An acellular human amnionic membrane model for in vitro culture of type II pneumocytes: the role of the basement membrane in cell morphology and function

Clinical Outcomes and Indications of In-Office Sutureless Dried Gamma Ray-Sterilized Human Amniotic Membrane Transplantation With Bandage Contact Lenses in Various Ocular Surface Disorders

PURPOSE

The purpose of this study was to investigate the efficacy and indications of using dried gamma ray-sterilized amniotic membrane (AM) transplantation with bandage contact lenses for the in-office sutureless treatment of various ocular surface diseases.

METHODS

This study retrospectively included 56 eyes of 52 patients with various ocular surface diseases treated with sutureless dehydrated AM-assisted therapeutic contact lenses. The patients were followed up and assessed 1, 2, and 4 weeks after the treatment, and the therapeutic contact lenses were removed after 1 or 2 weeks. The size of the corneal lesion and the degree of pain experienced by each patient before and after the procedure were measured. Corneal re-epithelization rate and clinical aspects related to re-epithelialization were evaluated.

RESULTS

Complete corneal re-epithelialization of the epithelial defect was observed in 40 eyes. The complete re-epithelialization rates of persistent epithelial defect caused by infectious ulcers, neurotrophic ulcers, ulcers due to burn, toxic keratopathy, previous penetrating keratoplasty or other corneal surgeries, and severe dry eye disease were 69.2%, 80.0%, 77.8%, 100%, 75.0%, and 100%, respectively. Five patients with uncontrolled infectious ulcers, neurotrophic ulcer, bullous keratopathy, and ulcers associated with rheumatic disease did not show significant improvement. Patient pain scores significantly decreased after the procedure.

CONCLUSIONS

Dehydrated AM transplantation with a bandage contact lens is minimally invasive and easily applicable through in-office procedures. This approach is expected to be more actively applied in various ocular surface disorders, not only in severe corneal diseases but also in relatively mild corneal surface diseases.

Biomimetic Human Lung Alveolar Interstitium Chip with Extended Longevity

Determining the mechanistic causes of lung diseases, developing new treatments thereof, and assessing toxicity whether from chemical exposures or engineered nanomaterials would benefit significantly from a preclinical human lung alveolar interstitium model of physiological relevance. The existing preclinical models have limitations because they fail to replicate the key anatomical and physiological characteristics of human alveoli. Thus, a human lung alveolar interstitium chip was developed to imitate key alveolar microenvironmental factors including an electrospun nanofibrous membrane as the analogue of the basement membrane for co-culture of epithelial cells with fibroblasts embedded in 3D collagenous gels, physiologically relevant interstitial matrix stiffness, interstitial fluid flow, and 3D breathing-like mechanical stretch. The biomimetic chip substantially improved the epithelial barrier function compared to transwell models. Moreover, the chip having a gel made of a collagen I-fibrin blend as the interstitial matrix sustained the interstitium integrity and further enhanced the epithelial barrier, resulting in a longevity that extended beyond eight weeks. The assessment of multiwalled carbon nanotube toxicity on the chip was in line with the animal study.

Nanoparticle targeting of mechanically modulated glycocalyx

The mechanical properties and forces in the extracellular environment surrounding alveolar epithelial cells have the potential to modulate their behavior. Particularly, breathing applies 3-dimensional cyclic stretches to the cells, while the stiffness of the interstitium changes in disease states, such as fibrosis and cancer. A platform was developed that effectively imitates the active forces in the alveolus, while allowing one to control the interstitium matrix stiffnesses to mimic fibrotic lung tumor microenvironments. Alveolar epithelial cancer cells were cultured on these platforms and changes in the glycocalyx expression were evaluated. A complex combination of stiffness and dynamic forces altered heparan sulfate and chondroitin sulfate proteoglycan expressions. Consequently, we designed liposomal nanoparticles (LNPs) modified with peptides that can target heparan sulphate and chondroitin sulfates of cell surface glycocalyx. Cellular uptake of these modified nanoparticles increased in stiffer conditions depending on the stretch state. Namely, chondroitin sulfate A targeting improved uptake efficiency in cells experiencing dynamic stretches, while cells seeded on static stiff interstitium preferentially took up heparan sulfate targeting LNPs. These results demonstrate the critical role that mechanical stiffness and stretching play in the alveolus and the importance of including these properties in nanotherapeutic design for cancer treatment.

Alveolar wars: The rise of in vitro models to understand human lung alveolar maintenance, regeneration, and disease

Diseases such as idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, and bronchopulmonary dysplasia injure the gas-exchanging alveoli of the human lung. Animal studies have indicated that dysregulation of alveolar cells, including alveolar type II stem/progenitor cells, is implicated in disease pathogenesis. Due to mouse-human differences, there has been a desperate need to develop human-relevant lung models that can more closely recapitulate the human lung during homeostasis, injury repair, and disease. Here we discuss how current single-cell RNA sequencing studies have increased knowledge of the cellular and molecular composition of human lung alveoli, including the identification of molecular heterogeneity, cellular diversity, and previously unknown cell types, some of which arise specifically during disease. For functional analysis of alveolar cells, in vitro human alveolar organoids established from human pluripotent stem cells, embryonic progenitors, and adult tissue from both healthy and diseased lungs have modeled aspects of the cellular and molecular features of alveolar epithelium. Drawbacks of such systems are highlighted, along with possible solutions. Organoid-on-a-chip and ex vivo systems including precision-cut lung slices can complement organoid studies by providing further cellular and structural complexity of lung tissues, and have been shown to be invaluable models of human lung disease, while the production of acellular and synthetic scaffolds hold promise in lung transplant efforts. Further improvements to such systems will increase understanding of the underlying biology of human alveolar stem/progenitor cells, and could lead to future therapeutic or pharmacological intervention in patients suffering from end-stage lung diseases.

Acellular human lung scaffolds to model lung disease and tissue regeneration

Recent advances in whole lung bioengineering have opened new doors for studying lung repair and regeneration ex vivo using acellular human derived lung tissue scaffolds. Methods to decellularise whole human lungs, lobes or resected segments from normal and diseased human lungs have been developed using both perfusion and immersion based techniques. Immersion based techniques allow laboratories without access to intact lobes the ability to generate acellular human lung scaffolds. Acellular human lung scaffolds can be further processed into small segments, thin slices or extracellular matrix extracts, to study cell behaviour such as viability, proliferation, migration and differentiation. Recent studies have offered important proof of concept of generating sufficient primary endothelial and lung epithelial cells to recellularise whole lobes that can be maintained for several days ex vivo in a bioreactor to study regeneration. In parallel, acellular human lung scaffolds have been increasingly used for studying cell–extracellular environment interactions. These studies have helped provide new insights into the role of the matrix and the extracellular environment in chronic human lung diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. Acellular human lung scaffolds are a versatile new tool for studying human lung repair and regeneration ex vivo.

Acellular human lung scaffolds can be used as diverse tools to study lung disease and tissue regeneration ex vivohttp://ow.ly/ZS0l30k9MEH

Application of Euclidean distance mapping for assessment of basement membrane thickness distribution in asthma

Abnormal thickening of the airway basement membrane is one of the hallmarks of airway remodeling in asthma. The present protocols for measuring the basement membrane involve the use of stained tissue sections and measurements of the basement membrane thickness at certain intervals, followed by the calculation of the geometric mean thickness for each airway. This report describes an automated, unbiased approach which uses color segmentation to identify structures of interest on stained sections and Euclidean distance mapping to measure the thickness distribution of airway structures. This method was applied to study the thickness distribution of the basement membrane and airway epithelium in lungs donated for research from seven nonasthmatic and eight asthmatic age- and sex-matched donors. A total of 60 airways were assessed. We report that the thickness and thickness distribution of the basement membrane and airway epithelium are increased in large and small airways of asthmatics compared with nonasthmatics. Using this method we were able to demonstrate the heterogeneity in the thickness of the basement membrane and airway epithelium within individual airways of asthmatic subjects. This new computational method enables comprehensive and objective quantification of airway structures, which can be used to quantify heterogeneity of airway remodeling in obstructive lung diseases such as asthma and chronic obstructive pulmonary disease.NEW & NOTEWORTHY The described application of Euclidean distance mapping provides an unbiased approach to study the extent and thickness distribution of changes in tissue structures. This approach will enable researchers to use computer-aided analysis of structural changes within lung tissue to understand the heterogeneity of airway remodeling in lung diseases.

Transplantation of human adipose stem cell-derived hepatocyte-like cells with restricted localization to liver using acellular amniotic membrane

Introduction

Adult stem cell-derived hepatocytes transplantation holds considerable promise for future clinical individualized therapy of liver failure or dysfunction. However, the low engraftment of the available hepatocytes in the liver disease microenvironment has been a major obstacle.

Methods

Acellular human amniotic membrane was developed as a three-dimensional scaffold and combined with hepatocyte-like cells derived from human adipose stem cells to engineer a hepatic tissue graft that would allow hepatocyte engraftment in the liver effectively.

Results

The hepatic tissue grafts maintained hepatocyte-specific gene expression and functionality in vitro. When transplanted into the surgical incision in livers for engraftment, the engineered hepatic grafts significantly decreased the degree of liver injury caused by a carbon tetrachloride treatment and generated cords that were similar to the ductal plates in the liver between the acellular human amniotic membrane and the liver of receipts at day 3 post-transplantation. The hepatic tissue grafts maintained the expression of human hepatocyte-specific markers albumin, hepatocyte nuclear factor 4α, and cytochrome P450 2B6 in the liver of receipts, and acquired human-specific drug metabolism ability at eight weeks post-transplantation.

Conclusions

The acellular human amniotic membrane has the ability to maintain the functional phenotype of the hepatocyte-like cells derived from human adipose stem cells. Functional acellular human amniotic membrane-hepatocytes grafts integrated with the liver decreases the acute liver injury of mice. These engineered tissue constructs may support stem cell-based individualized therapy for liver disease and for bioartificial liver establishment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0208-9) contains supplementary material, which is available to authorized users.

Production and utilization of acellular lung scaffolds in tissue engineering

Pulmonary disease is a worldwide public health problem that reduces the quality of life and increases the need for hospital admissions as well as the risk for premature death for those affected. For many patients, lung transplantation is the only chance for survival. Unfortunately, there is a significant shortage of lungs for transplantation and since the lung is the most likely organ to be damaged during procurement many lungs deemed unacceptable for transplantation are simply discarded. Rather than discarding these lungs they can be used to produce three-dimensional acellular (AC) natural lung scaffolds for the generation of engineered lung tissue. AC scaffolds are lungs whose original cells have been destroyed by exposure to detergents and physical methods of removing cells and cell debris. This creates a lung scaffold from the skeleton of the lungs themselves. The scaffolds are then used to support adult, stem or progenitor cells which can be grown into functional lung tissue. Recent studies show that engineered lung tissues are capable of surviving after in vivo transplantation and support limited gas exchange. In the future engineered lung tissue has the potential to be used in clinical applications to replace lung functions lost following injury or disease. This manuscript discusses recent advances in development and use of AC scaffolds to support engineering of lung tissues.

The impact of compositional topography of amniotic membrane scaffold on tissue morphogenesis of salivary gland

Amniotic membrane (AM) has been widely used in the reconstruction of oral epithelial defects. However, whether it is also effective in facilitating tissue formation of salivary gland, an appendix of oral epithelia, has never been explored. To investigate the effects and the underlying mechanism of AM on salivary gland morphogenesis, murine fetal submandibular gland (SMG) explants were cultured on different preparations of AM scaffolds. It was found that, on AM stromal scaffold, SMG demonstrated well-developed branching morphogenesis. Nonetheless, on AM epithelial scaffold, SMG epithelial cell converted to a spindle-shape, lost intercellular connection, changed cytoskeletal organization, and exhibited scattering behaviors. Meanwhile, the integrity of SMG basement membrane was dismantled as well. However, when acellular AM epithelial scaffold was used, cultured SMG demonstrated organized morphology, indicating that AM epithelial component provided specific surface features for SMG morphogenesis. To further investigate AM scaffold morphogenetic effect, it was found hepatocyte growth factor (HGF), an epithelial scattering factor, was expressed abundantly in cultivated AM epithelia. After blocking HGF function of AM, cultured SMG regained branching activity, reorganized cell adhesion and subcellular organization, and reproduced basement membranes. Therefore, AM-derived bioactive factor profoundly influences cell behaviors and structure formation of SMG. Together, this study showed that compositional topography of AM scaffold is important in affecting SMG morphogenesis. By understanding the effects of AM scaffold on SMG morphogenesis, it provides important information for rationally designing and fabricating AM scaffold for salivary gland regeneration.

Characterization of pulmonary cell growth parameters in a continuous perfusion microfluidic environment

In vitro models of the alveolo-pulmonary barrier consist of microvascular endothelial cells and alveolar epithelial cells cultured on opposing sides of synthetic porous membranes. However, these simple models do not reflect the physiological microenvironment of pulmonary cells, wherein cells are exposed to a complex milieu of mechanical and soluble stimuli. In this report, we studied alveolar epithelial (A549) and microvascular endothelial (HMEC-1) cells within varying microfluidic environments as a first step towards building a microfluidic analog of the gas-exchange interface. We fabricated polydimethylsiloxane (PDMS) microdevices for parallel studies of cell growth under multiple flow rates. Cells adhered and proliferated in the microculture chambers for shear stresses up to approximately 2 x 10(-3) dynes/cm(2), corresponding to media turnover rates of approximately 53 seconds. Proliferation of these cells into confluent monolayers and expression of cell-specific markers (SP-A and CD-31) demonstrated successful pulmonary cell culture in microscale devices, a first for alveolar epithelial cells. These results represent the initial steps towards the development of microfluidic analogs of the alveolo-pulmonary barrier and tissue engineering of the lung.

Immunosuppressive properties of human amniotic membrane for mixed lymphocyte reaction

The combination of allograft limbal transplantation (ALT) and amniotic membrane transplantation (AMT) has been applied in the treatment of severe ocular surface diseases. The beneficial effect of this combination has been thought to result from possible immunosuppressive ability of amniotic membrane (AM). However, the mechanisms of any such ability remain unknown. In this study, we investigated whether human AM has the ability to suppress allo-reactive T cell responses in vitro. For mixed lymphocyte reaction (MLR), lymphocytes isolated from lymph nodes of C57BL/6 mice (Mls1b, Vbeta6+) were cultured with irradiated splenocytes from DBA/2 mice (Mls1a, Vbeta6-) with or without human AM. For carboxyfluorescein diacetate succinimidyl ester (CFSE) experiments, responder lymph node cells were labelled with a stable intracellular fluorescent dye and cultured with irradiated stimulator cells. The ratio of responder Vbeta6+ T cells was then determined by FACS analysis, and the division profiles of responder Vbeta6+ T cells were analysed by CFSE content. Furthermore, Th1 and Th2 cytokine synthesis by allo-reactive T cells in MLR culture supernatants was determined by enzyme-linked immunosorbent assay (ELISA). Addition of AM to the MLR culture resulted in the significant inhibition of thymidine incorporation compared with control culture lacking AM. The population of responder CD4+Vbeta6+ T cells was significantly reduced in the AM-treated culture in comparison to control. CFSE analysis revealed less division and lower proliferation of responder CD4+Vbeta6+ T cells in cultures with AM than without. In addition, allo-rective T cell synthesis of both Th1 (IL-2 and IFNgamma) and Th2 (IL-6 and IL-10) type cytokine was significantly decreased in the presence of AM. These results indicate that human AM has the ability to suppress allo-reactive T cells in vitro. This inhibitory effect likely contributes to the success of the ALT-AMT combination.

Laminin alpha 5 is required for lobar septation and visceral pleural basement membrane formation in the developing mouse lung

Laminin alpha/beta/gamma heterotrimers are the major noncollagenous components of all basement membranes. To date, five alpha, three beta, and three gamma chains have been identified. Laminin alpha 5 is expressed early in lung development and colocalizes with laminin alpha1. While laminin alpha1 expression in the lung is restricted to the embryonic period, laminin alpha 5 expression persists throughout embryogenesis and adulthood. Targeted mutation of the mouse laminin alpha 5 gene Lama5 causes embryonic lethality at E14-E17 associated with exencephaly, syndactyly, placentopathy, and kidney defects, all attributable to abnormal basement membranes. In this investigation, lung development in Lama5(-/-) mice up to E16.5 was examined. We observed normal lung branching morphogenesis and vasculogenesis, but incomplete lobar septation and absence of the visceral pleura basement membrane. Preservation of branching morphogenesis was associated with ectopic deposition of laminin alpha 4 in the airway basement membrane. Perturbation of pleural basement membrane formation and right lung septation correlated with absence of laminin alpha 5, which was found to be the only laminin alpha chain present in the normal visceral pleura basement membrane. Our finding of normal lung branching morphogenesis with abnormal lobar septation demonstrates that these processes are not obligatorily linked.

Laminin alpha-chain expression and basement membrane formation by MLE-15 respiratory epithelial cells

Basement membranes have a critical role in alveolar structure and function. Alveolar type II cells make basement membrane constituents, including laminin, but relatively little is known about the production of basement membrane proteins by murine alveolar type II cells and a convenient system is not available to study basement membrane production by murine alveolar type II cells. To facilitate study of basement membrane production, with particular focus on laminin chains, we examined transformed murine distal respiratory epithelial cells (MLE-15), which have many structural and biochemical features of alveolar type II cells. We found that MLE-15 cells produce laminin-alpha5, a trace amount of laminin-alpha3, laminins-beta1 and -gamma1, type IV collagen, and perlecan. Transforming growth factor-beta1 significantly induces expression of laminin-alpha1. When grown on a fibroblast-embedded collagen gel, MLE-15 cells assemble a basement membrane-like layer containing laminin-alpha5. These findings indicate that MLE-15 cells will be useful in modeling basement membrane production and assembly by alveolar type II cells.

Maintenance of the differentiated type II cell characteristics by culture on an acellular human amnion membrane

We have developed a Culture system for guinea pig alveolar type II cells using an epithelium-denuded human amnion membrane as a substratum. The differentiated morphology was maintained for 3 wk by both air-interface feeding and immersion feeding when type II cells were cultured on the basement membrane side of the amnion with fibroblasts on the opposite side (coculture). Functionally high levels of surfactant protein B (SP-B) and C (SP-C) messenger ribonucleic acids (mRNAs) were expressed even after the 3-wk cultivation and surfactant protein A mRNA was detected on day 10 of the culture. The differentiation was also maintained when fibroblasts were cultured on lower chambers of the culture plates (separate culture). In contrast, culture of type II cells without fibroblasts (monoculture) could not preserve the mature morphology. When the monoculture was supplemented with keratinocyte growth factor or hepatocyte growth factor, a monolayer of rather cuboidal type II cells with apical microvilli was maintained. However, the percent area of lamellar bodies in these cells was significantly less than that in freshly isolated type II cells, and mRNA expressions of SP-B and SP-C were also considerably suppressed. These findings suggest that other growth factors or combinations of these factors are necessary for the maintenance of the differentiated phenotype. As substratum, a permeable collagen membrane or a thin gel layer of Engelbreth-Holm-Swarm mouse sarcoma extracts did not preserve the mature characteristics. This culture system using an acellular human amnion membrane may provide novel models for research in type II cells.

Amniotic membrane transplantation in infectious corneal ulcer

PURPOSE

To evaluate the efficacy of amniotic membrane transplantation in the management of treated infectious corneal ulcer in which inflammatory reactions were responsible for corneal damage.

METHOD

A prospective study of 21 consecutive eyes (21 patients) was performed. Sufficient antibacterial, antifungal, or antiviral agents were applied to eradicate causative organisms before permanent or temporary amniotic membrane transplantation, or a combination of the two in few patients. The amniotic membrane was soaked in antiinfective agents before transplantation in all cases.

RESULTS

After amniotic membrane transplantation, follow-up times ranged from 4 to 28 months (mean, 18 months). Clinical indications included Staphylococcus species (four cases), Pseudomonas species (five cases), Acanthamoeba species (three cases), fungus (two cases), and herpesvirus (seven cases). The corneal surface was healed successfully and recurrences of microbial infection were not noted in any case. Visual acuity was improved in cases that were nonscarring or after additional penetrating keratoplasty.

CONCLUSION

Amniotic membrane transplantation seems to be a useful adjunctive surgical procedure for the management of infectious corneal ulcer by promoting wound healing and reducing inflammation.

Maintenance of the differentiated type II cell characteristics by culture on an acellular human amnion membrane

We have developed a Culture system for guinea pig alveolar type II cells using an epithelium-denuded human amnion membrane as a substratum. The differentiated morphology was maintained for 3 wk by both air-interface feeding and immersion feeding when type II cells were cultured on the basement membrane side of the amnion with fibroblasts on the opposite side (coculture). Functionally high levels of surfactant protein B (SP-B) and C (SP-C) messenger ribonucleic acids (mRNAs) were expressed even after the 3-wk cultivation and surfactant protein A mRNA was detected on day 10 of the culture. The differentiation was also maintained when fibroblasts were cultured on lower chambers of the culture plates (separate culture). In contrast, culture of type II cells without fibroblasts (monoculture) could not preserve the mature morphology. When the monoculture was supplemented with keratinocyte growth factor or hepatocyte growth factor, a monolayer of rather cuboidal type II cells with apical microvilli was maintained. However, the percent area of lamellar bodies in these cells was significantly less than that in freshly isolated type II cells, and mRNA expressions of SP-B and SP-C were also considerably suppressed. These findings suggest that other growth factors or combinations of these factors are necessary for the maintenance of the differentiated phenotype. As substratum, a permeable collagen membrane or a thin gel layer of Engelbreth-Holm-Swarm mouse sarcoma extracts did not preserve the mature characteristics. This culture system using an acellular human amnion membrane may provide novel models for research in type II cells.

Reconstruction of ocular surface with heterologous limbal epithelium and amniotic membrane in a rabbit model

PURPOSE

To report in vivo reconstruction of the ocular surface using amniotic membrane and heterologous transplants of epithelial limbal cells in rabbits with chemical burns.

METHODS

After severe damage to the ocular surface with n-heptanol and keratectomy, 15 rabbits developed total limbal deficiency with conjunctival epithelialization, vascularization, and chronic inflammation. One month later, a complete keratectomy was performed in all eyes: 12 received additional transplantation of human amniotic membrane and heterologous limbal epithelial cells in a double amniotic membrane layer, 2 received amniotic membrane only, and 1 control eye received no procedure.

RESULTS

After 1 month of follow-up, corneas in eight of the operated eyes presented minimal vascularization, without signs of rejection. Corneal surface reconstruction was demonstrated with the growth of new corneal-like epithelial phenotype and integration of amniotic membrane to the basal corneal surface. A superficial amniotic membrane (with the amnion side up as a dressing) peeled off after 7 to 10 days. The epithelialization with heterologous limbal epithelial cells was evident underneath. The other four operated eyes were followed for 6 months; the ocular surface was also stable with a corneal-like epithelial phenotype.

CONCLUSION

Simultaneous transplantation of amniotic membrane and heterologous limbal epithelial cells in severe ocular surface disorders could restore ocular surface and may be useful in patients with severe bilateral limbal epithelial loss, giving new perspectives for the treatment of severe ocular surface disorders.

Triggering the induction of myofibroblast and fibrogenesis by airway epithelial shedding

Myofibroblasts have been thought to participate in subepithelial fibrosis in asthma, but the mechanism of myofibroblast induction has not been fully understood. In this study we investigated injury-related myofibroblast induction in a coculture system of guinea-pig epithelial cells and fibroblasts cocultured in a human amnion chamber. After pseudostratified epithelial cells were mechanically scraped, migrated flat epithelial cells differentiated into cuboidal appearances on Day 4 and then returned to their original shapes on Day 8. During the course of the epithelial redifferentiation, it was found by Northern blot analysis, immunohistochemistry for alpha-smooth muscle actin, and electron microscopic observation that the myofibroblasts were transiently induced on Day 4. The myofibroblast induction was inhibited by the blocking of transforming growth factor (TGF)-beta1 and thrombospondin (TSP)-1, indicating that the activation of TGF-beta1 by TSP-1 would induce myofibroblasts. This finding was also supported by a transient upregulation of TSP immunoreactivity and TSP-1 messenger RNA (mRNA) in fibroblasts. Interestingly, epithelial injury reduced TGF-beta1 immunoreactivity in the amnion membrane but did not affect TGF-beta1 mRNA in epithelial cells and fibroblasts, indicating that TGF-beta1 supplied from the extracellular matrix can participate in myofibroblast induction. Concurrently with myofibroblast induction, procollagen type I and III mRNAs were upregulated in fibroblasts, and obvious collagen deposition was observed ultrastructurally around the myofibroblasts compared with the fibroblasts. These results indicate that induced myofibroblasts can be functionally more active in producing collagen than are resting fibroblasts. The present study suggests that epithelial injury stimulates TGF-beta1 release from the extracellular matrix and its activation via TSP-1 production, causing collagen synthesis through myofibroblast induction.

Amniotic membrane patching promotes healing and inhibits proteinase activity on wound healing following acute corneal alkali burn

Amniotic membrane (AM) contains basement membrane components and various proteinase inhibitors. Furthermore, when used as a graft, the basement membrane of AM could block inflammatory insults to a damaged corneal surface. Thus, we evaluated whether amniotic membrane patching could promote the healing process by inhibiting proteolytic damage. Alkali wounds were inflicted on the central corneas of rabbits by applying a round filter paper, 6.0 mm in diameter, soaked in 1 N NaOH for 30 sec. Amniotic membrane patching was performed over the perilimbal sclera immediately after wounding. A total of 115 rabbits were divided into four groups: (1) immediately covered by AM with the amnion cell side down up to the perilimbal sclera (n =26); (2) covered by AM with the stromal side down up to the perilimbal sclera (n =19); (3) anchored to the fornix (n =29); and (4) uncovered as a control (n =41). AM was removed 3 days postoperatively. During follow-ups, epithelial defects, corneal thickness and its opacity of each eye were measured. Some corneas were removed for histopathologic studies and for proteinase activity assay and zymography. The epithelial healing was faster and the corneal thickness was thicker in all three AM-covered groups than in the control (P<0.05). No significant difference was found between covered and anchored groups (P>0.05). Corneal opacity was least in the amnion cell side down group. Infiltration of polymorphonuclear leukocytes (PMNs) was much less in AM-covered groups than in the control. Pathological results were associated with zymographic findings, which revealed much higher proteinase activity in uncovered group than AM-covered groups. Immediate intervention for acute alkali burns with AM as a temporary patch promotes wound healing by inhibiting proteinase activity and PMNs infiltration.

Amniotic membrane as a substrate for cultivating limbal corneal epithelial cells for autologous transplantation in rabbits

PURPOSE

To examine the viability of using human amniotic membrane as substrate for culturing corneal epithelial cells and transplanting them onto severely injured rabbit eyes.

METHODS

An ocular-surface injury was created in the right eye of eight rabbits by a lamellar keratectomy extending 5 mm outside the limbus. Next, from the limbal region of the uninjured left eyes of five of these animals, a small biopsy of corneal epithelial cells was taken and cultured on acellular human amniotic membrane. One month later, the invading conjunctiva that covered the corneal surface of all eight injured eyes was surgically removed. Five of the eyes then received grafts of amniotic membrane containing autologous cultured epithelial cells, whereas the other three received grafts of acellular amniotic membrane alone.

RESULTS

A confluent primary culture of limbal corneal epithelial cells was established on acellular human amniotic membrane after 14 days. Cells were partially stratified and fairly well attached to the underlying amniotic membrane, although a fully formed basement membrane was not evident. The three rabbits that received amniotic membrane transplantation alone all had total epithelial defects on the graft in the early postoperative period. Eyes that were grafted with amniotic membrane that contained cultivated epithelial cells, however, were all successfully epithelialized up to 5 days after surgery.

CONCLUSION

Autologous transplantation of cultivated corneal epithelium is feasible by using acellular amniotic membrane as a carrier.

Amniotic membrane transplantation for ocular surface reconstruction in patients with chemical and thermal burns

OBJECTIVE

The purpose of the study is to examine the usefulness of preserved human amniotic membrane transplantation in patients with chemical and thermal burns.

DESIGN

The study design was a nonrandomized clinical trial.

PARTICIPANTS

Seven eyes of six patients with severe chemical (n = 5) and thermal (n = 2) burns were studied.

INTERVENTION

Eyes were treated with excision of cicatricial tissues followed by a placement of amniotic membrane on the sclera. Transplantation of limbal grafts from an opposite eye (n = 4) or from donor eyes preserved at -80 degrees C (n = 2) was performed simultaneously.

MAIN OUTCOME MEASURES

Reconstruction of ocular surface epithelia and visual acuity were measured.

RESULTS

With the mean observation period of 53.3 weeks, central corneal epithelium was reconstructed successfully in all eyes. Neither amniotic membrane nor limbal grafts were rejected. A persistent epithelial defect developed in one eye, which was treated successfully by tarsorrhaphy. After surgery, the corneal epithelium showed normal arrangements on specular microscopy, and its barrier function recovered to seminormal. Corrected visual acuity markedly improved in each eye. Regenerated conjunctiva on the amniotic membrane was stable and uninflammed with minimum-to-mild scarring. Slight recurrence of conjunctivalization was noted in three eyes. However, because these eyes were stable and central cornea was clear, no further surgery was needed.

CONCLUSIONS

Amniotic membrane transplantation promotes normal conjunctival epithelialization while suppressing fibrosis formation. The procedure, especially when performed with limbal autograft transplantation, appears to be effective for the treatment of chemical or thermal burns of the ocular surface.

Mononuclear phagocytes egress from an in vitro model of the vascular wall by migrating across endothelium in the basal to apical direction: role of intercellular adhesion molecule 1 and the CD11/CD18 integrins

Little is known about how mononuclear phagocytes (MP) are cleared from sites of inflammation as inflammatory lesions resolve. In this study, the possibility that MP could be cleared from tissues by migrating across endothelium in the basal to apical direction was investigated. In an in vitro model of a blood vessel wall consisting of human umbilical vein endothelial cells (HUVEC) grown on amniotic tissue, a majority of MP that initially transmigrated into the amnion later exited by migrating back across the endothelium in the basal to apical direction. MP that egressed from these cultures adhered to the apical surface of the endothelium or were found nonadherent in the medium above the endothelium. Egression of MP continued throughout the 4-d period examined, displaying higher than first order kinetics and a t(1/2) of approximately 24 h. These kinetics were decreased by increasing the volume of medium bathing the cultures, suggesting that a soluble factor(s) regulates the rate of egression. In contrast, the kinetics were accelerated by pretreating the endothelium with IL-1. The initial phase of this increased rate of egression was inhibited by antibodies to inter- cellular adhesion molecule 1 (ICAM-1) or CD18 by 100 and 71%, respectively. Immunostaining revealed that ICAM-1 was present on the apical and basal surfaces of umbilical vein endothelium in vitro and in situ. These data demonstrate that MP can traverse endothelium in the basal to apical direction, and lend insight into the mechanisms by which this process occurs.

Type IV collagen production by rat pulmonary alveolar epithelial cells

Maintenance and repair of the pulmonary alveolar basement membrane are critical processes for preserving normal alveolar structure and function. To elucidate the mechanisms that control type IV collagen production by pulmonary alveolar epithelial cells, type II pneumocytes from rat lungs were isolated and maintained in tissue culture as monolayers. Using Northern blot analysis and metabolic labeling, we found that the cultured epithelial cells express type IV collagen mRNA and incorporate [3H]proline into type IV collagen. To determine the effects of phenotype on control of type IV collagen synthesis, we took advantage of the well-described observation that isolated type II cells lose their distinctive type II features when cultured on plastic in serum-containing medium for 7 days and assume an appearance more like that of type I epithelial cells. We found that [3H]proline incorporation into type IV collagen increased from day 1 to day 2 and thereafter decreased gradually up to day 7. Despite this decrease in [3H]proline incorporation, type IV collagen mRNA increased over the same period. If the loss of type II cell characteristics was prevented by culturing the cells in EHS matrix, a basement membrane-like extracellular matrix, there was little increase in relative abundance of type IV collagen mRNA as compared with cells cultured on plastic. We therefore conclude that type IV collagen production by isolated pulmonary alveolar epithelial cells is controlled, at least in part, by translational or post-translational mechanisms. Furthermore, the full type II cell phenotype is not required for expression of type IV collagen genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulmonary epithelial cell proliferation in primary culture of alveolar type II cells

A small subpopulation of pulmonary epithelial cells (PE) proliferates in low-density primary culture of alveolar type II cells and forms colonies of cells that could be passaged for several generations and that in some respects maintain a differentiated phenotype of the alveolar type II cells. At this time it is not known if these cells are some form of progenitor epithelial cells or type II cells that are not fully differentiated in vitro. The proliferation of the PE cells was dependent on serum, alveolar macrophage-conditioned medium, and insulin being included in the culture medium. Under these conditions, approximately 0.5-1.0% of the seeded cells that adhered to the culture dishes were capable of forming colonies. Efficiency of colony formation increased to 5-10% in subsequent passages. PE cells maintained a high level (> 40%) of saturated phosphatidylcholine (PC) as a percentage of total PC throughout the culture period (> 28 days). However, the saturated PC content was not constant throughout the long-term culture period and the subsequent passages (41.3% at 29 days and 37.3% in the 3rd passage). These cells also contained numerous lamellar bodies and were able to bind the Maclura pomifera lectin. PE cells also expressed cytokeratin No. 19, as well as alkaline phosphatase activity, both possible markers for differentiated type II cells. However, PE cell synthesized low levels of Pg (approximately 2%), were squamous, and tended to form multiple strata, unlike the cuboidal type II cells in vivo. The cells did not exhibit immunocytochemically demonstrable surfactant-associated protein A (SP-A). Additional factors and culture requirements may be necessary for complete maturation of cultured PE cells. This was demonstrated by culturing PE cells on EHS matrix. Aggregates of cells surrounding a central lumen were formed after a few hours in culture and were maintained for 20 days. The cells contained lamellar bodies and some intercellular junctions. PE cells can be regarded as a highly selected subpopulation of pulmonary epithelial cells that concomitantly maintain proliferation and aspects of differentiated alveolar type II cells in long-term culture.

Reactivity of alveolar epithelial cells in primary culture with type I cell monoclonal antibodies

An understanding of the process of alveolar epithelial cell growth and differentiation requires the ability to trace and analyze the phenotypic transitions that the cells undergo. This analysis demands specific phenotypic probes to type II and, especially, type I pneumocytes. To this end, monoclonal antibodies have been generated to type I alveolar epithelial cells using an approach designed to enhance production of lung-specific clones from a crude lung membrane preparation. The monoclonal antibodies were screened by a combination of enzyme-linked immunosorbent assay and immunohistochemical techniques, with the determination of type I cell specificity resting primarily on immunoelectron microscopic localization. Two of these new markers of the type I pneumocyte phenotype (II F1 and VIII B2) were used to analyze primary cultures of type II cells growing on standard tissue culture plastic and on a variety of substrata reported to affect the morphology of these cells in culture. On tissue culture plastic, the antibodies fail to react with early (days 1 to 3) type II cell cultures. The cells become progressively more reactive with time in culture to a plateau of approximately 6 times background by day 8, with a maximum rate of increase between days 3 and 5. This finding is consistent with the hypothesis that type II cells in primary culture undergo at least partial differentiation into type I cells. Type II cells grown on laminin, which reportedly delays the loss of type II cell appearance, and on fibronectin, which has been reported to facilitate cell spreading and loss of type II cell features, develop the type I cell markers during cultivation in vitro with kinetics similar to those on uncoated tissue culture plastic. Cells on type I collagen and on tissue culture-treated Nuclepore filters, which have been reported to support monolayers with type I cell-like morphology, also increase their expression of the II F1 and VIII B2 epitopes around days 3 to 5. Taken together with available morphologic information, these data suggest that expression of different alveolar epithelial cell phenotypic markers by type II cells in primary culture may be independently regulated. The monoclonal antibody probes described in this report should prove useful in the continued investigation of the mechanisms and regulation of alveolar epithelial cell differentiation.

Structural and functional relationships between type II pneumocytes and components of extracellular matrices

Type II pneumocytes of the pulmonary alveolus are dynamic cells with multiple functional capabilities in vivo, including secretion of surface-active lipoproteins and cell renewal of the epithelial lining of the alveolus, involving its differentiation into another cell type (the type I pneumocyte). The factors that influence and control these processes, which are vital to the function of the alveolus, have begun to be more clearly understood in recent years, in large part because of the development of adequate in vitro systems, which permit the manipulation of relevant variables. These appear to be a complex interaction between insoluble components of extracellular matrices, principally of the basement membrane, and soluble factors that include hormones and growth factors. This review focuses particularly on those components of extracellular matrices that specifically and nonspecifically impact on type II cell function, and it attempts to bring together the diverse technical approaches used to define and examine these relationships cytochemically and functionally.

Effects of transforming growth factor-beta on collagen synthesis by fetal rat lung epithelial cells

Studies were performed to characterize the effects of acute and chronic exposure to transforming growth factor-beta (TGF-beta) on collagen biosynthesis by fetal rat lung epithelial (FRLE) cells, a cell line established from the fetal rat lung alveolar epithelial cell. Neither condition of exposure to TGF-beta stimulated cell growth, but both conditions increased total protein synthesis. Quantitative evaluation by carboxymethyl-Trisacryl chromatography revealed that FRLE cells synthesized types I, III, IV, and V collagen under all circumstances. Acute and chronic exposure to TGF-beta increased total collagen production approximately 50% and 300%, respectively, with the increases in total collagen production exceeding those of total protein synthesis. In addition, these analyses indicated that the production of types I and III molecules was stimulated to a greater extent than was the synthesis of types IV and V molecules. Both experimental conditions increased the ratio of secreted to cell-associated molecules for types I and III molecules, decreased this ratio for type IV collagen, but minimally affected the culture distribution of type V collagen. Additionally, both conditions of exposure to TGF-beta were found to increase the proportion of the homotrimeric forms of types I and V molecules relative to their heterotrimeric counterparts. Thus, these studies establish that TGF-beta selectively and type-specifically alters collagen production without affecting growth in an epithelial cell line of fetal rat lung origin.

Expression of urokinase-type plasminogen activator by rat pulmonary alveolar epithelial cells

Intra-alveolar fibrin deposition accompanies many forms of inflammatory lung injury. Appropriate clearance of this fibrin matrix is important for normal healing and remodeling. The local generation of plasmin by the action of plasminogen activators (PAs) represents a pivotal step in the fibrinolytic process. To investigate whether the alveolar epithelium plays a role in the modulation of intra-alveolar fibrinolysis, we have studied PA regulation by rat pulmonary alveolar epithelial cells. We have found large quantities of PA activity both in conditioned media and cell lysates from epithelial monolayers in culture. Casein-plasminogen zymography reveals that this PA activity migrates as a tight doublet with an apparent mol wt of 45 kD, clearly distinct from rat tissue-type PA (tPA, greater than 68 kD). Analysis of freshly isolated type II alveolar epithelial cells demonstrates readily measurable PA activity in cell lysates, as well as expression of urokinase-type PA (uPA) mRNA on Northern blot analysis. Upregulation of PA activity occurs progressively with time in culture as the alveolar epithelial cells lose type II cell characteristics and become more flattened. Stimulation of alveolar epithelial cell monolayers with lipopolysaccharide or tumor necrosis factor increases levels of secreted PA activity. The relative abundance of uPA mRNA was shown to change in parallel with PA activity during in vitro differentiation or after exposure to inflammatory mediators. Thus, alveolar epithelial cells are likely an important source of uPA in the lung, the expression of which is influenced by the state of cellular differentiation as well as the presence of inflammatory mediators.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum accelerates the loss of type II cell differentiation in vitro

The differentiated phenotype of the alveolar type II cell is rapidly altered in vitro. To evaluate factors that might influence this process, we isolated and plated rat type II cells in serum-supplemented media to promote adherence and then maintained the cells in a simple nutrient medium in the absence (S- cells) or presence (S+ cells) of serum for 5 to 7 d. The type II S- cells remained metabolically active. Despite protein synthesis that was 50% that of S+ cells, S- cells continued to synthesize a broad spectrum of proteins and to express several features of type II cell differentiation. They synthesized an apical integral membrane glycoprotein, Maclura pomifera agglutinin (MPA)-gp200, and a cytokeratin, No. 19, while S+ cells did not. When supplemented with linoleic acid, S- cells contained lamellar and multivesicular bodies, incorporated cell surface MPA into these structures, and secreted their phosphatidylcholine (PC) in response to mastoparan. Despite the relative synthesis of higher levels of total and saturated PC in S- cells supplemented with linoleic acid, phosphatidylglycerol remained diminished. A surfactant protein (SP-A) was present in S- cells, but synthesis was not detected. These studies demonstrate that serum accelerates the loss of type II cell differentiation in vitro and that the expression of type II cell markers of differentiation is not inherently linked.

Improved maintenance of adult rat alveolar type II cell differentiation in vitro: effect of serum-free, hormonally defined medium and a reconstituted basement membrane

We have developed a serum-free, hormonally defined medium for maintenance of differentiation of adult type II cells cultured on Engelbreth-Holm-Swarm (EHS) tumor basement membrane gels. This defined medium consists of 1:1 (vol/vol) mixture of Ham's F12 and Dulbecco's modified Eagle's media supplemented with insulin, dibutyryl cyclic AMP, hydrocortisone, epidermal growth factor, selenium, and albumin/linoleic acid complex. Compared to cells cultured on EHS gels in serum-supplemented medium, type II cells cultured on EHS gels in this defined medium showed increased acetate incorporation into total lipids (10-fold) and an increase in the relative percentage of acetate incorporated into phosphatidylcholine (PC) (87.8 +/- 0.4% versus 78.5 +/- 1.0% [mean +/- SE]; P less than 0.01), saturated phosphatidylcholine (SPC) (61.4 +/- 0.5% versus 55.2 +/- 0.9%; P less than 0.01), and phosphatidylglycerol (PG) (5.3 +/- 0.3% versus 0.8 +/- 0.1%; P less than 0.01) and decreased acetate incorporation into neutral lipids (9.7 +/- 0.8% versus 62.6 +/- 1.9%; P less than 0.01). No response to this defined medium was seen when type II cells were cultured on tissue culture plastic. Type II cells cultured on EHS gels in serum-supplemented medium for 4 d had numerous neutral lipid droplets in their cytoplasm. In contrast, neutral lipid droplets were not commonly observed within the cytoplasm of the cells cultured in serum-free, hormonally defined medium on EHS gels. This morphologic finding was consistent with the result that cells cultured in serum-supplemented medium significantly increased the relative percentage of acetate incorporated into neutral lipids. These data indicate that adult type II cells cultured on a reconstituted basement membrane (EHS gels) can be maintained in synthetic culture medium without serum. These culture conditions permit the expression of a pattern of differentiated phospholipid biosynthesis and cell morphology more similar to normal type II cell differentiation.

Effects of substrata on the polarization of bovine endometrial epithelial cells in vitro

Epithelial-cell function requires cellular polarity in which apical membrane surfaces have unique characteristics and cellular organelles are stratified. Physiological investigations of endometrial epithelial cells would be enhanced greatly by the ability of a method to polarize cells in culture. This study investigates the effects of different substrata on polarization of cultured bovine endometrial epithelial cells. Fetal bovine endometrial epithelial-cell lines were developed from explant outgrowth. Epithelial monolayers were subcultured onto amniotic membranes, Millicell-HA membranes, or Millicell-CM membranes coated with rat-tail collagen, Matrigel, laminin, Vitrogen, or fibronectin. Cultures on these substrata were maintained at the air/liquid interface. Cells grown on either collagen-coated or uncoated Millicell membranes also were maintained submerged in medium. Excellent polarized morphology was attained in cultures grown at the air/liquid interface on amniotic membranes and rat-tail collagen-coated membranes. Lectin-binding patterns to apical membranes of polarized epithelial cell cultures paralleled patterns of binding to bovine endometrial surfaces in vivo. Cultures on rat-tail collagen were maintained for several weeks. These methods provide a valuable system for studying the endometrium in vitro.

Modulation of alkaline phosphatase activity in alveolar type II like cells

Alveolar type II like cells (ALT II) represent a small subpopulation of alveolar type II cells, which is able to proliferate, can be passaged and possess many characteristics of differentiated adult type II cells. A correlation was found between the growth and development of ALT II cells in culture and their alkaline phosphatase activity. Unlike alveolar type II cells, which lose the activity in culture, ALT II cells regain the activity and maintain it for a long culture period. Quantitative histochemical analysis of the stained cells indicate that 80% of the cells at days 15-20 in culture are alkaline phosphatase positive. Inhibition studies indicate that alkaline phosphatase from ALT II cells and freshly isolated type II cells were similar. The inhibition of ALT II alkaline phosphatase by L-levamisole and its heat stability are similar to that of the bone enzyme and differ from the intestinal enzyme. Alkaline phosphatase expression is considered part of the differentiated phenotype of these cells. Therefore, the presence of this enzyme in ALT II cells adds support to the notion that these cells maintain many aspects of mature alveolar type II cells.

Modulation of bioelectric properties across alveolar type II cells by substratum

Rat alveolar type II cells were cultured on collagen-coated filters (CCF) and human amnionic basement membrane (ABM) to determine the effect of culture substratum on the development of monolayer bioelectric properties. Monolayers cultured on both substrata rapidly developed bioelectric properties with similar time courses, monolayer capacitance values (approximately 1 muF/cm2), current-voltage relationships, and responses to stimulants and inhibitors of active ion transport. Increasing seeding densities tended to increase monolayer bioelectric properties regardless of culture substratum. Monolayers cultured on ABM had higher resistance values (491 vs. 291 omega.cm2) and lower short-circuit currents (2.85 vs. 4.51 muA/cm2) than monolayers with similar cell densities cultured on CCF. These differences in monolayer bioelectric properties were not due to differences in substratum resistance or capacitance effects. The relationships between monolayer bioelectric properties were also affected by the culture substratum. In additional experiments, cells cultured on contracted gels formed monolayers with high short-circuit currents (9.25 muA/cm2). Cell morphology varied depending on the culture substratum, with cells cultured on contracted gels appearing the most cuboidal, whereas the flattest and most attenuated cells were those cultured on ABM. On the basis of these observations, we conclude that culture substratum significantly affects the development of bioelectric properties across alveolar type II cell monolayers. In vivo the bioelectric properties across the alveolar epithelium may also vary with changes in cellular substratum or cell density (e.g., after acute lung injury) and possibly with cell morphology (e.g., alveolar type I vs. alveolar type II cells).

Type I cell-like morphology in tight alveolar epithelial monolayers

The pulmonary alveolar epithelium separates air spaces from a fluid-filled interstitium and might be expected to exhibit high resistance to fluid and solute movement. Previous studies of alveolar epithelial barrier properties have been limited due to the complex anatomy of adult mammalian lung. In this study, we characterized a model of isolated alveolar epithelium with respect to barrier transport properties and cell morphology. Alveolar epithelial cells were isolated from rat lungs and grown as monolayers on tissue culture-treated Nuclepore filters. On Days 2-6 in primary culture, monolayers were analyzed for transepithelial resistance (Rt) and processed for electron microscopy. Mean cell surface area and arithmetic mean thickness (AMT) were determined using morphometric techniques. By Day 5, alveolar epithelial cells in vitro exhibited morphologic characteristics of type I alveolar pneumocytes, with thin cytoplasmic extensions and protruding nuclei. Morphometric data demonstrated that alveolar pneumocytes in vitro develop increased surface area and decreased cytoplasmic AMT similar to young type I cells in vivo. Concurrent with the appearance of type I cell-like morphology, monolayers exhibited high Rt (greater than 1000 omega.cm2), consistent with the development of tight barrier properties. These monolayers of isolated alveolar epithelial cells may reflect the physiological and morphological properties of the alveolar epithelium in vivo.

Influence of the extracellular matrix on type 2 cell differentiation

Growth and division of type II pulmonary epithelial cells are important components of the pathway by which the alveolar surface is repaired following several forms of lung injury. These processes, which result in reepithelialization of the denuded alveolar basement membrane, involve loss of type II cell differentiation and transition to a type I epithelium. As in other cells, the extracellular matrix appears to be an important determinant of type II cell differentiation. This effect on the type II cell is exerted by both simple and complex matrices and may be modulated by active synthesis and remodeling of the matrix components by the pneumocytes themselves. In general, laminin or laminin-rich complex surfaces favor cellular differentiation; fibronectin or fibronectin-rich complex matrices accelerate loss of differentiated form and function. In both cases, matrix-initiated changes in the type II cell involve regulation of cell shape and morphology, hormone responsiveness, secretory activity, phospholipid synthesis, protein turnover, and gene expression. These influences of the extracellular matrix, along with the effects of locally acting soluble factors, likely direct the cellular transitions required for restoration of a physiologically competent alveolar surface during the repair of lung injury.

Influence of extracellular matrix and collagen components on alveolar type 2 cell morphology and function

The effects of various extracellular matrices and collagenous components on the morphology, growth, and function of cultured alveolar type 2 cells is examined. Cells grown on an endothelial matrix (EC) showed the greatest adherence, some cell division, and spreading to reach confluence sooner than cells grown on an epithelial matrix or on various types of collagen. The attenuated cells from all cultures were not true type 1 cells because, on trypsinization, they detached as sheets, reverted immediately to a cuboidal shape held together by junctional complexes, and showed an apparently normal content of lamellar bodies. The greatest synthesis of disaturated phosphatidylcholine (DSPC) was seen in cells grown on EC soon after confluence, but all cultures showed reduced but equal levels of DSPC-DNA by Day 4. This occurred whether cells were attenuated or cuboidal in shape. The results suggest that some component(s) of the endothelial matrix at the alveolar basement membrane facilitates epithelial cell growth. However, over longer culture periods the matrix preparations had little effect on type 2 cell proliferation whereas function diminished. This suggests that maintenance of these cells as normal type 2 cells or their further differentiation to the type 1 form requires some additional cell derived factor(s).

Matrix-derived soluble components influence type II pneumocytes in primary culture

Type II pulmonary epithelial cells cultured on a plastic surface fail to retain differentiated form and function. During the first 3 days in primary culture, the cells flatten and lose characteristic lamellar inclusions; they increase in size and exhibit accelerated rates of protein synthesis and thymidine incorporation. These transitions are inhibited markedly if the cells are plated on matrigel (MG), a laminin-rich surface derived from the Englebreth-Holm-Swarm (EHS) sarcoma. Soluble components released from matrigel (MGS) mimic some of the effects of the solid gel. As on a plastic surface, the cells flatten when exposed to MGS during culture. In contrast, MGS inhibits thymidine incorporation and protein synthesis; it is most effective when added early in the culture interval. Direct contact of the cells with the matrigel surface itself is always more effective than maximal MGS activity. The effects of MGS are not reproduced by purified laminin or by transforming growth factor-beta, both of which are present in matrigel. These results indicate that the effects of the solid matrigel surface on cell morphology are caused in part by direct cell-matrix contact but that additional effects, such as decreased DNA synthesis, can be mediated by activity of solubilized gel components. They further provide a model wherein changes in type II cell morphology and function, which typically occur in parallel during primary culture, can be separated experimentally.

Modulation of keratin expression in type II pneumocytes by the extracellular matrix

The expression of specific keratin intermediate filaments during differentiation of rat type II pneumocytes in primary culture on various matrices was investigated. Changes in keratin expression were assessed using a monoclonal antikeratin antibody, 24A3, known to react strongly with alveolar epithelial cells in injured lung. Type II cell differentiation was modulated by culture on extracellular matrices known to either accelerate or retard loss of differentiated morphology and metabolic function. During culture on a plastic or fibronectin-rich surface, loss of cell differentiation correlates with increased staining with 24A3 antikeratin antibody by indirect immunofluorescence and with increased abundance of a family of acidic 46,000-dalton keratin isoforms detected in two-dimensional polyacrylamide gels of type II cell cytoskeletal extracts. Loss of type II cell differentiation is retarded or prevented by culture on substrata of purified laminin or of EHS tumor-derived basement membrane (matrigel). 24A3-linked fluorescence and expression of the 46 kDa keratins are reduced in parallel, although at 7 days in culture on matrigel or laminin, keratin expression increases. The results show that changes in type II cell differentiation effected in primary culture by the extracellular matrix correlates with changes in expression of the 24A3-reactive keratins. Loss of differentiated shape and function favors expression of these cytoskeletal antigens, which may provide quantifiable markers of the type II to type I cell transition that occurs during alveolar remodeling.

Improved maintenance of adult rat alveolar type II cell differentiation in vitro: effect of hydrocortisone and cyclic AMP

We have examined the effect of hydrocortisone and cyclic AMP on the maintenance of lipid synthesis in primary cultures of adult rat alveolar type II cells. These hormones were tested in the presence of either 1% or 5% charcoal-stripped rat serum (CS-rat serum). The effect of substratum on responsiveness to these hormones was evaluated by comparing cells cultured for 4 days on tissue culture plastic, on floating type I collagen gels, on rat lung fibroblast feeder layers on floating collagen gels (floating feeder layers), and on Engelbreth-Holm-Swarm (EHS) tumor basement membrane gels. Type II cells cultured on floating feeder layers in medium containing 1% CS-rat serum and 10(-5) M hydrocortisone plus 0.5 mM dibutyryl cyclic AMP exhibited significantly increased incorporation of [14C]acetate into total lipids (238% of control). The hormone combination also increased the relative percentage of acetate incorporated into phosphatidylglycerol (PG; 7.3% versus 1.9%) and saturated phosphatidylcholine (PC; 43.6% versus 37.6%). The percentage of acetate incorporated into neutral lipids was significantly decreased by the addition of hormones (28.6% versus 70.0%). The addition of hydrocortisone and cyclic AMP to medium containing 5% CS-rat serum resulted in an increase in the relative incorporation of acetate into saturated PC (51.2% versus 46.4%), but had no effect on the relative incorporation of acetate into PG or on the incorporation of acetate into total lipids. Type II cells cultured on EHS gels in medium containing 1% CS-rat serum plus hydrocortisone and cyclic AMP showed increased acetate incorporation into total lipids (204% of control) and a relative decrease in the percentage of acetate incorporated into neutral lipids (16.9% versus 47.0%). The hormone combination also increased the relative incorporation of acetate into PG (4.4% versus 2.5%) and saturated PC (49.9% versus 42.1%). Hydrocortisone and cyclic AMP added to medium containing 5% CS-rat serum concentration increased the relative incorporation of acetate into saturated PC by type II cells on EHS gels, but these additions had no effect on acetate incorporation into PG. No responses to these soluble factors were seen when type II cells were cultured on floating type I collagen gels without feeder layers or on tissue culture plastic. These data indicate that there are positive interactions between substratum, soluble factors and serum in the maintenance of differentiated function of adult rat alveolar type II cells in vitro.

The pattern of cytokeratin synthesis is a marker of type 2 cell differentiation in adult and maturing fetal lung alveolar cells

During the last stages of fetal life, the immature epithelial cells of the rat lung alveolus develop the properties of mature type 2 cells. Adult type 2 cells rapidly lose these same properties when isolated and maintained in cell culture. We have examined the synthesis of cytokeratin proteins by adult type 2 cells as they lose their differentiated characteristics during 1 week in culture, and of immature fetal alveolar epithelial cells as they differentiate either in utero or when cultured on an extracellular matrix. Freshly isolated adult type 2 cells synthesize four cytokeratins which by electrophoretic mobilities and Western blot analysis correspond to human cytokeratins Nos. 7, 8, 18, and 19. During 7 days in culture synthesis of cytokeratin No. 19 is dramatically decreased and cytokeratin No. 18 becomes the predominant acidic cytokeratin produced. Fetal lung epithelial cells at 18 days gestation lack most characteristics of mature type 2 cells. When freshly isolated, these cells synthesize cytokeratins Nos. 7, 8, and 18 but make only minimal amounts of cytokeratin No. 19. When these cells are allowed to mature either in utero or in culture on a whole basement membrane extract, they develop both the morphological characteristics and the pattern of cytokeratin synthesis of fully developed type 2 cells, with cytokeratins No. 19 being the major acidic cytokeratin produced.

A cell culture model of chemically and spontaneously derived mouse lung alveologenic carcinoma

Malignant cell lines related to mouse lung alveologenic carcinoma have been established from urethane-induced tumors and after in vitro spontaneous transformation of preneoplastic cell lines. Both the chemically and spontaneously transformed cell lines formed invasive, poorly differentiated carcinomas with secondary lung deposits when implanted subcutaneously in immune-suppressed mice. They differed from the related preneoplastic cell line in coordinately exhibiting anchorage-independent growth, reduced epidermal growth factor receptor activity and absence of pericellular fibronectin. These data suggest that similar molecular events may occur in type 2 pneumocyte-related cells in order to generate mouse lung alveologenic adenomas and carcinomas by both spontaneous and chemical carcinogen induction mechanisms. A reduced level of pericellular fibronectin was also demonstrated in an in situ compressive urethane-induced mouse lung adenoma. Loss of pericellular fibronectin may therefore be an early and persistent phenotypic alteration during transformation to the alveologenic adenoma and carcinoma.

Fetal type 2 pneumocytes form alveolar-like structures and maintain long-term differentiation on extracellular matrix

We investigated the effects of reconstituted basement membrane (a crude extract of the Engelbreth-Holm-Swarm tumor) on type 2 pneumocyte differentiation during long-term culture. Cells were derived from mature 29 d fetal rabbits. Morphology was studied by light and electron microscopy. On thin gel, the cells initially segregated into clumps; they were cuboidal with apical microvilli and contained lamellar bodies, but dedifferentiated by 8 d. On thick gel, epithelial cells associated into spherical clusters surrounding a central lumen. These alveolarlike structures persisted at least 22 d. The cells were cuboidal and had lamellar bodies and intercellular tight junctions; they exhibited polarity, with apical microvilli facing the lumen, basally located nuclei, and gel matrix abutting the basal surface. In contrast, cells cultured on plastic formed colonies, then a monolayer, but dedifferentiated 5-7 d after plating. [14C]Acetate was used to label newly synthesized phospholipids. The amount of disaturated phosphatidylcholine (DSPC), expressed as a percentage of total phosphatidylcholine (PC), was used as an indicator of surfactant lipid production; percentage DSPC synthesized by cells cultured on thick gel did not change significantly, from 55 +/- 3 at 3 d, to 63 +/- 2 at 22 d in culture. DSPC synthesized by cells cultured on plastic decreased from 57 +/- 1% at 3 d to 45 +/- 2% at 22 d (p less than 0.001), which is consistent with the morphologic evidence of dedifferentiation. Synthesis of total PC compared with total phospholipid did not vary with either time in culture or substrate. This study emphasizes the importance of a complex extracellular matrix for maintenance of type 2 pneumocyte differentiation. The system should prove useful for studying the interaction of these cells with basement membrane, including the role of events occurring at the cell surface in modulating expression of a differentiated phenotype.

Synthesis of surfactant components by cultured type II cells from human lung

We examined the effect of monolayer culture on surfactant phospholipids and proteins of type II cells isolated from human adult and fetal lung. Type II cells were prepared from cultured explants of fetal lung (16-24 weeks gestation) and from adult surgical specimens. Cells were maintained for up to 6 days on plastic tissue culture dishes. Although incorporation of [methyl-3H]choline into phosphatidylcholine (PC) by fetal cells was similar on day 1 and day 5 of culture, saturation of PC fell from 35 to 26%. In addition, there was decreased distribution of labeled acetate into PC, whereas distribution into other phospholipids increased or did not change. The decrease in saturation of newly synthesized PC was not altered by triiodothyronine (T3) and dexamethasone treatment or by culture as mixed type II cell/fibroblast monolayers. The content of surfactant protein SP-A (28-36 kDa) in fetal cells, as measured by ELISA and immunofluorescence microscopy, rose during the first day and then fell to undetectable levels by the fifth. Synthesis of SP-A, as measured by [35S]methionine labeling and immunoprecipitation, was detectable on day 1 but not thereafter. Levels of mRNAs for SP-A and for the two lipophilic surfactant proteins SP-B (18 kDa) and SP-C (5 kDa) fell with half-times of maximally 24 h. In contrast, total protein synthesis measured by [35S]methionine incorporation increased and then plateaued. In adult cells, the content of SP-A and its mRNA decreased during culture, with time-courses similar to those for fetal cells. We conclude that in monolayer culture on plastic culture dishes, human type II cells lose their ability to synthesize both phospholipids and proteins of surfactant. The control of type II cell differentiation under these conditions appears to be at a pretranslational level.

The ultrastructure of epithelial cells of the distal lung

This review has focused on the structural and functional characteristics of those epithelial cells that line the walls of the lower respiratory bronchioles, alveolar ducts, and alveoli. In all, five cells types were considered: Clara cells, types I, II, and III pneumocytes, and alveolar macrophages. In addition, a very brief mention of the structure and influence of the basement membrane in alveolar development and repair was included, as well as a brief review of the role of epithelial cells in response to selected deleterious influences. No attempt was made to extend this review to cover the structure and functions of the epithelial lining of the conducting portions of the respiratory system, or the exciting and expanding complexities and interrelationships of the septal stroma. Since the volume of literature encircling this subject has virtually exploded during the last 15 years, it becomes almost impossible to review all reports. However, attempts were made to be selective in citations. Insofar as future developments are concerned, much remains to be understood concerning (1) the responses of all cell types to cytotoxic influences, including their respective abilities to repair induced damage, (2) cell-cell and cell-extracellular matrix relationships in response to injury, (3) the uniqueness of the basement membrane in the lung in controlling permeability and gaseous exchange, (4) the role(s) of alveolar macrophages in response to injury and their relationships to the septal macrophage population, (5) the aberrations in the respective cell types that can give rise to neoplastic growth, and (6) the role of the immune system in responding to the general defense of the lung. Indeed much has been learned in the past 2 decades, and it is expected that a review of this sort 1 or 2 decades hence will elucidate many of the functions and structural modifications of the lung.

Culture of type II pneumocytes on a type II cell-derived fibronectin-rich matrix

Recent evidence suggests that during primary culture, type II pneumocytes synthesize and deposit components of an extracellular matrix. The present study investigated the response of freshly isolated type II cells to a preformed, fibronectin-rich matrix synthesized by type II cells over a 6-day interval of primary culture on a plastic surface. Type II cells on 6-day matrix (M6) degraded the preformed matrix and deposited newly synthesized fibronectin more rapidly than cells on plastic, suggesting that M6 itself stimulated type II cell-mediated matrix turnover. In type II cells on plastic, incorporation of radiolabeled thymidine into DNA increased 620 and 1,880% after 2 and 3 days in culture, respectively, as the cells assumed a more flattened phenotype. Although cells on M6 did not divide, both basal rates of thymidine labeling and sensitivity to serum modulators of DNA synthesis were enhanced by the M6 surface, as compared with plastic. Culture of type II cells on surfaces of purified fibronectin enhanced the rate of DNA synthesis in a manner similar to that observed on M6; this effect was blocked by antifibronectin. The data suggest that more rapid fibronectin synthesis and deposition are important components of the response of type II cells to primary culture. Extracellular matrix produced by type II cells appears to be similar to the basement membrane onto which these cells proliferate in vivo after lung injury. A fibronectin-rich surface in itself may thus induce additional extracellular matrix synthesis and further direct cellular differentiation and proliferation.

Role of laminin in maintenance of type II pneumocyte morphology and function

Loss of differentiated function by type II pneumocytes plated on plastic surfaces was demonstrated by decreased lamellar body content, increased cellular protein, and rapid cellular flattening, changes that were retarded modestly by plating cells on laminin-coated surfaces. Laminin surfaces also inhibited [3H]thymidine (THM) incorporation into cellular DNA by 40% compared with plastic at 40 h, but did not alter an additional mitogenic effect of rat serum over fetal calf serum. In contrast, cells plated on the laminin-rich basement membrane-like gel formed from an extract of EHS mouse sarcoma, matrix gel (MG), maintained a high content of intracellular lipids in lamellar inclusions and retained a rounded morphology for at least 3 days. MG markedly inhibited THM incorporation and morphological changes when cells were cultured on this surface or when MG was formed over cells initially plated on plastic for various intervals. The importance of the laminin component of MG was demonstrated when these surfaces were pretreated with a highly specific antilaminin serum. Type II cells commenced flattening on the treated MG surface, and THM incorporation increased with the same time course as did control cells on plastic. The data suggest that short-term culture and study of differentiated type II pneumocytes may require a laminin-rich substratum. THM incorporation into type II cell DNA provides an important early and sensitive index of cell-basement membrane interaction and subsequent maintenance of function.