG-BB) in the treatment of chronic full thickness pressure ulcers was compared to that of placebo gel. The authors concluded that within the setting of a comprehensive wound managemen<sup>t</sup> program, becaplermin gel once daily increases the incidence of complete healing and≥90% healing in patients with full thickness pressure ulcers. Another randomized double blinded study [8] suggested that treatment with PDGF-BB before surgery enhances the ability to achieve a closed wound over surgery alone.

The growth hormone (GH), anabolic hormone, regulates growth through hypertrophy, hyperplasia, and as a result of tissue di fferentiation, cell proliferation and protein synthesis. It can exert its e ffects directly on the tissues, or indirectly through mediators: the so-called insulin-like growth factors IGF-I and IGF-II. [9,10] GH activity, sometimes mediated via IGF-I, is involved in skin homeostasis and wound healing acting at di fferent stages and including dermal–epidermal communication [11]. During the inflammatory phase of healing, macrophages deliver growth factors that attract fibroblasts and facilitate the proliferative phase. GH promotes the release of some of these factors, such as EGF, VEGF, and FGF. Furthermore, IGF-I and II mRNAs are modulated during the healing process. The highest levels of IGF-I are seen in human wound fluids within 24 h after injury, and they return to their baseline when healing ends. [12,13] Animal models reveal that systemic GH promotes cellular proliferation, especially fibroblasts; granulation tissue formation; increases collagen formation and extracellular matrix; and enhances keratinocyte migration, shortening the time for healing, as well as providing increased mechanical strength of the wounds. [14–23] GH is also an inducer of the immune system. It mainly acts on macrophages and T-lymphocytes, that also play a key role in wound healing. [24] Studies in burns show that rhGH treatment could result in improved wound healing and reduced length of hospital stay. [25] Most of the studies have been developed applying systemic GH, and adverse e ffects cannot be ignored. [26]

The aim of this study is to compare the healing rate and di fferent histological parameters on a previously described PU human skin model with and without the application of the recombinant human growth hormone (rhGH). We hypothesize that the local administration of the rhGH should increase the healing rate of PU and improve the quality of the human skin.
