Scar formation: the spectral nature of fetal and adult wound repair

Influence of transforming growth factor-beta3 on fibrous capsule formation around microgrooved subcutaneous implants in vivo

Previous studies have shown that addition of transforming growth factor-beta3 (TGF-beta3) is capable of reducing scar tissue formation in skin defects. Therefore, we examined whether TGF-beta3 can also influence the organization of a fibrous capsule around implants in vivo. For this reason, 24 silicone implants with microgrooves with a groove depth of 1.0 microm and a ridge and groove width of 10.0 microm were made and loaded with human recombinant TGF-beta3 (0, 5, 50, and 250 ng). An in vitro release enzyme-linked immunosorbent assay (ELISA) test was done with another 10 implants to estimate the amount of TGF released from the implants. The implants were inserted subcutaneously in the backs of 6 guinea pigs. Each animal received four implants, which were left in place for 10 weeks. At the end of the implantation time, the implants were retrieved, embedded, and processed for histology. Histomorphometrical measurements were done on the capsule formation and the implant-cell interface quality and quantity. The results showed a fibrous capsule of 15 microm up to 50 microm thickness around all implants. There were no significant differences between the TGF-beta3-loaded implants or the controls. Frequently, inflammatory cells were present in the capsule. The implant-tissue interface was on average between 5 and 15 microm thick and consisted mostly out of one or two layers of macrophages or foreign body giant cells. Statistical analysis again showed no significant differences between the various TGF-beta3-coated implants and controls. Finally, we concluded that a microtextured surface can indeed be used for the release of TGF-beta3. On the other hand, this did not result in major differences in wound healing between implants loaded with 5, 50, or 250 ng of TGF-beta3 and controls.

A new era of discovery in facial plastic surgery

OBJECTIVE

To review recent advances in our understanding of molecular biology and wound healing relevant to facial plastic surgery.

DATA SOURCES

Recent basic science literature relevant to molecular biology and wound healing and its clinical implications.

CONCLUSIONS

During the 21st century, we will experience a new biological and informational age that will have profound implications for facial plastic surgery. This modern era will be driven by discoveries in molecular biology and wound healing that will result in new diagnosis and treatment modalities.

Treatment outcome in cleft lip and palate: issues and perspectives

In the last 40 years, great progress has been made toward a better understanding of many aspects of the cleft lip and palate defect, but there is still a long way to go before there is agreement on the optimal treatment procedures. With regard to the primary operations, it can be stated, in a somewhat simplified form, that there are two main schools of thought in cleft treatment. One advocates early closure of the lip and palate, a procedure which imparts a high priority to early speech function. The other recommends delayed closure of the hard palate, thereby according a high priority to the growth of the maxilla. A number of intercenter and multicenter studies have been carried out recently in an effort to elucidate which procedures give the best result, both esthetically and functionally. The results are ambiguous, and this has led a number of researchers to suggest that the randomized clinical trial is the only way to resolve the ambiguity. The fact that it has proved difficult to identify the optimal procedures in the field of cleft lip and palate treatment need not only be due to a less than optimal research design; a contributory factor might also be the great variability in craniofacial morphology and in the response to treatment in patients who have exactly the same cleft lip and palate diagnosis. Intensive research has made it possible to state categorically that clefts occur due to many different factors in an interplay between genetics and environment. Therefore, it is not likely that a single gene can be responsible for clefting. Since scar tissue presents many problems-for instance, impairment of growth-the reduction or prevention of scar formation has long been a desirable goal. The discovery that a fetus can heal without scar formation has led to many animal experiments. The timing of the surgical intervention on fetuses is critical, since late-gestation fetuses heal with adult-like scarring. There are still many unsolved problems connected with fetal surgery, and at present prenatal surgery for repair of cleft lip and palate is not ethically defensible in humans. On the other hand, it appears that there are considerable possibilities for the reduction of human scarring after surgery with the introduction of various wound-healing medications.

Cellular mechanisms for diminished scarring with aging

The study of an age-dependent spectrum of scar formation is driven by the desire to understand and recapitulate scarless healing. Although focus in the past has been directed toward scarring in the fetus, less exuberant scarring is a common clinical observation in the elderly. Cell turnover is a major contributor to the development of scar tissue and is governed by the proliferative and apoptotic cellular fractions within a healing wound. We hypothesize that the balance between cell proliferation and apoptosis during late stages of excisional wound healing is, at least in part, responsible for age-related variations in scarring potential. Full-thickness 7-mm ulcers (four per ear), exposing bare cartilage, were made on the inner surface of the ear on 12 young and 12 aged New Zealand White rabbits. Analyses were performed at days 15, 21, and 28 postwounding. A previously described Scar Elevation Index was derived from histomorphometric analysis, along with the quantification of epithelial ingrowth and total cellularity. Apoptotic cellular fractions were derived from TdT-mediated dUTP nick end-labeling assay-stained histologic sections; proliferative fractions were derived from proliferating cell nuclear antigen-labeled serial sections. Young rabbits demonstrated significantly greater scar elevation/area. Apoptosis was strongly associated with progress of epithelialization in both groups. Significantly higher proliferative indices were seen in the young and were sustained through day 28, by which time levels had substantially declined in the aged. No differences in apoptotic indices were demonstrated between groups at any time point. The clinical observation of less exuberant scarring with aging is supported by this animal model. Apoptosis follows the progression of epithelialization but does not appear to independently influence scar morphology. A diminished proliferative response during later stages of healing is an important contributing mechanism for the decrease in scar formation seen in the elderly.

Differential expression of receptor tyrosine kinases and Shc in fetal and adult rat fibroblasts: toward defining scarless versus scarring fibroblast phenotypes

The remarkable ability of the fetus to heal early gestation skin wounds without scarring remains poorly understood. Taking advantage of recent advances in signal transduction, the tyrosine phosphorylation patterns of fetal rat fibroblasts, representing the scarless cutaneous repair phenotype, and adult rat fibroblasts, representing scarforming phenotype, were examined whether there were inherent differences in cellular signaling. Specifically, correlation of the phosphorylation patterns with the expression levels of the signaling molecules that transmit information from the plasma membrane receptor to the nucleus was sought. By using three different cell lines of explanted fibroblasts from gestational day 13 fetal rat skin (n = 24) and 1-month-old postnatal adult rat skin (n = 3), immunoblotting was performed to compare tyrosine phosphorylation patterns. The results revealed five major protein bands of interest in fetal rat fibroblasts, but not in the adult rat fibroblasts. These phosphorylated protein bands are of interest because of their possible role in wound repair and may have the potential to regulate cellular responses to the extracellular matrix and their secondary signaling molecules. It was hypothesized that these bands represented receptor tyrosine kinases, epidermal growth factor receptor, and discoidin domain receptor 1, and their downstream adaptor protein Shc that binds receptor tyrosine kinases to transduce signals intracellularly. Furthermore, elevated expression of platelet-derived growth factor receptor-beta in adult compared with fetal fibroblasts was demonstrated, suggesting that decreased expression of certain growth factors may also be important for the scarless phenomenon to occur.

Hyaluronan induces scarless repair in mouse limb organ culture

BACKGROUND/PURPOSE

Wounded fetal mouse limbs harvested from two distinct time points in gestation heal differently in organ culture. The healing of a gestational day 14 limb is by scarless repair, whereas gestational day 18 (gd 18) limbs heal by scarring. The persistence of elevated levels of hyaluronic acid (HA) is a major difference in the extracellular matrix of scarless repair. The purpose of this study was to demonstrate that chronic additions of HA to incisional wounds of gd 18 limbs induces scarless repair.

METHODS

Time-dated pregnant CD-1 mice (term, 20 days) were killed on gestational day 18 and fetuses were harvested via laparotomy. A through and through stab wound was made in each forelimb with a 1-mm microscapel, and the wound was closed with a single 10-0 nylon suture. The forelimbs were amputated at the level of the shoulder and placed in organ culture. Daily medium changes with 1 mL of BGJb (devoid of serum) were made. Half the cultures received 10 microL of HA (4 mg/mL) directly to the wound site with each medium change. The other half of the cultures received 10 microL of phosphate-buffered saline (PBS-control). At day 7, the limbs were harvested, fixed in methyl Carnoys solution, paraffin embedded, and 5-microm serial sections cut. The sections were stained with H&E or Sirius red/fast green. The sections were viewed in a blinded fashion by two observers. Suture defined the wound site, and the sections were graded for healing by scarring.

RESULTS

Minimal limb growth occurred in both control and HA-treated limbs. Grossly, both control and treated limbs healed incisional wounds by 7 days in culture. Limbs from both treatment and control groups showed viability by microscopic analysis. The limbs treated with HA had no appreciable scar morphologically in sections in which epithelial dimpling and suture were evident. The orientation of the collagen fiber bundles in the control wounds were in parallel arrays perpendicular to the incision. The orientation of the collagen fiber bundles in the HA-treated limbs had a basket weave pattern that was indistinguishable from unwounded dermis. The direct repeated additions of HA to healing organ cultured limb explants of gestational day 18 fetal mice promoted scarless repair.

CONCLUSIONS

This result demonstrates that chronic elevation of HA in the microenvironment of a wound affects healing by promoting the deposition of a more dermal-like connective tissue matrix in the wound site. The maintenance of elevated levels of HA could have utility in the clinical setting to improve the organization of connective tissue, leading to the reduction of scar complications.

Hyaluronic acid induces tumour necrosis factor-alpha production by human macrophages in vitro

Foetal wounds heal with minimal or no scar formation. High levels of hyaluronic acid (HA) have been implicated as a contributory factor. Macrophages are essential for normal wound healing, a role facilitated by secretion of an array of cytokines. Of these, tumour necrosis factor alpha (TNF-alpha) has been shown to reduce wound collagen levels and thus scarring. This study examines the ability of HA to stimulate TNF-alpha production by human macrophages. The human U937 myelomonocytic cell line was differentiated into DU937 adherent macrophages. DU937 monolayers were exposed to HA at concentrations of 0.1, 1, 10 and 100 micrograms/ml. Conditioned media from HA-exposed monolayers were assayed for TNF-alpha activity using a standard L929 fibroblast bioassay. TNF-alpha activities of HA-exposed DU937 culture supernatants were compared to those of controls and expressed as % cytotoxicity. Exposure of macrophages to HA at concentrations of 10 micrograms/ml and 100 micrograms/ml significantly stimulated TNF-alpha production, as demonstrated by % cytotoxicities expressed as median (interquartile range) of 33.5 (29-34.5)% (P = 0.03) and 77.5 (67-85)% (P = 0.029) respectively (Mann-Whitney U test). This effect was specifically associated with TNF-alpha generated during HA exposure, as these cytotoxic effects could be abolished by addition of anti-TNF-alpha antibody, reducing cytotoxicity to 9 (6.5-13.5)% and 8.5 (6-12)% respectively. These observations indicate that HA stimulates TNF-alpha production by human macrophages. TNF-alpha is known to downregulate fibroblastic collagen synthesis within experimental wounds. We suggest that the high levels of HA within foetal wounds may play a part in limiting fibroplasia, and thereby limit scarring, via an upregulation of TNF-alpha production from wound macrophages.

Transforming growth factor beta s and wound healing

The Transforming Growth Factor beta superfamily (TGF beta) is one of the most complex groups of cytokines with widespread effects on many aspects of growth and development. The TGF beta isoforms and other family members, e.g. Activins and BMPs, have diverse effects in similar physiological situations. TGF beta is involved in the wound healing process. The three mammalian isoforms (TGF beta 1, 2 and 3) and recently other family members, e.g. Activin, have been localised in healing wounds. Manipulation of the ratios of TGF beta superfamily members, particularly the ratio of TGF beta 1 relative to TGF beta 3, reduces scarring and fibrosis. Such manipulations include reducing the levels of TGF beta 1/TGF beta 2 using neutralising antibodies or preventing the activation of TGF beta s. In chronic or impaired wounds the exogenous addition of TGF beta superfamily members accelerates aspects of the healing process. This review summarises evidence for the role of TGF beta superfamily members in wound healing and how modulation of TGF beta levels can prevent scarring and fibrosis.