Age-Dependent Transformation of Skin Biomechanical Properties and Micromorphology during Infancy and Childhood (2018)

It is well known that skin continues to undergo structural and functional changes after birth. These include skin surface acidification (Fluhr et al., 2012Fluhr J.W. Darlenski R. Lachmann N. Baudouin C. Msika P. De Belilovsky C. et al.Infant epidermal skin physiology: adaptation after birth.Br J Dermatol. 2012; 166: 483-490Crossref PubMed Scopus (116) Google Scholar); increased hydration over the first few weeks of life, followed by decreased hydration (Nikolovski et al., 2008Nikolovski J. Stamatas G.N. Kollias N. Wiegand B.C. Barrier function and water-holding and transport properties of infant stratum corneum are different from adult and continue to develop through the first year of life.J Invest Dermatol. 2008; 128: 1728-1736Abstract Full Text Full Text PDF PubMed Scopus (263) Google Scholar); and changes in cell organization and structure (Bensaci et al., 2015Bensaci J. Chen Z.Y. Mack M.C. Guillaud M. Stamatas G.N. Geometrical and topological analysis of in vivo confocal microscopy images reveals dynamic maturation of epidermal structures during the first years of life.J Biomed Optics. 2015; 20: 095004Crossref PubMed Scopus (7) Google Scholar, Fluhr et al., 2014Fluhr J.W. Lachmann N. Baudouin C. Msika P. Darlenski R. De Belilovsky C. et al.Development and organization of human stratum corneum after birth: electron microscopy isotropy score and immunocytochemical corneocyte labelling as epidermal maturation's markers in infancy.Br J Dermatol. 2014; 171: 978-986Crossref PubMed Scopus (21) Google Scholar). In terms of biomechanical properties, skin elasticity decreases over the first decade of life (Sugihara et al., 1991Sugihara T. Ohura T. Homma K. Igawa H.H. The extensibility in human skin: variation according to age and site.Br J Plast Surg. 1991; 44: 418-422Abstract Full Text PDF PubMed Scopus (41) Google Scholar) and collagen and elastin fiber density increases for several decades (Vitellaro-Zuccarello et al., 1994Vitellaro-Zuccarello L. Cappelletti S. Dal Pozzo Rossi V. Sari-Gorla M. Stereological analysis of collagen and elastic fibers in the normal human dermis: variability with age, sex, and body region.Anat Rec. 1994; 238: 153-162Crossref PubMed Scopus (72) Google Scholar). Studies of the biomechanical maturation of the skin have focused primarily on changes that occur in adults during aging, and most studies of skin maturation in children have focused on the maturation of the epidermis, which increases in thickness with age and has high cell turnover in the first months of life (Stamatas et al., 2010Stamatas G.N. Nikolovski J. Luedtke M.A. Kollias N. Wiegand B.C. Infant skin microstructure assessed in vivo differs from adult skin in organization and at the cellular level.Pediatr Dermatol. 2010; 27: 125-131Crossref PubMed Scopus (174) Google Scholar). One recent study showed that skin biomechanical properties evolve throughout infancy (Visscher et al., 2017Visscher M.O. Burkes S.A. Adams D.M. Hammill A.M. Wickett R.R. Infant skin maturation: preliminary outcomes for color and biomechanical properties.Skin Res Technol. 2017; 23: 545-551Crossref PubMed Scopus (13) Google Scholar), but such studies remain rare. Here, we investigated the correlation between the maturation of biomechanical properties of the skin and the evolution of skin topography and micromorphology from infancy to early adulthood (study approved by Provincial Ethical Committee of Modena, University Hospital of Modena). We recruited a cohort of 70 subjects in seven age groups: 1–15 days, 5 weeks, 5–7 months, 2 years, 4–5 years, 7–8 years, and 20–35 years (Supplementary Table S1 online); all patients or their parent or guardian gave their informed written consent. Skin properties were examined by cutometry and reflectance confocal microscopy in vivo, and by immunohistochemistry in a limited number of foreskin biopsy samples (see Supplementary Material online for detailed methods). Cutometry showed that skin elasticity, as measured by the ratio of immediate retraction to maximum distention (Ur/Uf), increased from infancy to 2 years of age and then plateaued (Figure 1a). The viscoelastic component, calculated as the ratio of immediate to delayed distension (Uv/Ue), decreased from infancy to adulthood (Figure 1b). Total recovery (Ua) was slightly higher at older ages (Figure 1c) and total deformation (Uf) did not vary between age groups (Figure 1d). The parameters related to skin elasticity and recovery (Ur/Uf, Ua/Uf, Ur/Ue, and Ua) were positively correlated with age and body surface area, whereas Uv/Ue was negatively correlated with both of these and Uf did not have any significant correlations (Figure 1e). None of the parameters were correlated with stratum corneum hydration. The viscoelastic properties of the skin are related to the presence of interstitial fluid in the dermal extracellular matrix, and thus changes in Uv/Ue may reflect the water content of the epidermis and the dermis (Dobrev, 2002Dobrev H.P. A study of human skin mechanical properties by means of Cutometer.Folia Med. 2002; 44: 5-10Google Scholar). Although we cannot completely rule out this possibility, none of the parameters examined were significantly (ie, P < 0.01) correlated with stratum corneum hydration as measured by cutometer (Figure 1e), suggesting that the changes in these skin biomechanical properties are related to structural maturation rather than hydration. This is consistent with reports that skin biomechanical properties are related to the structure of the extracellular matrix in adults, specifically elastin fibers, fibrillin microfibrils, rete ridges, and, to a lesser extent, collagen fibrils (Langton et al., 2017Langton A.K. Graham H.K. McConnell J.C. Sherratt M.J. Griffiths C.E.M. Watson R.E.B. Organization of the dermal matrix impacts the biomechanical properties of skin.BrJ Dermatol. 2017; 177: 818-827Crossref PubMed Scopus (37) Google Scholar). Skin microstructure was examined by reflectance confocal microscopy, which allows fast, in vivo imaging of the cyto-architectural aspects of the skin. The thickness of the stratum corneum and the supra papillary epidermis increased with age (Figure 2a), consistent with previous findings (Stamatas et al., 2010Stamatas G.N. Nikolovski J. Luedtke M.A. Kollias N. Wiegand B.C. Infant skin microstructure assessed in vivo differs from adult skin in organization and at the cellular level.Pediatr Dermatol. 2010; 27: 125-131Crossref PubMed Scopus (174) Google Scholar). The homogeneity and furrow architecture of the stratum corneum changed dramatically between infants and older children (Supplementary Figure S1a–S1d). Reflecting spheroids were observed throughout childhood and were most prevalent during infancy (Supplementary Figure S1e–S1f), and the nature of these structures warrants further investigation. The observed evolution from poorly defined to well-defined keratinocyte outline (Supplementary Table S2 online) might reflect the fact that keratinocyte proliferation is high during the first months of life (Stamatas et al., 2010Stamatas G.N. Nikolovski J. Luedtke M.A. Kollias N. Wiegand B.C. Infant skin microstructure assessed in vivo differs from adult skin in organization and at the cellular level.Pediatr Dermatol. 2010; 27: 125-131Crossref PubMed Scopus (174) Google Scholar). Dermal papillae increased in number with age, consistent with a previous study by Miyauchi et al., 2016Miyauchi Y. Shimaoka Y. Fujimura T. Koike Y. Yatabe M. Nishikawa M. et al.Developmental changes in neonatal and infant skin structures during the first 6 months: in vivo observation.Pediat Dermatol. 2016; 33: 289-295Crossref PubMed Scopus (15) Google Scholar, but contrary to this previous study, rete ridge thickness was stable across age groups (Figure 2a). This contradiction could be explained by a difference in age group stratification in the studies. Interestingly, collagen fibers were fibrillar and showed a parallel orientation in newborns, whereas they were thicker, coarse, and multidirectional in older infants through adults (Figure 2a–2c). Mechanical forces are known to increase the diameter of collagen fibrils (Sanders and Goldstein, 2001Sanders J.E. Goldstein B.S. Collagen fibril diameters increase and fibril densities decrease in skin subjected to repetitive compressive and shear stresses.J Biomech. 2001; 34: 1581-1587Crossref PubMed Scopus (63) Google Scholar). Our data support a contribution of mechanical forces to dermal maturation postnatally, including the reorganization of the collagen matrix and increased collagen fiber thickness (Sanders and Goldstein, 2001Sanders J.E. Goldstein B.S. Collagen fibril diameters increase and fibril densities decrease in skin subjected to repetitive compressive and shear stresses.J Biomech. 2001; 34: 1581-1587Crossref PubMed Scopus (63) Google Scholar). We also noticed a structural pattern that has not been reported previously. Circular “cuffing” of the follicle by collagen fibers was observed in newborns only, and was very rare or completely absent in all other age groups (Figure 2a, 2d, 2e). This interesting feature may reflect the unique properties of postnatal hair follicle growth (Zhou et al., 2016Zhou L. Yang K. Wickett R.R. Andl T. Zhang Y. Dermal sheath cells contribute to postnatal hair follicle growth and cycling.J Dermatol Sci. 2016; 82: 129-131Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar). The elastin component of the skin cannot be observed by reflectance confocal microscopy. Therefore, we analyzed its structural maturation by immunohistochemistry on foreskin samples from patients of various ages. Both fibrillin and elastin fibers increased in length and intensity with age, especially at the dermal–epidermal junction (Supplementary Figure S2 online). The strength of the present study is the analysis of both structural and biomechanical properties of the skin in the same subjects over a wide range of age groups. In addition to evidence supporting the relationship between structural and biomechanical properties during skin maturation, we introduce the observation of collagen fiber cuffing around hair follicles in newborns. Collectively, these data demonstrate the biochemical and structural evolution of the dermis during postnatal development, as well as the translation of these changes into maturation of the biomechanical properties of the skin. GB, CDB, GB, NL, PM, and CB are current or former employees of Expanscience Laboratories; JF and RD provided consulting services for Expanscience; CPMG, NF, and BC provided services on a commercial basis for this study; GP and CM received a research grant; GP was investigator of the clinical study. This study was funded by Expanscience Laboratories. The authors thank BioScienceWriters for diligence in the full review and editing of the manuscript. Download .pdf (1.09 MB) Help with pdf files Supplementary Data