The mechanism of excisional fetal wound repair in vitro is responsive to growth factors

Epithelial bridges maintain tissue integrity during collective cell migration

The ability of skin to act as a barrier is primarily determined by the efficiency of skin cells to maintain and restore its continuity and integrity. In fact, during wound healing keratinocytes migrate collectively to maintain their cohesion despite heterogeneities in the extracellular matrix. Here, we show that monolayers of human keratinocytes migrating along functionalized micropatterned surfaces comprising alternating strips of extracellular matrix (fibronectin) and non-adherent polymer form suspended multicellular bridges over the non-adherent areas. The bridges are held together by intercellular adhesion and are subjected to considerable tension, as indicated by the presence of prominent actin bundles. We also show that a model based on force propagation through an elastic material reproduces the main features of bridge maintenance and tension distribution. Our findings suggest that multicellular bridges maintain tissue integrity during wound healing when cell-substrate interactions are weak and may prove helpful in the design of artificial scaffolds for skin regeneration.

Cytoskeletal regulation of dermal regeneration

Wound healing results in the repair of injured tissues however fibrosis and scar formation are, more often than not the unfortunate consequence of this process. The ability of lower order vertebrates and invertebrates to regenerate limbs and tissues has been all but lost in mammals; however, there are some instances where glimpses of mammalian regenerative capacity do exist. Here we describe the unlocked potential that exists in mammals that may help us understand the process of regeneration post-injury and highlight the potential role of the actin cytoskeleton in this process. The precise function and regulation of the cytoskeleton is critical to the success of the healing process and its manipulation may therefore facilitate regenerative healing. The gelsolin family of actin remodelling proteins in particular has been shown to have important functions in wound healing and family member Flightless I (Flii) is involved in both regeneration and repair. Understanding the interactions between different cytoskeletal proteins and their dynamic control of processes including cellular adhesion, contraction and motility may assist the development of therapeutics that will stimulate regeneration rather than repair.

Decreased expression of Flightless I, a gelsolin family member and developmental regulator, in early-gestation fetal wounds improves healing

Up until late in the third trimester of gestation and through to adulthood, the healing response acts more to regenerate than to repair a wound. The mechanisms underlying this "scar-free" healing remain unknown although the actin cytoskeleton has a major role. Flightless I (Flii), an actin-remodelling protein and essential developmental regulator, negatively affects wound repair but its effect on scar-free fetal healing is unknown. Using fetal skin explants from E17 (regenerate) and E19 (repair) rats, the function of Flii in fetal wound repair was determined. Expression of Flii increased between E17 and E19 days of gestation and wounding transiently increased Flii expression in E17 but not E19 wounds. However, both confocal and immunofluorescent analysis showed E17 keratinocytes immediately adjacent to the wounds downregulated Flii. As a nuclear coactivator and inhibitor of proliferation and migration, the absence of Flii in cells at the edge of the wound could be instrumental in allowing these cells to proliferate and migrate into the wound deficit. In contrast, Flii was strongly expressed within the cytoplasm and nucleus of keratinocytes within epidermal cells at the leading edge of E19 wounded fetal skin explants. This increase in Flii expression in E19 wounds could affect the way these cells migrate into the wound space and contribute to impaired wound healing. Neutralising Flii protein improved healing of early- but not late-gestation wounds. Flii did not colocalise with actin cables formed around E17 wounds suggesting an independent mechanism of action distinct from its actin-binding function in scar-free wound repair.

Wound healing in a fetal, adult, and scar tissue model: a comparative study

Early gestation fetal wounds heal without scar formation. Understanding the mechanism of this scarless healing may lead to new therapeutic strategies for improving adult wound healing. The aims of this study were to develop a human fetal wound model in which fetal healing can be studied and to compare this model with a human adult and scar tissue model. A burn wound (10 x 2 mm) was made in human ex vivo fetal, adult, and scar tissue under controlled and standardized conditions. Subsequently, the skin samples were cultured for 7, 14, and 21 days. Cells in the skin samples maintained their viability during the 21-day culture period. Already after 7 days, a significantly higher median percentage of wound closure was achieved in the fetal skin model vs. the adult and scar tissue model (74% vs. 28 and 29%, respectively, p<0.05). After 21 days of culture, only fetal wounds were completely reepithelialized. Fibroblasts migrated into the wounded dermis of all three wound models during culture, but more fibroblasts were present earlier in the wound area of the fetal skin model. The fast reepithelialization and prompt presence of many fibroblasts in the fetal model suggest that rapid healing might play a role in scarless healing.

Promoting the proliferative and synthetic activity of knee meniscal fibrochondrocytes using basic fibroblast growth factor in vitro

BACKGROUND

Meniscal tears situated within the inner avascular region do not heal despite suturing. New approaches need to be developed to augment surgical repair.

HYPOTHESIS

To demonstrate that basic fibroblast growth factor, used as a single agent or in combination with serum, stimulates the activity of fibrochondrocytes by enhancing proliferation and extracellular matrix synthesis in all meniscal zones, including the inner (avascular) zone of the meniscus.

STUDY DESIGN

Controlled laboratory study.

METHODS

Monolayer cell cultures were prepared from the inner, middle, and outer zones of the lateral meniscus. Various concentrations of basic fibroblast growth factor were used in the presence or absence of 10% fetal calf serum. The authors measured the uptake of radiolabeled thymidine to assess cell proliferation and radioactive sulfur and proline to assess extracellular matrix formation.

RESULTS

Overall, basic fibroblast growth factor-stimulated cells from all meniscal zones to proliferate and to form new extra-cellular matrix (P <.05). The basic fibroblast growth factor (in the absence of serum) increased DNA formation and protein synthesis by cells from the inner meniscal zone by 7- and 15-fold, respectively (P <.001).

CONCLUSIONS

These results indicate that meniscal cells and, more important, cells from the avascular zone are capable of responding favorably to the addition of basic fibroblast growth factor by expressing their intrinsic potential to proliferate and generate new extracellular matrix.

CLINICAL RELEVANCE

The results suggest that it may be possible to augment surgical repair of the meniscus in the future.

Regulation of MAPK Activation, AP-1 Transcription Factor Expression and Keratinocyte Differentiation in Wounded Fetal Skin

Fetal epithelium retains the ability to re-epithelialize a wound in organotypic culture in a manner not dependent on the presence of underlying dermal substrata. This capacity is lost late in the third trimester of gestation or after embryonic day 17 (E(17)) in the rat such that embryonic day 19 (E(19)) wounds do not re-epithelialize. Moreover, wounds created in E(17) fetuses in utero heal in a regenerative, scar-free fashion. To investigate the molecular events regulating re-epithelialization in fetal skin, the wound-induced expression profile and tissue localization of activator protein 1 (AP-1) transcription factors c-Fos and c-Jun was characterised in E(17) and E(19) skin using organotypic fetal cultures. The involvement of mitogen-activated protein kinase (MAPK) signaling in mediating wound-induced transcription factor expression and wound re-epithelialization was assessed, with the effect of wounding on the expression of keratinocyte differentiation markers determined. Our results show that expression of AP-1 transcription factors was induced immediately by wounding and localized predominantly to the epidermis in E(17) and E(19) skin. c-fos and c-jun induction was transient in E(17) skin with MAPK-dependent c-fos expression necessary for the re-epithelialization of an excisional wound in organotypic culture. In E(19) skin, AP-1 expression persisted beyond 12 h post-wounding, and marked upregulation of the keratinocyte differentiation markers keratin 10 and loricrin was observed. No such changes in the expression of keratin 10 or loricrin occurred in E(17) skin. These findings indicate that re-epithelialization in fetal skin is regulated by wound-induced AP-1 transcription factor expression via MAPK and the differentiation status of keratinocytes.

Differential effect of wounding on actin and its associated proteins, paxillin and gelsolin, in fetal skin explants

Skin from the embryonic day 17 rat retains the ability to epithelialize an excisional wound when isolated in serum-supplemented suspension culture. This ability is lost by embryonic day 19. We have investigated this effect of gestational age on fetal epithelial wound closure by correlating the involvement of filamentous actin (F-actin) and its associated proteins, paxillin and gelsolin, in the wound margins of embryonic day 17 and 19 rat skins, with the ability to close a full thickness excisional wound. Using fluorescent-phalloidin histochemistry and scanning confocal microscopy, actin polymerization was observed some five to six cells back from the margin of wounds in the embryonic day 17 skin as early as 3 h postwounding. As the wounds closed over the following 48-72 h, the actin further condensed around the epithelial margin before dispersing after wound closure. In contrast, no organization of actin was seen in the epithelial margin of wounds in skin from the embryonic day 19 embryos. Instead, actin filaments were observed surrounding the dermal wound margins. Chemical or mechanical disruption of the actin in wounded embryonic day 17 skins prevented epithelial closure, although wound repair was independent of cell division. In particular, incising the wound margin 24 h after wounding resulted in the "springing-open" of the embryonic day 17 wound but not the embryonic day 19 wound, reflecting the development of tension in the embryonic day 17 wound margin. Expression of paxillin mRNA was upregulated following wounding at embryonic day 17 but not at embryonic day 19. Paxillin was also observed to colocalize with actin in embryonic day 17 wounds, but not embryonic day 19 wounds, indicating a potential role for paxillin in epithelial repair of the fetal wound. In contrast, gelsolin mRNA was upregulated in embryonic day 19 fetal skin but not at embryonic day 17 and gelsolin protein was observed surrounding actin filaments at embryonic day 19 but not embryonic day 17. These results demonstrate a change in the mechanism of wound epithelialization at the same gestational age that fetal wounds change from scar-free to scar-forming wound repair.

Fetal rat amniotic fluid: transforming growth factor beta and fibroblast collagen lattice contraction

BACKGROUND

In several mammalian animal models, early-gestational-age fetal wounds heal without scar, but wounds of late gestational age heal with scar. This change in wound healing phenotype can be a result of both intrinsic (i.e., cellular characteristics) and extrinsic (i.e., environmental) factors. Our question was: Does amniotic fluid (AF) influence the change from scarless to scar-forming repair in the rat?

METHODS

Rat AF was investigated for its modulation of fibroblast-populated collagen lattice (FPCL) contraction and morphological changes of adult fibroblasts. AF was also assayed for transforming growth factor beta (TGF-beta) levels. Adult rat dermal fibroblasts in monolayer and incorporated into FPCLs were incubated with AF additions from gestational age 14, 16, 18, and 21 days at 10% (v/v).

RESULTS

Day 14 AF significantly stimulated FPCL contraction, but AF of 16, 18, and 21 days inhibited FPCL contraction. Fluorescence histology identified microtubules and microfilaments in AF treated adult rat dermal fibroblasts. The staining pattern of microtubules in Day 14 AF-treated fibroblasts showed denser structures at the cell center. Cells incubated with Day 16 or 18 AF showed fine peripheral microtubules. A mink lung epithelial cell bioassay was used to analyze concentrations of TGF-beta in AF. TGF-beta levels were greatly elevated in Day 14 AF, but were relatively low in Day 16, 18 and 21 AF. The inhibitor of FPCL contraction from AF of Days 16, 18, and 21 was not identified.

CONCLUSION

It is proposed that the robust expression of TGF-beta or cytoskeletal changes induced by Day 14 AF contributes to enhanced FPCL contraction.

Mitogenic whey extract stimulates wound repair activity in vitro and promotes healing of rat incisional wounds

The ability of single growth factors to promote healing of normal and compromised wounds has been well described, but wound healing is a process requiring the coordinated action of multiple growth factors. Only the synergistic effect on wound healing of combinations containing at most two individual growth factors has been reported. We sought to assess the ability of a novel milk-derived growth factor-enriched preparation ¿mitogenic bovine whey extract (MBWE), which contains six known growth factors, to promote repair processes in organotypic in vitro models and incisional wounds in vivo. MBWE stimulated the contraction of fibroblast-populated collagen lattices in a dose-dependent fashion and promoted the closure of excisional wounds in embryonic day 17 fetal rat skin. Application of MBWE increased incisional wound strength in normal animals on days 3, 5, 7, and 10 and reversed the decrease in wound strength observed following steroid treatment. Wound histology showed increased fibroblast numbers in wounds from normal and steroid-compromised animals. These data suggest the mixture of factors present in bovine milk exerts a direct action on the cells of cutaneous wound repair to enhance both normal and compromised healing.

Immunohistochemical expression of growth factors in subacute thyroiditis and their effects on thyroid folliculogenesis and angiogenesis in collagen gel matrix culture

The inflammatory-mechanistic basis of subacute thyroiditis remains unclear. To elucidate the roles of vascular endothelial cell growth factor (VEGF), basic fibroblast growth factor (bFGF), platelet-derived growth factor-BB (PDGF), transforming growth factor-beta1 (TGF-beta1) and epidermal growth factor (EGF) in the inflammatory process, their immunoexpression was examined in biopsy specimens of ten cases. At the granulomatous stage, all cases expressed VEGF, bFGF, PDGF, and TGF-beta1 in monocytes/macrophages infiltrating into follicle lumina, and in both epithelioid histiocytes and multinucleated giant cells of the granulomas. In fibroblasts and endothelial cells around the granulomas, all cases displayed VEGF, bFGF, and PDGF, but TGF-beta1 was detected only in fibroblasts in two cases. No cases expressed EGF in any of the above cell types. At the regenerative stage, all cases expressed VEGF, bFGF, and EGF in regenerating thyrocytes, whereas three and no cases displayed PDGF and TGF-beta1, respectively. Ten, seven and six cases expressed PDGF in fibroblasts, endothelial cells, and monocytes, respectively. In these cell types, all cases expressed VEGF and bFGF, whereas no cases displayed TGF-beta1 and EGF. To estimate the roles of these growth factors in thyroid tissue regeneration, their effects on thyroid folliculogenesis and angiogenesis were examined using collagen gel culture of thyrocytes and endothelial cells, respectively. Cell proliferation was also studied by bromodeoxyuridine (BrdU) uptake. EGF decreased follicle formation and TGF-beta1 drastically inhibited it, but the others had no effect. VEGF showed the greatest effect on vessel formation, although all of the others promoted it. EGF and VEGF or bFGF caused the highest BrdU uptake in thyrocytes and endothelial cells, respectively. The data suggest firstly, that at the granulomatous stage of subacute thyroiditis, growth factor-rich monocytes/macrophages infiltrating into follicle lumina trigger the granulomatous reaction, and VEGF, bFGF, PDGF, and TGF-beta1 produced by the stromal cell types tested mediate the reaction; secondly, that at the regenerative stage, EGF serves follicle regeneration through its mitogenic effect on thyrocytes, although some cofactors with EGF are involved in folliculogenesis and the decreased expression of TGF-beta1, a fibrogenic factor, contributes to thyroid tissue repair; and thirdly, that VEGF and bFGF are more responsible for the angiogenesis at both stages than the other factors studied.