Pharmacology of skin aging. Stimulation of glycosaminoglycan biosynthesis by L-fucose and fucose-rich polysaccharides, effect of in vitro aging of fibroblasts

The effect of L-fucose and fucose-rich polysaccharides (FROP-s [Biomed. Pharmacother., 2003; 57: 187-94]) was investigated, using human skin fibroblast cultures at several passages. The cells were incubated with radioactive glucosamine for 24 h, followed by the determination of individual glycosaminoglycans (GAG-s) by selective hydrolysis using specific enzymes. The effect of L-fucose and of FROP-3 [Biomed. Pharmacother., in 2003; 57: 187-94], both at 1 and 10 microg/ml, added to the culture medium, was investigated. L-Fucose stimulated the incorporation of the tracer in heparan sulfates by fibroblasts at the ninth passage by 20%. FROP-3 stimulated incorporation in keratan sulfates by 45% by fibroblasts of the fifth passage. This effect was identical at both concentrations tested. For fibroblasts at the ninth passage FROP-3 stimulated incorporation in dermatan sulfate. This effect was dose dependent of the order of +67% at 1 microg/ml and +128% at 10 microg/ml. Incorporation in hyaluronan was also stimulated by about +27%. These stimulations of GAG-biosynthesis might play a role in the increase of total skin thickness of hairless rats treated with L-fucose, as well as in several other favorable results recorded for FROP-3 such as the increased hydration (resistance to pressure) and elasticity of human skin (Robert C, Robert AM, Robert L).

Effect of sulfated GAGs on the expression and activation of MMP-2 and MMP-9 in corneal and dermal explant cultures

It has been shown previously that hyaluronan (HA) added to fibroblast and keratocyte cell cultures or corneal explant cultures produces an up-regulation of MMP-2 and MMP-9 expression and activation. Here, we examine the effect of sulfated GAG-s, chondroitin 4 and 6 sulfate (CS4, CS6), dermatan sulfate (DS), keratan sulfate (KS) and heparan sulfate (HS) on MMP-2 and 9 expression and activation under the same culture conditions. It appears that CS4 has only minor effects, KS inhibits MMP-2 activation and CS6, DS and HS increase MMP-2 activation in corneal explant cultures. For skin explant cultures, DS, KS and HS strongly increase MMP-9 activation, whereas KS inhibits and DS increases MMP-2 activation. All these effects can be strongly inhibited by the addition of an antibody to CD44, except CS6 and DS. Activation by these two GAGs was only slightly affected, supporting the contention that the effects of HA, CS4, KS and HS are mediated by one of the isoforms of this CD44 receptor. The physio-pathological significance of these results is discussed for cornea and skin ageing, because of the divergent evolution with in vitro ageing of the relative proportions of GAGs synthesised by these two cell types.

Effect of L-fucose and fucose-rich polysaccharides on elastin biosynthesis, in vivo and in vitro

With increasing age elastic fibres in human skin are progressively lysed and skin elasticity is also decreasing. Still there is an age-dependent increase of elastic fibre surface density, mostly due to an alteration of the fibres. The present experiments were undertaken to explore if L-fucose and fucose-rich polysaccharides (FROP-s) could influence elastin biosynthesis. We show here, that topical application of a fucose-containing preparation to the skin of hairless rats increased after 4 weeks the elastic fibre surface density by about 40%, shown by quantitative morphology. Using human skin fibroblasts in explant cultures, the addition of L-fucose or of FROP-3 increased the biosynthesis of immunoprecipitable tropoelastin by about 40%. No increase was found however of desmosine-isodesmosine in skin explant cultures after 72 h of incubation. The effect of L-fucose and FROP-3 on the biosynthesis of collagen and non-collagen proteins excreted by the skin explant cultures was also investigated. L-fucose, but not FROP-3, decreased collagen biosynthesis but both increased non-collagen protein biosynthesis. These results show that L-fucose and FROP-3 stimulate tropoelastin biosynthesis in vitro, and elastic fibre formation in vivo. This stimulation concerns also several non-collagen proteins secreted by skin explant cultures. Elastic fibre formation necessitates the simultaneous synthesis of several microfibrillar glycoproteins as well as of tropoelastin. The increased elastic fibre density in the in vivo experiments suggests that this is indeed achieved by L-fucose and FROP-3, further demonstrating their efficiency in the control of age-dependent modifications of connective tissues in general and of skin in particular.

Effects of L-fucose and fucose-rich oligo- and polysaccharides (FROP-s) on collagen biosynthesis by human skin fibroblasts. Modulation of the effect of retinol, ascorbate and α-tocopherol

The action of L-fucose and a fucose-rich oligosaccharide (FROP-3) on skin explant cultures and fibroblast cell cultures, alone or together with three vitamins (A, C, E, often used in topical preparations) was studied, using as criteria collagen accumulation and collagen and non-collagen protein biosynthesis. In order to confirm the results observed, several variants of the chemical procedure were used. Collagen accumulation in cell cultures and (3)H-proline incorporation in proteins and collagen gave comparable results. L-Fucose produced a slight but significant inhibition of collagen accumulation and of incorporation of radioactive proline in cell associated collagen and also modified the ratio of excreted (soluble) to cell- or tissue-bound collagen and non-collagen proteins. The oligosaccharide (FROP-3) did not produce a comparable inhibition of collagen biosynthesis. Both L-fucose and FROP-3 modulated the action of the two above-mentioned vitamins in most experimental conditions used. The combined action of the three vitamins (all-trans retinol, ascorbate, alpha-tocopherol) with L-fucose and even more so with FROP-3 can be considered as favourable for the modulation of the biosynthetic activity of fibroblasts.

Tropoelastin Biosynthesis by Corneal Cells

The vertebrate cornea is an avascular tissue and does not contain elastic fibers. We tested the capacity of corneal epithelial cells and stromal keratocytes to synthesize tropoelastin. Explant cultures and cell cultures were obtained from these two cell types in standard culture conditions. Their elastin-synthetic activity was compared to skin explant cultures and to dermal fibroblast cell cultures. Both corneal cell types synthesized tropoelastin as shown by the incorporation of a radioactive precursor followed by immunoprecipitation of tropoelastin. When serial cultures of keratocytes were tested, tropoelastin biosynthesis strongly increased after the 3rd passage and was at the 9th passage more than the double of that of the first passage. When cocultures were studied with or without cell contact, epithelial cells partially inhibited tropoelastin biosynthesis by keratocytes. This inhibition was somewhat stronger (-36%, p < 0.005) with cell-to-cell contact than keeping separate epithelial cells and keratocytes bathing in the same medium (-18%, p < 0.005). When human skin fibroblasts were substituted for keratocytes with cell-to-cell contact, their tropoelastin biosynthesis was also inhibited by corneal epithelial cells (-42%, p < 0.005), to the same extent as for keratocytes. In Transwell culture, this inhibition was again somewhat lower (-36%, p < 0.005). Some diffusible factor produced by epithelial cells is apparently involved. The epithelial inhibition of tropoelastin biosynthesis by stromal keratocytes might represent one of the mechanisms keeping corneal stroma exempt of elastin fibers.

Effect of a fucose-rich polysaccharide preparation on the age-dependent evolution of the skin surface micro-relief

It was demonstrated previously that the evaluation of the microdepressionary skin surface relief by semi-automated computerised morphometry enables the determination of several of its geometrical parameters, such as, the average number of polygons in a given microscopic field, and/or the average surface of the polygons. Using this procedure, it could be shown in a study on about 100 persons, males and females, aged from 6 months to 89 years that these parameters undergo age-related alterations. These alterations are faster at younger ages, slow down at middle ages and accelerate after 50-60 years again. Taken at a sun-protected site, the interior face of the forearm, close to the elbow, this test reflects the effect of chronological ageing of the skin surface micro-relief. In the present work we tested the effect of a preparation containing a fucose-rich polysaccharide as active principle on the microdepressionary skin surface micro-relief evaluated by a morphometric method, comparing results before treatment and after 4 weeks of treatment. We could demonstrate on 20 female volunteers, aged from 39 to 71 years, that after 4 weeks of treatment with the abovementioned preparation there was a significant improvement of the skin surface relief, as shown by the displacement of geometrical characteristics of 17 treated skins out of 20, towards a "younger" pattern, as for instance the increase of the average number of polygons in a given microscopic field. This improvement was less than 15% of the untreated value for two persons, and more than 15% for 15 women out of 20. The average improvement was 37.16%, and for the 15 persons showing more than 15% improvement, it was nearly 50%, corresponding to a decrease of apparent age by 10-15 years. These results suggest that a treatment with the fucose-rich polysaccharide preparation can slow down human skin ageing and even can reverse age-dependent skin alterations.

Aging, from basic research to pathological applications

This introductory chapter tentatively draws a parallel between some of the highlights of basic aging research and their incidence for the understanding of age-associated pathologies. Some of the most important mechanisms described for cell- and tissue-aging, such as limited cell-proliferation, "parasitic" reactions, such as non-enzymatic glycation, ROS-production, uncontrolled proteolysis involving matricryptic sites, receptor loss and uncoupling appear to be the most promising links between those two approaches to fundamental and clinical gerontology. Some of the "parasitic", epigenetic, post-translational reactions lead to vicious circles which over the years might well produce cell- and tissue-damage as seen in most age-associated diseases. There is little doubt that the better understanding of these relationships will also prove helpful for pharmacological research as related to the age-associated pathologies.

Oxidative and premature skin ageing

To elucidate the scientific state of the art with respect to the role of nutrition in skin ageing, nine experts from different disciplines discussed the role of micronutrients on 'oxidative and premature skin ageing'. In this 25th Hohenheim Consensus Meeting, 13 questions were discussed and, based on published valid data, answered by mutual agreement. The consensus answers achieved during the meeting are justified by a scientific background text. The importance of in vitro and in vivo models regarding oxidative and premature skin ageing was critically evaluated. There was a special focus on prevention and intervention of skin ageing with nutrition. Finally, the paper summarizes the scientific background from different areas related to oxidative and premature skin ageing.

Studies on skin aging. Preparation and properties of fucose-rich oligo- and polysaccharides. Effect on fibroblast proliferation and survival

Skin aging represents an important chapter of connective tissue aging and concerns an organ of vital importance. Here we describe the preparation as well as the biological properties of fucose-rich oligo- and polysaccharides (FROPs), composed of polymers of a trisaccharide containing galactose, acetyl galacturonic acid and fucose, from the original high molecular weight bacterial polysaccharide (Fucogel), Solabia, France). Using endoglycosidases, oligo- and polysaccharides were prepared and characterized by physical and chemical procedures. The non-reducing end-groups comprise equal amounts of galactose and fucose. The here-described biological properties are: stimulation of cell proliferation of cultured human skin fibroblasts, protection of cells against ascorbate-induced cytotoxicity due to the release of reactive oxygen species (ROS). Properties elsewhere described concern the inhibition of matrix metallo-proteinases 2 and 9 (MMP-2 and MMP-9), their expression and activation. Using fluorescein isothiocyanate (FITC)-labeled polysaccharides, their interaction with cell membranes and also their penetration and accumulation in cells, especially in the cell nucleus could be demonstrated, probably via cell-membrane receptor-mediated mechanisms. We describe some of the symptoms of skin aging and show, that the here-described polysaccharide preparations are susceptible to slow down some of the cellular and molecular mechanisms involved, partly by the mediation of the above-mentioned receptors, partly by acting directly on the regulation of gene expression.

Effect of L-fucose and fucose-rich oligo- and polysaccharides (FROP-s) on skin aging: penetration, skin tissue production and fibrillogenesis

Skin thickness is decreasing with age. This loss concerns both dermis and epidermis, cells and extracellular matrix. We could show here that percutaneous application of an L-fucose-containing preparation produced an increase of skin thickness and a densification of collagen bundles. We also could show that 3H-L-fucose penetrates in the dermis, a prerequisite for the above mentioned favorable pharmacological activities. These results, together with the previous favorable activities on the downregulation of matrix-degrading enzymes, free radical scavenging and increased cell proliferation confirm the favorable action of fucose and fucose-rich polysaccharides (FROP-s) on the skin by slowing down its aging.

Pharmacological properties of fucose. Applications in age-related modifications of connective tissues

Fucose is the only component of glycoconjugates of vertebrates in the L-configuration. It exhibits a number of unique and interesting biological properties reviewed briefly in this article. Its constant end-standing position on glycan chains predisposes fucose to play a key role in cell-cell and cell-matrix interactions, mediated by several receptors such as those recognising the Lewis-type blood group substances, fucose-recognising lectines and the mannose-fucose receptors. Some of the as yet unstudied or less well understood properties of L-fucose were explored in the present study, as its non-enzymatic interaction with amine-groups on macromolecules, its cellular uptake attributed to specific transport mechanisms and its effect on fibroblast cell cultures. We could document the stimulation of cell-proliferation and the inhibition of MMP-expression and activation, both for MMP-2 and MMP-9. These and the other shortly reviewed properties of L-fucose may play an important role in its biological applications and actions.

Protection by L-fucose and fucose-rich polysaccharides against ROS-produced cell death in presence of ascorbate

It was shown previously, that millimolar concentrations of ascorbate have cytotoxic and anti-proliferative effects (Eur. J. Clin. Invest. 32 (2002) 372). With increasing concentrations of ascorbate an increasing number of fibroblasts was detached from the culture dish and shown to be lysed by the effect of ascorbate-induced generation of reactive oxygen species (ROS-s). It also could be shown, that this cytotoxic effect is partly due to the dose-dependent inhibition by ascorbate of fibronectin biosynthesis. Superoxide dismutase (SOD) and catalase were shown to salvage cells from ROS-induced cell-death by preventing the inhibition of fibronectin biosynthesis. We used this model system to test the cyto-protective effect of L-fucose and fucose-rich oligo- and polysaccharides (FROP-s). It appeared that relatively low concentrations of L-fucose and FROP-3 (Biomed. Pharmacother. in press) could efficiently protect fibroblasts from the ascorbate-induced cell-death. These novel pharmacological properties of L-fucose and FROP-3 might well be related to their accelerating effect of wound healing.

Combination of raltitrexed and oxaliplatin is an active regimen in malignant mesothelioma: results of a phase II study

PURPOSE

The aim of this open-label phase II study was to evaluate the activity of raltitrexed (Tomudex; AstraZeneca, Cergy, France) and oxaliplatin combination therapy in patients with diffuse malignant pleural mesothelioma. PATIENT AND METHODSs: Fifteen pretreated and 55 chemotherapy-naive patients (median age, 60 years; World Health Organization performance status of < or = 2) were enrolled. Most patients (66%) had advanced disease. Patients received raltitrexed 3 mg/m2 followed by oxaliplatin 130 mg/m2 every 3 weeks.

RESULTS

Twenty-four patients (34%) were classified as having a poor prognosis. In the overall study population, 14 patients (20%) had a partial response, and 32 patients (46%) had stable disease. The symptomatic response rates were as follows: shortness of breath, 36%; pain, 30%; activity, 23%; appetite, 21%; and asthenia, 20%. Median time to disease progression was 18 weeks (95% confidence interval [CI], 13 to 22 weeks). In chemotherapy-naive patients, median survival was 31 weeks (95% CI, 23 to 40 weeks) from the start of treatment and 49 weeks (95% CI, 40 to 52 weeks) from diagnosis of mesothelioma. In pretreated patients, median survival was 44 weeks (95% CI, 24 to 40 weeks) from the start of treatment and 226 weeks (95% CI, 63 to 292 weeks) from the diagnosis of mesothelioma. Overall 1-year survival was 26% (95% CI, 15.5% to 36.4%), survival was 22% (95% CI, 10.9% to 33.2%) in chemotherapy-naive patients and 40% (95% CI, 15.2% to 64.8%) in pretreated patients. Hematologic toxicity was mild, and there was no alopecia. The most common adverse events were asthenia, nausea/vomiting, and paraesthesia, and no treatment-related deaths were reported.

CONCLUSION

The raltitrexed and oxaliplatin combination is an active outpatient regimen in malignant mesothelioma and has an acceptable tolerability profile.

Modulation of cell-phenotype during in vitro aging. Glycosaminoglycan biosynthesis by skin fibroblasts and corneal keratocytes

The aim of this study was to compare keratocyte and fibroblast phenotypes during in vitro aging by comparing their biosynthesis of glycosaminoglycans using explant and cell cultures. Human skin and corneal explant cultures were realised with Dulbecco Modified Eagle's medium containing 3H glucosamine. Sequential cell cultures were studied at different passages for GAGs biosynthesis by 3H glucosamine incorporation followed by selective degradation with specific hydrolases. Radioactivity was determined and each GAG fraction evaluated. KS and DS are the major components synthesised by corneal explant culture. During in vitro aging, keratocytes synthesised 41% less KS between passages 4-9 with a decrease by 26% of the proportion of DS observed in the same conditions. In skin explant cultures, as expected the major components are CS and hyaluronan (HA). In the first cell passage studied compared with skin organ cultures we could notice a strong decrease of the proportions of DS and KS compensated by an increase of the proportion of HA. During the successive passages of fibroblasts, the proportions of DS and HS decreased (-30 and -62%, respectively) and those of KS increased (+90%). These results indicate that there remain measurable differences between keratocyte and fibroblast phenotypes as far as GAG-synthesis is concerned all though the successive passages, starting from explant cultures and up to the limits of in vitro cell passages.

Regulation of elastase-type endopeptidase activity, MMP-2 and MMP-9 expression and activation in human dermal fibroblasts by fucose and a fucose-rich polysaccharide

Tissue loss during ageing and age-dependent pathologies are the result of a disturbed regulation of proteolytic activities. Elastase-type endopeptidases, especially MMP-2 and -9, play an important role in this respect. Dermal fibroblast cultures and skin explant cultures were used in order to measure the efficiency of fucose and fucose-rich polysaccharides to downregulate the elastase-type endopeptidase activity. Fucose and fucose-rich polysaccharides were shown to downregulate this elastase-type activity, the basic activity and also the hyaluronan or kappa-elastin-stimulated activity. In skin explant cultures, we could demonstrate that fucose and fucose-rich polysaccharides produced an inhibition of the activation of the pro-form to the active form of MMP-9. Here, we show that mono-, di-, oligo- and polysaccharides acting on the elastin-laminin receptor and/or on the fucose-mannose receptor are efficient inhibitors of such enzymes by downregulating elastase-type endopeptidase activity, both at the level of their biosynthesis and at the level of the activation of the pro-enzymes. Fucose and fucose-rich polysaccharide preparations were shown to be efficient modulators of MMP-2 and MMP-9, activity with potential therapeutic applications in age-related pathologies accompanied by tissue loss.

Inhibition of cell proliferation and fibronectin biosynthesis by Na ascorbate

BACKGROUND

The importance of ascorbate on the production of extracellular matrix proteins (as elastin and collagens) is now well documented, but no studies have been published concerning its effects on fibronectin biosynthesis. Fibronectin is important for cell attachment and for proliferation.

MATERIALS AND METHODS

The effects of Na ascorbate were investigated on cell attachment, proliferation, viability and fibronectin biosynthesis by human skin fibroblasts in vitro. Proliferation was followed by the monitoring of [(3)H]-thymidine incorporation; viability by the MTT-test, cell adherence by counting adherent and nonadherent cells and fibronectin biosynthesis by immunoprecipitation of biosynthetically labelled fibronectin.

RESULTS

In the presence of ascorbate, the fibroblasts showed a biphasic growth pattern. At 500 microM ascorbate, [(3)H]-thymidine incorporation was stimulated by 15% as compared to the controls. Higher concentrations gradually decreased proliferation up to 36% of the control value at 5 mM. These effects of ascorbate on DNA synthesis were followed to > 1.25 mM by a strong inhibition, cytotoxic effect and cell death. The non-adherent cell count increased to 10% of the total population at 2.5 mM and to 31% at 5.0 mM ascorbate.Increasing concentrations of ascorbate resulted in a dose-dependent decrease of fibronectin biosynthesis, both in the culture supernates and cell extracts. This inhibition mainly concerned cell membrane-associated fibronectin.Superoxide-dismutase or catalase could inhibit Na ascorbate-induced cytotoxicity and partially re-establish fibronectin biosynthesis. Desferrioxamine, ergothionein and vitamin E were inefficient.

CONCLUSIONS

Our results indicate that ascorbate decreases fibronectin biosynthesis of cultured human skin fibroblasts, thereby producing cell detachment and decreased proliferation. This effect is mainly mediated by the reactive oxygen species and can be inhibited by superoxide-dismutase and catalase.

[Protection of cornea against proteolytic damage. Experimental study of procyanidolic oligomers (PCO) on bovine cornea]

AIM OF THE STUDY

The purpose of this work was the development of an antiprotease treatment for the cornea and later for other ocular tissues. Indeed, many proteases participate in various ocular pathologies and no really effective treatments are currently available.

METHODS

We worked on bovine corneas in organ cultures, incubated in the presence and absence of bacterial collagenase. We used procyanidolic oligomers from grape seeds (PCO) as the antiproteolytic agent, added to the cultures in two different concentrations.

RESULTS

The corneas incubated in the presence of collagenase were quickly attacked and their degradation was practically complete after 24 hours. With the low concentration used (0.066 mg/ml), proteolysis was only slightly inhibited. With the higher concentration of PCO (1 mg/ml), the collagenolysis was completely prevented and the corneas were completely preserved.

CONCLUSION

These results show that the protective effect of the PCO against proteolytic attack, demonstrated previously on other tissues, proved to be effective also for the cornea in vitro. An adequate method of administration in clinical practice must now be developed in order to obtain an effective clinical antiproteolytic treatment.

[Elastin and elastases. Past, present, and future]

Extracellular matrix (ECM) is composed of a large number of macromolecules belonging to one of the four classes comprising ECM, collagens and elastin, the fibrous elements, proteoglycans and glycosaminoglycans and structural glycoproteins. Three of these 4 classes emerged during the Cambrian explosion, at the level of the first invertebrates, the sponges. Elastin appeared only in vertebrates. This protein is also the first to undergo post-synthetic modifications responsible for the progressive loss of its essential rheological properties involved in its physiological functions in the circulatory and respiratory system of vertebrates. The essential mechanisms of its emergence and decay are sufficiently understood in order to integrate them in the processes of development and age-dependent decay of organisms.

[The elastin-laminin receptor]

Elastin is a major component of the extracellular matrix. Elastin peptides derived from its degradation are present in human sera. Elastin peptides induce on fibroblasts, phagocytic cells, lymphocytes, smooth muscle cells and endothelial cells, a variety of biological effects mediated by the elastin-laminin receptor which has been demonstrated to be present on the membrane of these cells. The transduction pathway of the ELR receptor involves the activation of phospholipase C (PLC) by a pertussis toxin sensitive G-protein. PLC induces the production of inositol trisphosphate (IP3) leading to the increase of the intracellular free calcium on one hand, and of diacylglycerol (DAG) which stimulates the translocation to the membrane of PKC leading to the phosphorylation of members of the MAPK family, such as p42/p44 MAPK. Considering the multiple biological effects of ELR the elucidation of the complexity of the signaling pathways will help to better modulate it, mainly in pathological situations such as atherosclerosis.

Aging of connective tissues: from genetic to epigenetic mechanisms

The mechanisms of aging and of age-dependent pathologies can be studied at the tissue level. Such studies concern cell aging within tissues, where cells are surrounded by their matrix. Matrix components undergo post-synthetic modifications such as the Maillard reaction and proteolytic degradations. Finally, cell matrix interactions, mediated by cell membrane receptors also undergo age-dependent modifications. These three aspects of tissue-aging are discussed succinctly in this review with several examples, as the age-dependent increase of fibronectin and the potential harmful effects of its degradation products and the age-dependent degradation of elastin and the harmful effects of elastin peptides mediated by the elastin-laminin receptor. These examples clearly show the intricate cooperation of gene-mediated processes (increased expression of fibronectin and some elastolytic enzymes) and of post-synthetic processes as the novel properties of fibronectin and elastin derived peptides. Such epigenetic mechanisms appear to play a crucial role in age-dependent tissue alterations and pathologies.

Chemotactic penetration of keratocytes in ePTFE polymer in vitro

Expanded polytetrafluoroethylene (ePTFE) is used as a support for artificial corneas. Implanted in corneas, most of the time this polymer is colonized by corneal host cells. The absence of colonization often coincides with extrusion of the polymer. Therefore, we decided to introduce keratocytes into ePTFE in vitro before implantation. Because keratocytes do not spontaneously enter ePTFE, we used several chemoattractants, separately and in a mixture, to stimulate the penetration of cultured keratocytes into the polymer. The influence of the passage number on cell penetration was also studied. No significant differences were observed up to the seventh passage, although seventh-passage cells penetrated somewhat more slowly than younger cells. Satisfactory results were obtained with four of the tested chemotactic factors: IL-6, type alpha transforming growth factor (TGF-alpha), platelet derived growth factor isoform BB (PDGF-BB), and fibroblast growth factor-2 (FGF-2). Under our experimental conditions, two to more than six million keratocytes entered the polymer discs with a volume of 706.5 mm(3) in the presence of these four chemoattractants. TGF-alpha was the most efficient and was selected for further in vitro and in vivo studies.