Intrastromal invasion by limbal epithelial cells is mediated by epithelial-mesenchymal transition activated by air exposure

Corneal epithelial stem cells are located in the basal layer of the limbus between the cornea and the conjunctiva. Regulation of these limbal epithelial progenitor cells by the stromal niche dictates corneal surface health. To further characterize this process, limbal explants were cultured at the air-fluid interface, termed air-lifting, to stimulate the niche. As compared to submerged cultures, air-lifting significantly promoted epithelial stratification, migration, proliferation, and intrastromal invasion by limbal epithelial cells. Epithelial intrastromal invasion was noted when the limbal, but not corneal, epithelium was recombined with the limbal stroma containing live, but not dead, cells. Invading limbal basal cells displayed up-regulated nuclear expression of p63 and Ki67, down-regulated E-cadherin and cornea-specific keratin 3, and switched expression of beta-catenin from intercellular junctions to the nucleus and cytoplasm, indicating the activation of the Wnt/beta-catenin pathway. Invaded cells isolated by collagenase from the stroma of air-lifted, but not submerged, explants showed vivid clonal growth on 3T3 fibroblast feeder layers and complete epithelial-mesenchymal transition by expressing nuclear p63 and cytoplasmic S100A4. These findings collectively suggest that epithelial-mesenchymal transition via the Wnt/beta-catenin pathway influences the fate of limbal epithelial cells, likely to be progenitor cells, between regeneration and fibrosis when the stromal niche is activated.     

Limbal epithelial stem/progenitor cells attract stromal niche cells by SDF-1/CXCR4 signaling to prevent differentiation

Corneal epithelial stem cells (SCs) are an ideal model for investigating how adult lineage-committed epithelial SCs are regulated by an anatomically defined and accessible niche, that is, limbal palisades of Vogt, located between the cornea and the conjunctiva. We have used collagenase digestion to isolate the entire limbal epithelial SCs and subjacent mesenchymal cells, and we have demonstrated that their close association is crucial for promoting epithelial clonal growth, implying that the latter serves as niche cells (NCs). After their close association was disrupted by trypsin/EDTA, single SCs and NCs could reunite to generate sphere growth in three-dimensional Matrigel in the embryonic SC medium, and that such sphere growth initiated by SC-NC reunion was mediated by SDF-1 uniquely expressed by limbal epithelial progenitor cells and its receptor CXCR4, but not CXCR7, strongly expressed by limbal stromal NCs. Inhibition of CXCR4 by AMD3100 or a blocking antibody to CXCR4 but not CXCR7 disrupted their reunion and yielded separate spheres with a reduced size, while resultant epithelial spheres exhibited more corneal differentiation and a notable loss of holoclones. For the first time, these results provide strong evidence supporting that limbal SC function depends on close physical association with their native NCs via SDF-1/CXCR4 signaling. This novel in vitro model of sphere growth with NCs can be used for investigating how limbal SC self-renewal and fate decision might be regulated in the limbal niche.     

Human amniotic epithelial cells as novel feeder layers for promoting ex vivo expansion of limbal epithelial progenitor cells

Human amniotic epithelial cells (HAECs) are a unique embryonic cell source that potentially can be used as feeder layers for expanding different types of stem cells. In vivo, HAECs uniformly expressed pan-cytokeratins (pan-CK) and heterogeneously expressed vimentin (Vim). The two phenotypes expressing either pan-CK(+)/Vim(+) or pan-CK(+)/Vim(-) were maintained in serum-free media with high calcium. In contrast, all HAECs became pan-CK(+)/Vim(+) in serum-containing media, which also promoted HAEC proliferation for at least eight passages, especially supplemented with epidermal growth factor and insulin. Mitomycin C-arrested HAEC feeder layers were more effective in promoting clonal growth of human limbal epithelial progenitors than conventional 3T3 murine feeder layers. Cells in HAEC-supported clones were uniformly smaller, sustained more proliferation, and expressed less CK12 and connexin 43 but higher levels of stem cell-associated markers such as p63, Musashi-1, and ATP-binding cassette subfamily G2 than those of 3T3-supported clones. Subculturing of clonally expanded limbal progenitors from HAEC feeder layers, but not from 3T3 feeder layers, gave rise to uniformly p63-positive epithelial progenitor cells as well as nestin-positive neuronal-like progenitors. Collectively, these results indicated that HAECs can be used as a human feeder layer equivalent for more effective ex vivo expansion of adult epithelial stem cells from the human limbus. Disclosure of potential conflicts of interest is found at the end of this article.     

N-Cadherin is expressed by putative stem/progenitor cells and melanocytes in the human limbal epithelial stem cell niche

Corneal epithelial stem cells are known to be localized to the basal layer of the limbal epithelium, providing a model system for epithelial stem cell biology; however, the mechanisms regarding the maintenance of these stem cells in their specialized niche remain poorly understood. N-cadherin is a member of the classic cadherin family and has previously been demonstrated to be expressed by hematopoietic stem cells. In the present study, we demonstrate that N-cadherin is expressed by putative stem/progenitor cells, as well as melanocytes, in the human limbal epithelial stem cell niche. In addition, we demonstrate that upon in vitro culture using 3T3 feeder layers, loss of N-cadherin expression occurs with cell proliferation. These results indicate that N-cadherin may be a critical cell-to-cell adhesion molecule between corneal epithelial stem/progenitor cells and their corresponding niche cells in the limbal epithelium.     

Isolation and clonal analysis of human epidermal keratinocyte stem cells in long-term culture

We developed a procedure for growing normal epidermal keratinocyte stem cells isolated from a single punch biopsy of adult human skin in long-term culture. Primary skin epithelial cells were maintained in collagen-coated plates with irradiated human neonatal foreskin fibroblasts (line HPI.1) as a feeder for more than 120 days, approximately 115 population doublings, without signs of replicative senescence. Clonal analysis revealed the presence of holoclones, meroclones, and paraclones. Only emerging colonies with high proliferative potentials and extensive capacities for division (holoclones and meroclones) were subcultured, favoring the expansion of stem cells and progenitors capable of prolonged self-maintenance when subcloned, thus accounting for the prevailing long-term proliferation of the original culture. We found that meroclones included bipotent progenitors capable of generating both keratinocytes and mucin-producing cells. The numbers of these cells were greater after confluence, suggesting that commitment for their differentiation occurred late in the life of a single clone. On a three-dimensional gelatin matrix and on a collagen layer containing the fibroblast feeder, cells isolated from the expansion of holoclones and meroclones formed stratified cohesive layers of keratinocytes that were able to further differentiate, as in normal skin. These results indicate that our procedure will serve as a valuable tool to study expansion of epidermal stem cells as well as the growth mechanisms and cell products associated with their growth and differentiation.     

Monolayer culture of normal human esophageal epithelial cells

Summary A method is described for the isolation of epithelial cells from normal human esophagus and the establishment of the epithelial cells in monolayer cell culture using irradiated 3T3 fibroblast feeder layers as a substrate. The monolayer cultures were subsequently used to study the effects of a tumor promoter and carcinogen on human esophageal epithelial cells in vitro.     

Selective culture of epithelial cells from primary breast carcinomas using irradiated 3T3 cells as feeder layer

The main drawback of the selective culture of human mammary epithelial cells from primary breast cancer is the overgrowth of tumor-associated stromal fibroblasts. This drawback may be overcome by using, in primary culture, lethally irradiated 3T3 cells which act as a feeder layer to maintain tumor-derived epithelial cell proliferation. These 3T3 cells, exposed to 60 Gy at confluence, form a specific cellular substrate which constitutes an obstacle to fibroblast attachment. Enzyme-disaggregated breast cells from six primary breast carcinomas were cocultured over lethally irradiated but living 3T3 cells. The method led to the purification of tumor-derived epithelial cells from all six cancer samples, and long-term culture was obtained in one. The epithelial nature of these purified tumor-derived epithelial cells was demonstrated by their general morphology and by the expression of cytokeratins and Epithelial Membrane Antigen. These results confirm the stimulatory effect of a this stromal feeder layer on breast epithelial cell growth and show that this stromal feeder layer can also control the fibroblast overgrowth. Our results provide an alternative approach in the selective culture of epithelial cells from primary breast carcinoma.     

Ultrastructural localisation and size distribution of collagen fibrils in Glisson's sheath of rat liver: implications for mechanical environment and possible producing cells

The ultrastructure and size distributions of collagen fibrils in Glisson's sheath were investigated in the rat liver to analyse the mechanical environment around the fibrils and their possible cells of origin. Glisson's sheath was found to contain 2 populations of collagen fibrils with different diameters and distinct localisations, namely fibroblast‐associated and bile epithelium‐associated. Fibroblast‐associated collagen was composed of fibrils arranged in bundles and constituted the majority of the collagen in Glisson's sheath. Bile epithelium‐associated collagen was represented by small dispersed groups of fibrils just beneath the basement membrane of the bile duct. The basement membrane of the bile duct was frequently reduplicated into a few or as many as 10 layers of laminae densae, with scattered collagen fibrils between these laminae. The diameters of the fibrils of both groups of collagen increased in relation to the calibre of the bile duct, whereas at any given place in Glisson's sheath bile epithelium‐associated collagen fibrils had a smaller diameter compared with those of the fibroblast‐associated fibrils. The increment in fibril diameter along the bile duct is considered to be correlated with the increase in mechanical stress acting on Glisson's sheath. The difference in diameter between the 2 populations as well as the incorporation of fibrils between the laminae densae of the basement membrane of the bile duct supports the view that the bile epithelium‐associated collagen is produced by the epithelial cells of the bile duct, thus having a different origin from that of fibroblast‐associated collagen. These findings provide the first evidence that the epithelial cells of the interlobular bile duct produce fibril‐forming collagen. Furthermore, it is suggested that cholestasis stimulates the epithelial cells of interlobular bile duct to increased synthesis of fibril‐forming collagen that is also produced by these cells under physiological conditions.     

Growth and differentiation of human distal airway epithelial cells in culture. Effects of small amounts of serum in defined medium

We herein describe a new simple method for culturing primary epithelial cells derived from the distal airway of adult human lung. Peripheral lung tissue obtained from surgical materials was cultured as explants in Ham's F12 medium supplemented with hormones and growth factors. From days 10 to 14, outgrowth of epithelial cells started on the dish surface, and even after removal of explants at days 14 to 16, these cells continued to replicate during the 3rd and 4th week of culture, and eventually formed epithelial cell foci (10 to 20-fold increase in population). They then ceased to replicate and terminally differentiated in the 5th week. Addition of 1% serum to the culture medium enhanced the initial outgrowth of epithelial cells, whereas small amounts of serum had no effect on proliferation of cells after explant removal. On the other hand, serum modulated the differentiation phenotypes of epithelial cells. In the presence of 1% serum, numerous ciliated and secretory cells appeared, whereas the cells underwent epidermoid differentiation in the absence of serum. When replated onto 3T3 fibroblast feeder layers, the epithelial cells from earlier cultures showed a great replicative capability and formed colonies at higher frequencies (colony-forming efficiency, 8 to 27% at days 14 to 21), but the replicative capability was significantly reduced at the confluent stage (colony-forming efficiency, 0.44 to 2.0% at day 28). Morphologic examinations of explants strongly suggested that the primary cells were derived from the bronchiolar epithelium. We conclude that this new culture system provides an excellent model for studying the growth, differentiation, and function of human bronchiolar epithelial cells in vitro.     

Cell culture from rat renal glomeruli

The primary culture of rat renal glomeruli was found to result in the ready outgrowth of two cells types. One type designated c-cells were cytokeratin positive and exhibited microvilli and cilia. The second type designated f-cells were vimentin positive and showed rugose surfaces. C-cells were polygonal in culture on plastic surfaces and were derived from cells of parietal epithelial origin. F-cells assumed a more extended form on plastic and were judged to be a sub-set of parietal epithelial cells. Neither cell type was derived from the visceral epithelium which was found to have been destroyed during isolation of the glomeruli. When cultured on isolated glomerular basement membrane both the c-cells and f-cells assumed a polygonal morphology but when grown on Matrigel the cells assumed the form of long strands interconnecting the outgrowths between the glomeruli. The appearance of the cells in the strands, judged from scanning electron microscopy, suggested that these were formed from f-cells but other cell types were entrained in the structures. Glomeruli subjected to vigorous proteinase digestion of the basement membrane allowed culture of a wider variety of cells. These included endothelial cells, judged by OX-43 antibody and anti-von Willebrand Factor staining, and mesangial cells. In cultures from glomeruli polygonal cells are often assumed to be visceral epithelial cells, the results from this study indicate that this assumption is unsound. The very different behaviour of cells grown on isolated basement membrane as compared with cells grown on Matrigel suggests that Matrigel may not faithfully mimic basement membrane with respect to cell response in culture.     

A novel NIH/3T3 duplex feeder system to engineer corneal epithelial sheets with enhanced cytokeratin 15-positive progenitor populations

Corneal epithelial cell sheets co-cultivated with feeder cells are used to reconstruct the ocular surface in stem cell-depleted eyes. The present study was conducted to investigate the optimal method of using feeder cells in the interest of preserving progenitor cells in cultivated sheets. We compared the phenotype and secondary colony forming efficiency (CFE) of cell sheets that were engineered using 3T3 feeder cells as a separate layer or as a contact layer. We also devised a novel "duplex feeder" system that consists of two separate layers of feeder cells. After cells reached confluence, cells were cultured at the air-liquid interface to allow full stratification. Stratified sheets were then analyzed using immunohistochemistry and secondary colony formation. Contact feeder cultures and duplex feeder cultures yielded epithelial sheets with small, cuboid basal cells with strong expression of keratin (K)3, K12, and K 15. Furthermore, only duplex feeder layers reproduced the basal K 15, suprabasal K12 limbal phenotype where epithelial stem cells reside. A similar effect was observed when cornea stroma-derived progenitor cells were used as feeder cells. Duplex feeder sheets also produced significantly more secondary colonies than cells dissociated from single layer sheets, suggesting that the duplex feeder system produces transplantable sheets with a higher yield of progenitor cells.     

Feeder-free monolayer cultures of human embryonic stem cells express an epithelial plasma membrane protein profile

Human embryonic stem cells (hESCs) are often cocultured on mitotically inactive fibroblast feeder cells to maintain their undifferentiated state. Under these growth conditions, hESCs form multilayered colonies of morphologically heterogeneous cells surrounded by flattened mesenchymal cells. In contrast, hESCs grown in feeder cell-conditioned medium on Matrigel instead tend to grow as monolayers with uniform morphology. Using mass spectrometry and immunofluorescence microscopy, we showed that hESCs under these conditions primarily express proteins belonging to epithelium-related cell-cell adhesion complexes, including adherens junctions, tight junctions, desmosomes, and gap junctions. This indicates that monolayers of hESCs cultured under feeder-free conditions retain a homogeneous epithelial phenotype similar to that of the upper central cell layer of colonies maintained on feeder cells. Notably, feeder-free hESCs also coexpressed vimentin, which is usually associated with mesenchyme, suggesting that these cells may have undergone epithelium-to-mesenchyme transitions, indicating differentiation. However, if grown on a "soft" substrate (Hydrogel), intracellular vimentin levels were substantially reduced. Moreover, when hESCs were transferred back to feeder cells, expression of vimentin was again absent from the epithelial cell population. These results imply that on tissue culture substrates, vimentin expression is most likely a stress-induced response, unrelated to differentiation. Disclosure of potential conflicts of interest is found at the end of this article.     

人羊膜上皮细胞体外培养重建角膜上皮的实验研究

目的: 探讨人羊膜上皮细胞(human amniotic epithelial cells, HAECs)诱导分化为角膜上皮细胞的可塑性及其重建角膜上皮的可行性。方法: 取足月产人羊膜,通过胶原酶、胰蛋白酶和EDTA消化获得HAECs,将细胞在体外培养扩增、传代,将2~3代HAECs接种于去上皮的兔角膜基质上培养,待HAECs融合形成单层后,置于插入式培养皿中进行气液界面培养14 d。对角膜基质上培养的HAECs用光镜、扫描电镜和透射电镜进行形态学及超微结构观察,以及细胞角蛋白(CK)3/12的免疫组织化学检测。结果: HAECs可在角膜基质上生长,培养1~2 d可形成单层,气液界面培养14 d可形成4~5层细胞的复层上皮,扫描电镜观察细胞表面有丰富的微绒毛,透射电镜下可见上皮细胞之间有桥粒连接,免疫组化检测复层细胞表达CK3/12,所形成的复层结构与正常角膜上皮相似。结论: HAECs有可能作为种子细胞用于组织工程角膜上皮重建。     

Enamel tissue engineering using subcultured enamel organ epithelial cells in combination with dental pulp cells

We describe a strategy for the in vitro engineering of enamel tissue using a novel technique for culturing enamel organ epithelial (EOE) cells isolated from the enamel organ using 3T3-J2 cells as a feeder layer. These subcultured EOE cells retain the capacity to produce enamel structures over a period of extended culture. In brief, enamel organs from 6-month-old porcine third molars were dissociated into single cells and subcultured on 3T3-J2 feeder cell layers. These subcultured EOE cells were then seeded onto a collagen sponge in combination with primary dental pulp cells isolated at an early stage of crown formation, and these constructs were transplanted into athymic rats. After 4 weeks, complex enamel-dentin structures were detected in the implants. These results show that our culture technique maintained ameloblast lineage cells that were able to produce enamel in vivo. This novel subculture technique provides an important tool for tooth tissue engineering.     

The effect of micropores in the surface of temperature-responsive culture inserts on the fabrication of transplantable canine oral mucosal epithelial cell sheets

Primary canine oral mucosal epithelial cells were cultured on temperature-responsive dishes and cell culture inserts to fabricate transplantable epithelial cell sheets. When 3T3 feeder layers and fetal bovine serum were eliminated from dish culture, the harvested cell sheets became significantly more fragile. In contrast, when epithelial cells were cultured on inserts having submicron-scale pores, cell sheet fragility was eliminated. Keratin expression profiles showed no differences among the harvested cell sheets, but the expression of p63, a putative stem/progenitor marker, was strongly dependent on the presence of 3T3 feeder layers and serum. These results suggest that the maintenance of stem/progenitor cells is influenced by the apical/basal supply of nutrients as well as culture supplements.     

Fabrication of transplantable human oral mucosal epithelial cell sheets using temperature-responsive culture inserts without feeder layer cells

To exclude bacteria- or animal-derived factors from cultured fabrication of transplantable epithelial cell sheets, primary human oral mucosal epithelial cells were seeded on temperature-responsive culture inserts having submicron-scale pores. Supplying culture medium containing human autologous serum to both apical and basal sides of human epithelial cells allows these cells to grow to confluence. These proliferating cells created stratified epithelial layers even when 3T3 feeder layers and fetal bovine serum were eliminated from culture. Normal keratin expression profiles were obtained with these cells, and basal and midlayer cells expressed p63, a putative stem/progenitor marker. These results suggest that temperature-responsive culture inserts can be useful in clinical settings that require the exclusion of xenogeneic factors.     

Generation of human epidermal constructs on a collagen layer alone

Because engineered tissues are designed for clinical applications in humans, a major problem is the contamination of cocultures and tissues by allogenic molecules used to grow stem cells in vitro. The protocols that are commonly applied to generate epidermal equivalents in vitro require the use of irradiated murine fibroblasts as a feeder layer for keratinocytes. In this study, we report a simple procedure for growing human keratinocytes, isolated from adult skin, to generate an epidermal construct on a collagen layer alone. In this model, no human or murine feeder layers were used to amplify cell growth, and isolated keratinocytes were seeded directly at high cell density on the collagen-coated flasks or coverslips in an epithelial growth medium containing low calcium concentration. Morphological, immunochemical, and cytokinetic features of epithelial colonies grown on the collagen layer were typical of keratinocytes and were comparable with those reported for keratinocytes grown on a feeder layer. The stratification of keratinocytes generated 3-dimensional synthetic constructs displaying a tissue architecture comparable with that of natural epidermis. Epithelial cells expressed specific markers of keratinocyte terminal differentiation, including involucrin and filaggrin. Nevertheless, the number of cell layers was lower than in natural skin, and electron microscopical analysis revealed that the overall organization of these layers was poor compared with natural epidermis, including the formation of junctional complexes, basement membrane, and keratinization. The lack of epithelial-mesenchymal interactions that occur during skin histogenesis may account for such an incomplete maturation of epidermal constructs.     

Primary cultures of human livers and their albumin-producing capacity

Primary cultures of surgically obtained noncancerous portions of human liver tissues were made. Liver tissues were poorly dissociated with collagenase, but well dissociated with dispase. The yield and viability of cells were improved somewhat when dissociated with collagenase followed by dispase. The mean cell yield was 1.1 X 10(6) cells/g liver. The epithelial-like morphology of the dissociated liver cells was maintained for about one week, but thereafter degenerative alteration of cells was observed. In liver explant culture, an active outgrowth of cells was observed for more than one month. Albumin production in culture fluids from dissociated livers was detectable for about 2 weeks, but later became undetectable, while that from explant culture was detectable for at least one month. These data demonstrate that adult human hepatocytes can be isolated from noncancerous portions of livers with relatively high yield, and that albumin production of the dissociated cells is detectable for several days.     

A method for transforming human liver epithelial cells by transfection using a plasmid containing SV40 early region gene

Summary A method is described for establishing continuous cell lines of liver epithelial cells by transformation of cultured hepatocytes isolated by collagenase/dispase of adult human liver tissue. Between 2 to 5×10⁵ hepatocytes are inoculated into a 60 mm culture dish. The cells are incubated in a serum-free medium. Once the cells begin to divide, they are transformed by transfection with a plasmid containing SV40 early region genes.