Comparison between serum-free and fibroblast-cocultured single-cell clonal culture systems: evidence showing that epithelial anti-apoptotic activity is present in 3T3 fibroblast-conditioned media

Cell size correlates with phenotype and proliferative capacity in human corneal epithelial cells

This study investigated whether cell size correlates with phenotype and proliferative capacity in human corneal epithelial cells. Primary cultured human corneal epithelial cells were sorted by flow cytometry based on forward scatter profile in comparison with the profile of beads of known size. Four fractions (A, B, C, and D) of cells ranging in size from 10 to 16, 17 to 23, 24 to 30, and >or=31 microm in diameter, respectively, were collected to evaluate their 5-bromo-2-deoxyuridine (BrdU) label retention properties, cell phenotype, and clonal growth capacity on a 3T3 fibroblast feeder layer. Among these four populations, cell size was shown to positively correlate with the expression of the differentiation markers keratin (K) 3, K12, and involucrin and inversely with the levels of stem cell-associated markers DeltaNp63 and ABCG2 and with colony-forming efficiency (CFE) and growth capacity. Population A with the smallest size, accounting for 11.0%+/-4.5% of the entire population, contained the greatest number of BrdU label-retaining slow-cycling cells, displayed the highest percentage of cells immunopositive to p63 and ABCG2 and negative to K3 and involucrin, expressed the highest levels of DeltaNp63 and ABCG2 mRNA and the lowest levels of K3, K12, and involucrin, and possessed the highest CFE and growth capacity. These findings suggest that cell size correlates with cell differentiation phenotypes and proliferative capacity in human corneal epithelial cells. The smallest cells in population A seem to be enriched for putative stem cells, and small cell size may represent one of the important properties of adult corneal epithelial stem cells.

The use of human serum in supporting the in vitro and in vivo proliferation of human conjunctival epithelial cells

AIM

To evaluate the use of human serum (HS) in supporting the in vitro and in vivo proliferation of human conjunctival epithelial cells, and compare it with fetal bovine serum (FBS) and bovine pituitary extract (BPE).

METHODS

Conjunctival epithelial cells were cultivated in media supplemented with HS (5%, 10%), FBS (5%, 10%), and BPE (70 microg/ml, 140 microg/ml). The colony forming efficiency (CFE), bromodeoxyuridine (BrdU) ELISA proliferation assay, and cell generations were analysed. Cells were evaluated for keratin (K4, K19, and K3) and MUC5AC expression by immunostaining and RT-PCR. Conjunctival equivalents constructed on amniotic membranes were transplanted onto severe combined immune deficient (SCID) mice for 10 days and analysed histologically.

RESULTS

The proliferation assays of HS supplemented cultures (CFE, 6.7% (SD 1.8%); BrdU absorbance, 0.86 (0.16)) were comparable to FBS supplemented (CFE, 9.3% (1.8%); BrdU absorbance, 1.11 (0.18)) and BPE supplemented cultures (CFE, 5.9 (1.5); BrdU absorbance, 0.65 (0.12)). Goblet cell densities for HS, FBS, and BPE supplemented media were 52 cells/cm(2), 60 cells/cm(2), and 50 cells/cm(2), respectively. HS supplemented cultures formed stratified epithelial sheets in vivo following transplantation.

CONCLUSIONS

The proliferative capacity of conjunctival epithelial cells cultivated in HS supplemented cultures was comparable to FBS and BPE supplemented cultures. The elimination of animal material from the culture system is advantageous when cultivating cells for clinical transplantation.

ABCG2 transporter identifies a population of clonogenic human limbal epithelial cells

ABCG2, a member of the ATP binding cassette (ABC) transporters, has been identified as a molecular determinant for bone marrow stem cells and proposed as a universal marker for stem cells. This study investigates ABCG2 expression and its potential as a marker that identifies human limbal epithelial stem cells. ABCG2 expression was evaluated by immunofluorescent and immunohistochemical staining, laser scanning confocal microscopy, flow cytometry, and semiquantitative reverse transcription-polymerase chain reaction. Cells selected from primary limbal epithelial cultures by flow cytometry with ABCG2 monoclonal antibody (mAb) or Hoechst 33342 dye staining were evaluated for their gene expression and colony-forming efficiency (CFE). ABCG2 protein was mainly located in the basal cells of limbal epithelia but not in the limbal suprabasal and corneal epithelia. ABCG2 staining was also observed in primary limbal epithelial cultures. Limbal epithelia express higher levels of ABCG2 and DeltaNp63 mRNAs than corneal epithelia. Labeling with ABCG2 mAb yielded 2.5%-3.0% positive cells by flow cytometry. The ABCG2-positive cells exhibited greater CFE on a 3T3 fibroblast feeder layer than ABCG2-negative cells. A side population (SP) was detected by the Hoechst 33342 exclusion assay. SP cells displayed stronger expression of ABCG2 and DeltaNp63 mRNA and greater CFE than the non-SP cells. In conclusion, these findings demonstrate that ABCG2 transporter was exclusively expressed by limbal basal cells and that the ABCG2-positive and SP cells possess enriched stem cell properties, suggesting for the first time that ABCG2 could serve as a marker to identify the putative limbal epithelial stem cells.

Partial enrichment of a population of human limbal epithelial cells with putative stem cell properties based on collagen type IV adhesiveness

The concept that corneal epithelium stem cells reside in limbus has been recognized for more than a decade, but isolation of these stem cells has not been accomplished. This study was an initial attempt to isolate a population of human limbal epithelial cells enriched for certain putative stem cell properties based on their phenotype. Epithelial cells harvested from fresh human limbal rings and their primary cultures were allowed to adhere to collagen IV-coated dishes for 20 min and 2 hr, sequentially. The rapidly adherent cells (RAC), slowly adherent cells and non-adherent cells were evaluated for certain stem cell properties: (a) BrdU-label retention, (b) expression of basal cell (integrin beta1, p63, ABCG2) and differentiation (involucrin, keratin 12) markers, and (c) colony forming efficiency (CFE) and growth capacity on a 3T3 fibroblast feeder layer. Among unfractionated cells and the three selected populations, the RAC, accounting for about 10% of whole population, were enriched 5-fold in BrdU label-retaining cells, displayed the highest number of integrin beta1 and p63 positive and involucrin negative cells, expressed high levels of DeltaNp63 and ABCG2 mRNA, and lacked involucrin and K12 expression, and possessed the greatest CFE and growth capacity. These findings demonstrated for the first time that human limbal epithelial cells with stem cell properties can be partially enriched by their adhesiveness to collagen IV. The RAC population enriched for certain putative stem cell properties may prove useful in the future for transplantation to diseased and damaged corneas with limbal stem cell deficiency.

Phenotypic characterization of human corneal epithelial cells expanded ex vivo from limbal explant and single cell cultures

Cultivated human corneal epithelial cells have been successfully used for corneal reconstruction. Explant and single cell systems are currently used for human corneal epithelial cultivation. This study was conducted to characterize the phenotypes of human corneal epithelial cells expanded ex vivo by these two culture systems with regard to their growth potential, morphology and antigen expression patterns. Human corneal epithelial cells were expanded by limbal explant culture or limbal single cell suspension culture on a mitomycin C treated 3T3 fibroblast feeder layer. The phenotypes of primary cultured cells were evaluated by morphology and immunohistochemical staining with antibodies for proposed keratinocyte stem cell markers (p63, EGFR, K19 and integrin beta1) and differentiation markers (K3, involucrin and gap junction protein connexin 43). BrdU labeling was performed to identify the label-retaining cells. Human corneal epithelial cells were grown from limbal tissues preserved as long as 16 days by both culture systems. The growth rate depended on the tissue freshness, the time from death to preservation and the time from death to culture, but not on the donor age. Cell growth was observed in 96.2% (n = 43) of single cell suspension cultures and in 90.8% (n = 213) of explant cultures. The cell expansion was confluent in 10-14 days in single cell suspension cultures and 14-21 days in explant cultures. The cell morphology in single cell suspension culture was smaller, more compact and uniform than that in explant culture. Immunostaining showed a greater number of the small cells expressing p63, EGFR, K19 and integrin beta1, while more larger cells stained positively for K3, involucrin and connexin 43 in both culture systems. BrdU-label retaining cells were identified in 2.3+/-0.7% of explant cultures and 3.73+/-1.5% of single cell cultures chased for 21 days. In conclusion, the limbal rims are a great treasure for ex vivo expansion of human corneal epithelial cells. The phenotypes of corneal epithelial cells, ranging from basal cells to superficial differentiated cells, are well maintained in both culture systems. Slow-cycling BrdU-label retaining cells, that are characteristic of stem cells, were identified in the cultures.

The role of allogenic fibroblasts in an acute wound healing model

Skin is the first tissue-engineered organ to have been successfully developed in the laboratory, and it has been clinically available for use as epidermal sheets for some time. As refinements in this field of tissue engineering continue, several key issues give cause for concern. One issue is the need to form a more complete dermal analogue before grafting. To this end, fibroblasts may be used in vitro to deposit extracellular matrix components within a basic scaffold, laying down those molecules not endogenous to the material and thereby improving the quality of the skin replacement. Many studies have shown the benefits of in vitro seeding with fibroblasts, but there has been some debate regarding the longevity of such cells after allotransplantation into an immunocompetent host. In this study, the authors set out to determine the longevity of transplanted cells in an immunocompetent porcine model. A total of 24 wounds were made on four female animals, 12 of which were covered with acellular hyaluronic acid dermal matrices, and the remainder of which were covered with matrices seeded with allogenic (male) fibroblasts. After a week in vivo, the wounds were grafted with either split-thickness skin grafts or cultured epithelial autograft. Biopsy specimens were obtained from wounds at varying time intervals and assessed using genetic analysis to determine the survival of allotransplanted cells. No cells were detectable by polymerase chain reaction analysis (sensitivity, 1:100,000) after 7 days in vivo. Subsequent histologic examination demonstrated little difference in wound morphology. The authors conclude that allogenic fibroblasts do not survive transplantation in a porcine wound model.

Ex vivo expansion of corneal limbal epithelial/stem cells for corneal surface reconstruction

PURPOSE

The management of severe ocular surface disease due to limbal stem cell deficiency has changed dramatically. The concept of limbal stem cells, as the source of corneal epithelium revolutionised the therapeutic approach of ocular surface reconstruction. Deficiency of limbal stem cells results in blinding ocular surface diseases. Grafting viable limbal tissue, from either fellow healthy eye or a donor eye, with the resident stem cell population may replenish limbal stem cells and can restore the corneal surface to normality. Transplanting the limbal tissue can be achieved through a variety of procedures that include cadaveric keratolimbal allograft (KLAL), live or living related conjunctival limbal allograft (Ir-CLAL) and limbal autograft. Advances in tissue engineering techniques have offered a viable alternative to overcome the limitation of limbal tissue available for transplantation. Epithelial stem cells harvested from a small limbal biopsy can be expanded in vitro on a suitable carrier and then transplanted to the diseased cornea to successfully restore the corneal surface. This article is a chronological review of the important steps that brought ex vivo expanded stem cell transplantation in ocular, particularly corneal surface reconstruction.

METHODS

The MEDLINE data base was searched for the years 1966-2002, using key words cornea, cell culture, ex-vivo expansion, limbus, stem cell, ocular surface and transplantation. Several articles that were not found by MEDLINE search were taken from references from other articles. Inclusion or exclusion of article was based on the relevance to the subject.

CONCLUSIONS

Corneal epithelial reconstruction with ex vivo expanded limbal cells is a potential tool in ocular surface reconstruction, although the technique is currently investigational. Strategies to achieve conjunctival epithelial restoration and tear film replenishment will allow ophthalmic surgeons to truly reconstruct the ocular surface.

Ex vivo preservation and expansion of human limbal epithelial stem cells on amniotic membrane cultures

BACKGROUND/AIM

Amniotic membrane (AM) transplantation effectively expands the remaining limbal epithelial stem cells in patients with partial limbal stem cell deficiency. The authors investigated whether this action could be produced ex vivo.

METHODS

The outgrowth rate on AM was compared among explants derived from human limbus, peripheral cornea, and central cornea. For outgrowth of human limbal epithelial cells (HLEC), cell cycle kinetics were measured by BrdU labelling for 1 or 7 days, of which the latter was also chased in primary cultures, secondary 3T3 fibroblast cultures, and in athymic Balb/c mice following a brief treatment with a phorbol ester. Epithelial morphology was studied by histology and transmission electron microscopy, and phenotype was defined by immunostaining with monoclonal antibodies to keratins and mucins.

RESULTS

Outgrowth rate was 0/22 (0%) and 2/24 (8.3%) for central and peripheral corneal explants, respectively, but was 77/80 (96.2%) for limbal explants (p <0.0001). 24 hour BrdU labelling showed a uniformly low (that is, less than 5%) labelling index in 65% of the limbal explants, but a mixed pattern with areas showing a high (that is, more than 40%) labelling index in 35% of limbal explants, and in all (100%) peripheral corneal explants. Continuous BrdU labelling for 7 days detected a high labelling index in 61.5% of the limbal explants with the remainder still retaining a low labelling index. A number of label retaining cells were noted after 7 day labelling followed by 14 days of chase in primary culture or by 21 days of chase after transplantation to 3T3 fibroblast feeder layers. After exposure to phorbol 12-myristate 13-acetate for 24 hours and 7 day labelling, HLEC transplanted in athymic mice still showed a number of label retaining basal cells after 9 days of chase. HLEC cultured on AM were strongly positive for K14 keratin and MUC4 and slightly positive in suprabasal cells for K3 keratin but negative for K12 keratin, AMEM2, and MUC5AC. After subcutaneous implantation in athymic mice, the resultant epithelium was markedly stratified and the basal epithelial cells were strongly positive for K14 keratin, while the suprabasal epithelial cells were strongly positive for K3 keratin and MUC4, and the entire epithelium was negative for K12 keratin and MUC5A/C.

CONCLUSIONS

These data support the notion that AM cultures preferentially preserve and expand limbal epithelial stem cells that retain their in vivo properties of slow cycling, label retaining, and undifferentiation. This finding supports the feasibility of ex vivo expansion of limbal epithelial stem cells for treating patients with total limbal stem cell deficiency using a small amount of donor limbal tissue.

Corneal stem cells in review

The cornea provides the eye with protection and the refractive properties essential for visual acuity. The transparent epithelium is highly specialized with basal and stratified squamous cells that are renewed throughout life from a stem cell population. The stem cells are thought to reside at the corneal limbus and may be maintained by a variety of intrinsic and extrinsic factors such as the local environment, survival factors, and cytokines. A number of markers have been localized to the limbus in an attempt to identify stem cells; however, definite stem cell identification remains elusive. During homeostasis and following injury to the corneal epithelium, the limbal stem cells divide to produce daughter transient amplifying cells that proliferate, migrate, and differentiate to replace lost cells. However, this cannot occur if the stem cell population is depleted. Limbal stem cell deficiency then results in corneal re-epithelialization by the neighboring conjunctiva, causing pain, poor vision, and even blindness. This review will focus on corneal epithelial stem cells in ocular surface repair and regeneration. The current knowledge of stem cell biology in the corneal epithelium, clinical consequences of stem cell deficiency, and therapeutic strategies aimed at reversing stem cell deficiency will be discussed.

Can we produce a human corneal equivalent by tissue engineering?

Tissue engineering is progressing rapidly. Bioengineered substitutes are already available for experimental applications and some clinical purposes such as skin replacement. This review focuses on the development of reconstructed human cornea in vitro by tissue engineering. Key elements to consider in the corneal reconstruction, such as the source for epithelial cells and keratocytes, are discussed and the various steps of production are presented. Since one application of this human model is to obtain a better understanding of corneal wound healing, the mechanisms of this phenomenon as well as the function played both by membrane-bound integrins and components from the extracellular matrix have also been addressed. The analysis of integrins by immunohistofluorescence labelling of our reconstructed human cornea revealed that beta(1), alpha(3), alpha(5), and alpha(6) integrin subunits were expressed but alpha(4) was not. Laminin, type VII collagen and fibronectin were also detected. Finally, the future challenges of corneal reconstruction by tissue engineering are discussed and the tremendous applications of such tissue produced in vitro for experimental as well as clinical purposes are considered.

Effects of antimetabolite induced cellular growth arrest on fibroblast-fibroblast interactions

The use of single five-minute applications of antimetabolites during glaucoma filtration surgery has significantly reduced the occurrence of post-operative scarring and bleb failure. However, surgery for some patients is still unsuccessful, despite the use of antiproliferative agents, due to formation of scar tissue at the drainage site. It is not known if cells growth arrested in the treated area with a single application of antimetabolites influence the activity of adjacent non-treated cells. We hypothesise that the activity of non-treated cells recruited to the wound site may be involved in post-operative scarring. The aim of this study was to investigate the effects of antimetabolite induced cellular growth arrest on cell-cell interactions using in vitro techniques.Tenon's capsule fibroblast cultures were growth arrested by exposure for 5 minutes to mitomycin-C (0.001, 0.01 and 0.1 mg ml-1), 5-fluorouracil (0.25, 2.5 and 25 mg ml-1) or phosphate buffered saline (PBS). Following a period of serum-starvation, conditioned media (CM) were subsequently collected from the cells at intervals up to 29 days post-treatment. Correction for cell number was made prior to supplementation of serum-free medium with CM. CM were assessed for ability to support or inhibit normal non-treated fibroblast proliferation, migration and collagen contraction. Conditioned media collected from cells growth arrested with MMC or 5FU stimulated normal fibroblast proliferation, migration and collagen contraction in excess of non-conditioned serum-free medium. Peaks of fibroblast activity in CM differed according to which drug and concentration had originally been given to the treated cells. This study has demonstrated that CM collected from fibroblasts treated for 5 minutes with a range of concentrations of antimetabolites can differentially influence normal non-treated fibroblast activity. This in vitro data suggests that despite entering growth arrest, fibroblasts may still influence the behaviour of other cells via soluble mediators. They may have implications in the clinical setting, in that it may not be sufficient to suppress proliferation alone to prevent fibroblast behaviour associated with scarring after glaucoma filtration surgery.

Differential regulation of keratinocyte growth factor and hepatocyte growth factor/scatter factor by different cytokines in human corneal and limbal fibroblasts

Corneal epithelial stem cells and transient amplifying cells are located in the limbal and corneal regions, respectively. In a serum-free medium with or without different cytokines, limbal fibroblasts consistently produced greater levels of keratinocyte growth factor (KGF) transcript and protein than corneal fibroblasts, whereas corneal fibroblasts produced greater levels of hepatocyte growth factor/ scatter factor (HGF/SF) transcript and protein than limbal fibroblasts. Expression of HGF/SF transcript and protein was up-regulated mildly by epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), or platelet-derived growth factor B (PDGF-BB) but markedly by interleukin-1 beta (IL-1 beta) and was more pronounced in limbal than in corneal fibroblasts. Expression of KGF transcript was down-regulated by EGF, TGF-alpha, and PDGF-BB, was markedly up-regulated by IL-1 beta, and was more pronounced in limbal than in corneal fibroblasts. Expression of KGF protein was up-regulated markedly by IL-1 beta and moderately by PDGF-BB, especially in limbal fibroblasts. TGF-beta 1 uniquely turned off transcript and protein expression of HGF/SF and KGF in corneal fibroblasts. Although its transcript levels were similarly down-regulated in limbal fibroblasts, KGF protein levels were paradoxically up-regulated by TGF-beta 1 when added alone or with TGF-alpha or IL-1 beta. These data indicate that KGF and HGF/SF, two fibroblast-derived epithelial mitogens, are expressed differentially by limbal and corneal fibroblasts and are modulated by cytokines activated during epithelial-mesenchymal interactions, suggesting that they may play a different role in modulating corneal epithelial stem cells and transient amplifying cells.

Effects of adenine nucleosides and nucleotides on the isolated heart of the snail Helix aspersa and the slug Arion ater

1. Adenine nucleosides and nucleotides were examined for pharmacological activity in hearts isolated from the snail Helix aspersa and the slug Arion ater. 2. Adenosine, AMP, ADP and ATP (above 100 microM) produced either an excitation or an inhibition in the isolated hearts of the snail and slug. 3. 2-Chloroadenosine, alpha, beta-methylene ATP and 2-methylthio ATP were inactive at concentrations up to 1 mM. 4. Responses were not blocked by any commonly accepted vertebrate purinoceptor antagonists, indicating that these purinoceptors are dissimilar to vertebrate purinoceptors and cannot be classified according to accepted purinoceptor classifications. 5. Electrical field stimulation of the snail heart produced frequency-dependent responses: 1-4 Hz produced predominantly excitation, 8-32 Hz predominantly inhibition. These responses were unaffected by the purines up 3 mM.