Collagen structural alterations contribute to stiffening of tissue after split‐thickness skin grafting

Abstract

The gold standard treatment for full thickness injuries of the skin is autologous split-thickness skin grafting. This involves harvesting the epidermis and superficial dermis from healthy skin and transplanting it onto the prepared wound bed. The donor site regenerates spontaneously, but the appendages and cellular components from the dermal layer are excluded from the graft. As a result, the new tissue is inferior; the healed graft site is dry/itchy, has decreased elasticity, increased fragility, and altered sensory function. Because this dermal layer is composed of collagen and other extracellular matrix proteins, the aim was to characterize the changes in the dermal collagen after split thickness grafting that could contribute to a deficit in functionality. This will serve as a baseline for future studies designed to improve skin function using pharmacological or cell-based therapies for skin repair. A xenograft model whereby human split-thickness grafts were implanted into full-thickness defects on immunocompromised (athymic Nu/Nu) mice was used. The grafts were harvested 4 and 8 weeks later. The collagen microstructure was assessed with second harmonic generation with dual-photon microscopy and light polarization analysis. Collagen fiber stiffness and engagement stretch were estimated by fitting the results of biaxial mechanical tensile tests to a histo-mechanical constitutive model. The stiffness of the collagen fibril-proteoglycan complex increased from 682 ± 226 kPa/sr to 1016 ± 324 kPa/sr between 4 and 8 weeks postgrafting. At the microstructural level there were significant decreases in both thickness of collagen fibers (3.60 ± 0.34 μm vs. 2.10 ± 0.27 μm) and waviness ratio (2.04 ± 0.17 vs. 1.43 ± 0.08) of the collagen fibers postgrafting. The decrease of the macroscopic engagement stretch from 1.19 ± 0.11 to 1.09 ± 0.08 over time postgrafting mirrored the decrease in waviness measured at the microscopic level. This suggested that the integrity of the collagen fibers was compromised and contributed to the functional deficit of the skin postgrafting.

Determination of Spearman Correlation Coefficient(r)to Evaluate the Linear Association of Dermal Collagen and Elastic Fibers in the Perspectives of Skin Injury

“…A study has shown that, in the process of skin graft, an imbalance between collagen synthesis and its cross-linkage and/or the lack of dermal appendages make it relatively less elastic and vulnerable to immobility (contracture) [2]. The progression of healing following skin grafting/skin injury is similar [9].…”

“…In addition to age factor, chronic exposure of the skin to UV rays [18] or treatment with laser light [19] also affect the collagen content of scar. Therefore, the wounds would heal more slowly and the skin will be more vulnerable to environmental stressors [2]. …”

“…The major drawback of this approach is that the healed split skin graft usually lacks elasticity and increased fragility associated with sensory disturbances. These changes in the transplanted split skin graft may be attributed to changes in the arrangement and content (ratio) of collagen and elastic fibers together with the extracellular matrix proteins [2]. …”

Imaging Collagen in Scar Tissue: Developments in Second Harmonic Generation Microscopy for Biomedical Applications

“…Of particular note is that the proportion of collagen fibres that are aligned increases in keloid and surgical scar tissue [ 104 , 105 , 106 ]. The collagen fibre bundles in scarred skin after grafting have been shown to be straighter and thinner [ 85 ]. These factors appear to contribute to the decrease in the tensile strength of the scar compared with normal skin [ 85 , 107 ].…”

Quantification of collagen organization in histopathology samples using liquid crystal based polarization microscopy

“…We report here the first use of the birefringence mode of the LC-PolScope for quantitatively imaging fibrillar collagen in tissues, and comparing it against SHG microscopy. Previous studies [32,33] have used the LC-PolScope as a tool for looking at directionality of collagen fibers and alignment in collagen gels but not for quantitating collagen organization in pathology samples. This study proposes that the LC-PolScope could be a powerful tool for histopathology by measuring changes in collagen organization.…”