Ontogeny of expression of transforming growth factor-beta 1 (TGF-beta 1), TGF-beta 3, and TGF-beta receptors I and II in fetal rat fibroblasts and skin

Transforming growth factor-beta3 affects plasminogen activator inhibitor-1 expression in fetal mice and modulates fibroblast-mediated collagen gel contraction

For over two decades, the precise role of transforming growth factor-beta (TGF-beta) isoforms in scarless healing of mammalian fetal skin wounds has generated much interest. Although their exact role remains to be established, it has been suggested that high TGF-beta3 activity may correlate with a scarless phenotype. Previously, we showed that plasminogen activator inhibitor-1 (PAI-1), a known TGF-beta downstream molecule and marker of fibrosis, is also developmentally regulated during fetal skin development. In this study, the relationship between TGF-beta3 and PAI-1 was investigated using embryonic day 14.5 TGF-beta3 knockout (ko) mice. The results showed increased PAI-1 expression in the epidermis and dermis of ko mice, using an ex vivo limb-wounding study. Furthermore, increased PAI-1 expression and activity was seen in embryo extracts and conditioned media of ko dermal fibroblasts. When TGF-beta3 knockout fibroblasts were placed into three-dimensional collagen matrices, they were found to have decreased collagen gel contraction, suggesting altered cell-matrix interaction. These findings provide a further avenue for the interactive role of TGF-beta3 and PAI-1 during fetal scarless repair.

Profiling of genes differentially expressed in a rat of early and later gestational ages with high-density oligonucleotide DNA array

The early gestational fetus heals dermal wounds rapidly and scarlessly. This phenomenon appears to be intrinsic to fetal skin and is probably modulated by interplay of many genes. We ventured to study differences in gene expression between earlier gestational skin (EGS) and later gestational skin (LGS) with the aid of high-density oligonucleotide DNA array to explore the molecular mechanism underlying scarless healing. Total RNA was isolated from fetal Wistar rat skin of the scarless (E15) and scar-forming (E18) periods of gestation (term=21.5 days), and purified to mRNAs. Both the mRNAs from EGS and LGS were reversely transcribed to cDNAs, and were labeled with the incorporation of fluorescent dCTP for preparing the hybridization probes through single primer amplification reaction and Klenow labeling methods. The mixed probes were then hybridized to the oligonucleotide DNA arrays that contained 5,705 DNA fragments representing 5,705 rat genes. After highly stringent washing, the microarray was scanned for fluorescent signals to display the differentially expressed genes between two groups of tissues. Among 5,705 rat genes, there were 53 genes (0.93%) with differentially expressed levels between EGS and LGS; 27 genes, including fibroblast growth factor 8 and follistatin, were up-regulated (0.47%); and 26 genes, containing lymphoid enhancer binding factor-1 and beta-catenin, were down-regulated (0.46%) in fetal skin of scarless period vs. scar-forming period. Analyses of genes related to ion channels, growth factors, extracellular matrix and cellular skeleton, and movement confirmed that our molecular data obtained by oligonucleotide DNA array were consistent with the published biochemical and clinical findings of fetal scarless healing. Stronger expression of fibroblast growth factor 8, follistatin, and weaker expression of lymphoid enhancer binding factor-1 and beta-catenin in EGS vs. LGS were also testified with reverse transcription-polymerase chain reaction and Western blotting methods. Oligonucleotide DNA array was a powerful tool for investigating different gene expression between scarless and scar-forming periods of gestation in the rat fetal skin. Many genes were involved in the phenotypic transition from scarless to scar-forming wound repair during gestation. Further analysis of the obtained genes will help to understand the molecular mechanism of fetal scarless healing.

Ontogeny of expression of transforming growth factor-beta and its receptors and their possible relationship with scarless healing in human fetal skin

Fetal cutaneous wounds that occur in early gestation heal without scar formation. Although much work has been done to characterize the role of transforming growth factor-beta (TGF-beta) isoforms and their receptors in the wound healing process, their roles in scarless wound repair observed in early gestation and their functions in human fetal skin development, and structural and functional maintenance are still not well understood. In this study, we explore the expression and distribution characteristics of three TGF-beta isoforms and their receptors, TGF-betaRI (TBRI) and TGF-betaRII (TBRII), in fetal and postnatal skins to understand the relevance of these five proteins to skin development and elucidate the mechanism(s) underlying the phenotypic transition from scarless to scar-forming healing observed during fetal gestation. Fetal skin biopsies of human embryo were obtained from spontaneous abortions at different gestational ages from 13 to 32 weeks and postnatal skin specimens were collected from patients undergoing plastic surgery. Gene expression and positive immunohistochemical signals of TGF-beta(1), TGF-beta(2), TGF-beta(3), TBRI, and TBRII could all be detected in fetal and postnatal skins. In early gestation, gene expression of TGF-beta(1), TBRI, and TBRII was weaker and protein contents were less compared with postnatal skins (p < 0.05). In contrast, more TGF-beta(2) mRNA transcript was found in early gestation than in late gestation and in postnatal skins, whereas protein content of this growth factor increased during gestation. Lastly, mRNA transcript and protein contents of TGF-beta(3) were apparently higher in early gestation compared to postnatal skin (p < 0.05). In postnatal skin, granules containing the three TGF-beta isoforms were mainly distributed in the cytoplasm and extracellular matrix of epidermal cells, interfollicular keratinocytes, and some fibroblasts. TBRI and TBRII were chiefly located in the cellular membrane of epidermal keratinocytes and some fibroblasts. The endogenous three TGF-beta isoforms and their receptors may be involved in the development of embryonic skin and in the maintenance of cutaneous structure and function, and also in postnatal wound healing. The differential levels of TGF-beta isoforms may provide either a predominantly antiscarring or profibrotic signal upon wounding depending on the gestational period. Lower expression of their receptors in early gestational skins may be a reason for the reduced ability to perceive ligands, ultimately leading to scar-free healing.

Inhibiting Scar Formation in Rat Wounds by Adenovirus-Mediated Overexpression of Truncated TGF-?? Receptor II

The purpose of this study was to explore the possibility of inhibiting wound scarring by blocking TGFbeta signaling of wound cells by means of a gene therapy approach. Normal dermal fibroblasts were infected in vitro either with recombinant adenovirus encoding a truncated TGFbeta receptor II (Ad-tTGF-betaRII) or with [beta]-galactosidase adenovirus (Ad-beta-gal). TGF-beta1 gene expression in infected fibroblasts was analyzed by Northern blot. In vivo, 1x10(9) plaque-forming units of Ad-tTGF-betaRII were intradermally injected into the dorsal skin of 10-day-old newborn Sprague-Dawley rats (n = 10). For gene therapy, 1x10(9) plaque-forming units of Ad-tTGF-betaRII viruses were injected intradermally at the right side dorsal skin of another set of same aged Sprague-Dawley rats as the experimental group (n = 15). In the control group, 1x10(9) plaque-forming units of Ad-beta-gal (n = 11) or the same volume of saline (n = 4) was injected at the left side skin of the same rats. A 5-mm-long full-thickness incisional wound was created at the injection sites of each rat 2 days after injection. Wound tissues were harvested at day 3 (n = 2), day 7 (n = 2), and day 14 (n = 11) after wounding for histological analysis. Scar area of wound tissues harvested at day 14 was quantitatively analyzed. The results showed that TGF-beta1 gene expression was markedly down-regulated in Ad-tTGF-betaRII infected fibroblasts compared with Ad-beta-gal infected cells. In vivo, adenovirus-mediated transgene expression in rat skin reached a peak level at day 2 after injection and the expression gradually decreased afterward. Inhibited inflammatory reaction was also observed in the treated wounds with significantly reduced inflammatory cells (p < 0.05). Moreover, in all 11 rats, the experimental wound at day 14 had much less scarring than its control wound of the same rat, with an average of 49 percent reduction of the scar area (p < 0.05). Furthermore, more panniculus muscles were repaired in the experimental wounds (nine of 11) than in the control wounds (two of 11) (p < 0.05). These results indicate that gene therapy by targeting wound TGF-beta can effectively inhibit wound scarring and may potentially be applied to clinical scar treatment.

Advances in the Modulation of Cutaneous Wound Healing and Scarring

Cutaneous wounds inevitably heal with scars, which can be disfiguring and compromise function. In general, the greater the insult, the worse the scarring, although genetic make up, regional variations and age can influence the final result. Excessive scarring manifests as hypertrophic and keloid scars. At the other end of the spectrum are poorly healing chronic wounds, such as foot ulcers in diabetic patients and pressure sores. Current therapies to minimize scarring and accelerate wound healing rely on the optimization of systemic conditions, early wound coverage and closure of lacerations, and surgical incisions with minimal trauma to the surrounding skin. The possible benefits of topical therapies have also been assessed. Further major improvements in wound healing and scarring require an understanding of the molecular basis of this process. Promising strategies for modulating healing include the local administration of platelet derived growth factor (PDGF)-BB to accelerate the healing of chronic ulcers, and increasing the relative ratio of transforming growth factor (TGF)beta-3 to TGFbeta-1 and TGFbeta-2 in order to minimize scarring.

Reduced dermatopontin expression is a molecular link between uterine leiomyomas and keloids

Uterine leiomyomas are prevalent estrogen-responsive clonal tumors, but the specific genetic alterations that contribute to their development have not been elucidated. To identify genes involved in the formation of leiomyomas, we used global expression profiling to compare clonal tumors with normal myometrium. Contrary to expectation, genes involved in estrogen action were not differentially expressed between leiomyoma and normal myometrium. Genes encoding extracellular-matrix proteins were prominently featured, suggesting their involvement in formation of a myofibroblast phenotype. Analysis of the extracellular matrix in the leiomyomas revealed a disordered collagen fibril orientation. Expression of the collagen-binding protein dermatopontin was found to be consistently decreased in leiomyoma by both reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time RT-PCR (mean underexpression = 9.41-fold) regardless of leiomyoma size, leiomyoma location, patient race, and patient age. This expression pattern was observed in 11 subjects and a total of 23 leiomyoma:myometrium pairs. Decreased expression of dermatopontin was also associated with keloid formation, a fibrotic disease that shares epidemiologic similarities with leiomyoma. Immunohistochemical studies of leiomyomas and keloids demonstrated reduced levels of dermatopontin in both tissues. In addition, ultrastructural analysis revealed that the orientation of the collagen fibrils in the keloid tissues strongly resembled that in the leiomyomas. Reduction in dermatopontin was associated with an increase in transforming growth factor-beta3 (TGFB3) mRNA levels in leiomyomas, whereas other genes involved in dermatopontin signaling were not differentially expressed. These findings suggest that leiomyoma development involves a myofibroblast cell phenotype characterized by dysregulation of genes encoding extracellular-matrix proteins. In particular, decreased expression of dermatopontin represents a molecular link between the leiomyoma and keloid phenotypes.

Corticosteroids stimulate selectively transforming growth factor (TGF)-beta receptor type III expression in transdifferentiating hepatic stellate cells

BACKGROUND/AIMS

Transforming growth factor (TGF)-beta receptors mediate TGF-beta signaling in activated hepatic stellate cells (HSC). This leads to pleiotropic cellular effects, e.g. to the production of extracellular matrix which is a hallmark for the development of liver fibrosis. Glucocorticoids and their receptors interact with the TGF-beta signaling pathway on the transcriptional and translational level.

METHODS

To characterize TGF-beta receptor expression during HSC transdifferentiation and to study the influence of corticosteroids on receptor transcription in several liver cells, we established a real-time polymerase chain reaction procedure for mRNA quantification with gene-specific standards.

RESULTS

All three TGF-beta receptor mRNAs are present in HSC and myofibroblasts. Whereas TGF beta receptor type I (T beta RI) shows a comparable mRNA expression during HSC transdifferentiation, T beta RII and T beta RIII mRNA concentration decreases in the course of time. In comparison with activated HSC T beta RIII mRNA is very low expressed in freshly isolated Kupffer cells and hepatocytes. Eight hours after corticosteroid treatment T beta RIII mRNA increased significantly in a time-and dose-dependent manner while the mRNA expression of T beta RI and T beta RII is not altered. The degree of induction of T beta RIII mRNA levels is also dependent upon the nature of the stimulating hormone: dexamethasone, hydrocortisone and aldosterone show different effects.

CONCLUSIONS

The increase of T beta RIII by corticosteroids indicates that these hormones are important regulators of this receptor and thereby they can modulate TGF-beta signaling.

From scarless fetal wounds to keloids: Molecular studies in wound healing

Surgical researchers were among the first to describe the different phases of wound healing and the events in tissue repair and regeneration that were taking place during each phase. The understanding of these events has been significantly enhanced in recent years by modern techniques in molecular and cellular biology. In this article, we discuss new findings in scarless fetal repair, angiogenesis in wound healing, and keloid pathogenesis. This serves to highlight the advances that have been made and also how much remains to be understood.

Analysis of differentially expressed genes in keloids and normal skin with cDNA microarray

BACKGROUND

Microarray analysis is a popular tool to investigate the function of genes that are responsible for the phenotype of diseases. Keloid is an intricate lesion that is probably modulated by interplay of many genes. We ventured to study the differences of gene expressions between keloids and normal skin with the aid of a cDNA microarray to explore the molecular mechanism underlying keloid formation.

MATERIALS AND METHODS

The polymerase chain reaction products of 8400 human genes were spotted on a chip in array. The DNAs were then fixed on the glass plate by a series of treatments. Total RNAs were isolated from freshly excised human keloids and normal skins and then were purified to mRNAs by Oligotex. Both the mRNAs from keloids and normal skins were reversely transcribed to cDNAs with the incorporation of fluorescent dUTP for preparing the hybridization probes. The mixed probes were then hybridized to the cDNA microarray. After highly stringent washing, the cDNA microarray was scanned for the fluorescent signals to display the differences between two kinds of tissues.

RESULTS

Among 8400 human genes, there were 402 genes (4.79%) with different expression levels between the keloids and normal skins in all cases, 250 genes, including TGF-beta1 and NGF, were upregulated (2.98%) and 152 downregulated (1.81%). Analyses of collagen, fibronectin, proteoglycan, growth factors, and apoptosis-related molecule gene expression confirmed that our molecular data obtained by cDNA microarray were consistent with the published biochemical and clinical observations of keloids. Higher expression of TGF-beta(1) and NGF in keloids versus normal skins was also testified with reverse transcription polymerase chain reaction method.

CONCLUSIONS

DNA microarray technology is an effective technique in screening for differences in gene expression between keloid and normal skin. Many genes are involved in the formation of keloids. Further analysis of the obtained genes will help to understand the molecular mechanism of keloid formation.

Plasminogen activator/plasmin system: a major player in wound healing?

The role of the plasminogen activator/plasmin system in fibrinolysis has been well established. Indeed, clinicians worldwide have successfully utilized recombinant tissue-type plasminogen activator as first-line treatment of acute myocardial infarction for almost 2 decades. Outside the field of cardiology, there has been increasing excitement regarding the possible contribution of this system in many other important biological processes, including cell adhesion, cell migration, cell-cell signaling, tumor invasion and metastasis, ovulation, and wound healing. In this review, we present evidence in the current literature that the plasminogen activator/plasmin system does have a role in wound healing, looking at both normal and abnormal healing. Furthermore, the invaluable insights provided by numerous transgenic animal experiments are summarized.

TGF-beta 3 decreases type I collagen and scarring after labioplasty

Cleft lip is a common congenital malformation, and labioplasty performed on infants to repair such defects often results in severe scar formation. Since TGF-beta 3 has been implicated in wound healing, we therefore hypothesized that TGF-beta 3 functions to reduce scarring after cleft lip repair. In this investigation, we demonstrated that exogenous TGF-beta 3 reduced scar formation in an incised and sutured mouse lip in vivo. During labioplasty, endogenous TGF-beta 3 expression was also elevated. In vitro experiments showed that exogenous TGF-beta 3 reduced type I collagen accumulation. Furthermore, TGF-beta 3 inhibited alpha-smooth-muscle actin expression, a marker for myofibroblasts. In tandem, TGF-beta 3 induced the expression and activity of MMP-9. Analysis of our data suggests that TGF-beta 3 is normally secreted following labioplastic wound healing. An elevated level of TGF-beta 3 reduces type I collagen deposition by restricting myofibroblast differentiation and thereby collagen synthesis, and by promoting collagen degradation by MMP-9. In combination, these events lead to TGF-beta 3-mediated reduced scar formation.

Transforming growth factor-beta receptor-II up-regulation during wound healing in previously irradiated graft beds in vivo

Wound healing disorders may often present in patients with head and neck cancer after surgical interventions, particularly in preirradiated tissue. Inflammatory changes and the expression of cytokines can lead to induction of fibrosis. The isoforms of the transforming growth factor beta (TGFbeta1-3) play a key role for this process. It has been shown that radiation treatment associated fibrosis is induced by TGFbeta1 and TGFbeta2, although the influence of radiation on the expression of the TGFbeta receptor-II (TGFbetaR-II) involved in the signal transduction of TGFbeta remains elusive. The objective of this in vivo study was to analyze the expression profile of TGFbetaR-II in the graft bed and in the transition area between graft and graft bed after surgery with and without prior radiation treatment to compare with the expression profiles of activated TGFbeta1 and latency-associated peptide. A total of 48 Wistar rats (male, weight 300-500 g) were used in the study. Eighteen rats were irradiated in the neck region (3 x 10 Gy) without transplantation. A free myocutaneous gracilis flap was transplanted in 30 rats, of which 16 animals were preirradiated in the neck region (3 x 10 Gy) and 14 animals were not irradiated at all. Tissue samples were taken postoperatively from the transition area between the graft and the graft bed and from the graft bed itself after 3, 7, 14, and 28 days. Tissue samples were taken from the irradiated neck region and the non-irradiated groin region 0, 4, 7, 11, 14, and 28 days after the end of the exposure. The expression of TGFbetaR-II, activated TGFbeta1 and latency-associated peptide was analyzed immunohistochemically both qualitatively and quantitatively (labeling index). The success rate for graft healing was 75% in the previously irradiated group with 30 Gy, and 86% in the non-irradiated group. Following radiation alone a significantly (p = 0.04) increased TGFbetaR-II expression in the neck was revealed 2-4 weeks following irradiation compared to non-irradiated skin. Whereas only minor differences in TGFbetaR-II expression were observed following surgery between the groups with and without prior radiation in the transition area between the graft and the graft bed, the group undergoing prior radiation and subsequent grafting showed significantly increased expression in the bed compared to the non-preirradiated group with a maximum on postoperative day 7 (week 1, p = 0.003; week 2-4, p < 0.001). In irradiated tissues the up-regulation of TGFbetaR-II expression correlated with an increase of activated TGFbeta1 and latency-associated peptide expression compared to non-irradiated tissues. After irradiation, a significantly increased TGFbetaR-II expression was identified in the irradiated graft bed, which may be the reason for delayed reepithelialization and fibrosis. Exogenous blocking or TGFbetaR-II inhibitors could therefore represent a new therapeutic approach for improving wound healing after preoperative radiotherapy.

Pulmonary fibrosis: cellular and molecular events

Connective tissue remodeling of the interstitium is an important feature of chronic lung diseases encompassing interstitial inflammatory changes and subsequent pulmonary fibrosis. The early inflammatory phase is usually associated with the release of several cytokines and chemokines by activated resident cells and infiltrating cells which, in turn, help further recruit inflammatory mononuclear cells. Cytokines and growth factors secreted by inflammatory cells and by interstitial cells (fibroblasts and myofibroblasts) play an important role in the fibrogenic phase of pulmonary fibrosis by inducing matrix synthesis. In addition, matrix-degrading enzymes and their inhibitors also contribute to extracellular matrix (ECM) remodeling in pulmonary fibrosis. This review addresses the pathophysiology of wound healing and different phases of pulmonary fibrosis.

Fetal wound healing current perspectives

Early in gestation, fetal wounds are capable of healing scarlessly. Scarless healing in the fetus is characterized by regeneration of an organized dermis with normal appendages and by a relative lack of inflammation. Although there is a transition period between scarless and scar-forming repair, scarless healing also depends on wound size and the organ involved. The ability to heal scarlessly, furthermore, appears to be intrinsic to fetal skin. Unique characteristics of fetal fibroblasts, inflammatory cells, extra-cellular matrix, cytokine profile, and developmental gene regulation may be responsible for the scarless phenotype of early gestation fetal wounds. With the current knowledge, only minimal success has been achieved with the topical application of neutralizing antibodies, antisense oligonucleotides, and growth factors to improve wound-healing outcomes. Thus, further investigation into the mechanisms underlying scarless repair is crucial in order to devise more effective therapies for scar reduction and the treatment of cirrhosis, scleroderma, and other diseases of excessive fibrosis.

Differential expression of urokinase-type plasminogen activator and plasminogen activator inhibitor-1 in early and late gestational mouse skin and skin wounds

Early gestation fetal mouse skin heals without scars. Plasminogen activator inhibitor-1 (PAI-1) has been associated with postnatal organ fibrosis. We hypothesized that the relative balance between urokinase-type plasminogen activator (uPA) and PAI-1 expression in favor of uPA prevents scarring in early fetal skin wounds, whereas a change in favor of PAI-1 in late gestation results in wound scarring. To evaluate uPA and PAI-1 expression, 1-mm skin wounds were made in E14.5 and E18 mice and harvested 24, 48, or 96 hours postwounding. Aprotinin (2 mg/ml)-coated beads were injected into selected E14.5 wounds. Normal skin and skin wounds were evaluated for uPA, PAI-1, and collagen expression. We showed that in normal skin uPA level is higher in E14.5 than in E18 mice, while PAI-1 is lower in E14.5 than in E18 mice. After wounding, E14.5 wounds show a moderate increase in uPA and a minimal increase in PAI-1. E18 wounds show a transient increase in uPA but a significant, sustained increase in PAI-1. Addition of aprotinin to E14.5 wounds causes an increase in collagen deposition. We conclude that the differential expression of uPA and PAI-1 in the skin of early vs. late gestation mice may contribute to the degree of scar formation seen after cutaneous injury.

TGF-beta-3 promotes scarless repair of cleft lip in mouse fetuses

TGF-beta3 mediates epithelial-mesenchymal transformation during normal fusion of lip and palate, but how TGF-beta3 functions during cleft lip repair remains unexplored. We hypothesize that TGF-beta3 promotes fetal cleft lip repair and fusion by increasing the availability of mesenchymal cells. In this investigation, we demonstrated that cleft lips in mouse fetuses were repaired by fetal surgery, producing scarless fusion. At the site of the operation, we first observed an infusion of platelets expressing TGF-beta3, followed by increased expression of cyclin D1 and tenascin-C, and coupled with increased mesenchymal cell proliferation. In an ex vivo serumless culture system, cleft lip explants fused in the presence of exogenous TGF-beta3. Cultured lips also showed up-regulation in cyclin D1 and tenascin-C expression. These findings suggest that microsurgical repair of cleft lip in the fetus that produced scarless fusion is mediated by TGF-beta3 regulation of mesenchymal cell proliferation and migration at the site of repair.

Matrix metalloproteinases and the ontogeny of scarless repair: the other side of the wound healing balance

Early gestation mammalian fetuses possess the remarkable ability to heal cutaneous wounds in a scarless fashion. Over the past 20 years, scientists have been working to decipher the mechanisms underlying this phenomenon. Much of the research to date has focused on fetal correlates of adult wound healing that promote fibrosis and granulation tissue formation. It is important to remember, however, that wound repair consists of a balance between tissue synthesis, deposition, and degradation. Relatively little attention has been paid to this latter component of the fetal wound healing process. In this study, we examined the ontogeny of ten matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) in nonwounded fetal rat skin and fibroblasts as a function of gestational age. We used a semiquantitative polymerase chain reaction protocol to analyze these important enzymes at time points that represent both the scarless and scar-forming periods of rat gestation. The enzymes evaluated were collagenase-1 (MMP-1), stromelysin-1 (MMP-3), gelatinase A (MMP-2), gelatinase B (MMP-9), membrane-type matrix metalloproteinases (MT-MMPs) 1, 2, and 3, and TIMPs 1, 2, and 3. Results demonstrated marked increases in gene expression for MMP-1, MMP-3 and MMP-9 that correlated with the onset of scar formation in nonwounded fetal skin. Similar results were noted in terms of MMP-9 gene expression in fetal fibroblasts. These results suggest that differences in the expression of these matrix metalloproteinases may have a role in the scarless wound healing phenotype observed early in fetal rat gestation. Furthermore, our data suggest that the differential expression of gelatinase B (MMP-9) may be mediated by the fetal fibroblasts themselves.

The ontogeny of scarless healing II: EGF and PDGF-B gene expression in fetal rat skin and fibroblasts as a function of gestational age

Twenty years ago, surgeons noted the ability of early-gestation fetal skin to heal in a scarless manner. Since that time, numerous investigators have attempted to elucidate the mechanisms behind this phenomenon. As a result of this effort, it is now well established that many animals undergo a transition late in development from scarless cutaneous healing to a scar-forming, adultlike phenotype. The authors have been interested in the role played by cytokines known to be involved in the adult wound-healing process and how they relate to scarless repair. They therefore asked the following question: Are genes for epidermal growth factor (EGF) and platelet-derived growth factor-B (PDGF-B) expressed differentially as a function of gestational age in fetal rat skin and dermal fibroblasts? To answer this question, skin from fetal Sprague-Dawley rats (N = 56) at time points that represented both the scarless and scar-forming periods of rat gestation was harvested. In addition, fibroblasts derived from fetal rat skin were cultured in vitro at similar times. These cells were expanded in culture and, when confluent, total ribonucleic acid from both fibroblasts and whole skin was extracted and subjected to Northern blot analysis with probes for EGF and PDGF-B. Results demonstrated that neither EGF nor PDGF-B gene expression changed markedly as a function of gestational age in fetal fibroblasts alone. In whole skin, however, both EGF and PDGF-B demonstrated a marked decrease in gene expression with increasing gestational age. Furthermore, the most striking decrease in gene expression for both cytokines came between 16 and 18 days of gestation-the transition point between scarless and scar-forming repair in the fetal rat. These data suggest that EGF and PDGF may play a role in the mechanism of scarless cutaneous repair. Moreover, it appears that fetal fibroblasts are not the cell type responsible for this differential gene expression. These results raise questions about the unique cytokine milieu likely to be present during the time of scarless healing and the cells that ultimately guide the mechanisms leading to skin regeneration.