Epithelial cell characteristics of cultured human limbal explants

Morphological characteristics of the limbal epithelial crypt

AIM

In 2005 we reported the discovery of a novel anatomical structure at the limbus, which we termed the limbal epithelial crypt (LEC). The purpose of this study was to further evaluate the distribution, immunophenotypical, and ultra structural characteristics of the LEC as a putative niche of stem cells.

METHODS

Sequential histological sections of human corneo-scleral limbal rims were examined for the presence and distribution of the LEC. Immunophenotypical characterisation of the LEC cells using a panel of antibodies of interest was undertaken. Transmission electron microscopy of the LEC was used to examine the ultra structural and morphometric features of cells within the LEC and adjacent limbus.

RESULTS

A total of 74 LECs were identified in eight corneo-scleral rims. These varied in number, size and distribution within rims. Cells within the crypt demonstrated the following phenotype: CK3-/CK19+/CD 34-/Vimentin+/p63+/Connexin 43+/MIB1 (Ki67)-. Presence of Cx43 was also demonstrated in the rete pegs adjacent to the LEC. Basal cells of the LEC were significantly smaller than basal cells found in adjacent rete pegs and also smaller than suprabasal limbal and central corneal epithelial cells (p<0.05). Morphologically they had a high nuclear:cytoplasmic ratio and were adherent to the underlying basement membrane by means of complex convolutions of cytoplasmic processes.

CONCLUSIONS

LECs are sparse but a consistent finding in the human corneo-scleral limbus. The LEC contains a unique sub-population of cells expressing several characteristics that are consistent with it representing a putative stem cell niche.

Cultured human ocular surface epithelium on therapeutic contact lenses

BACKGROUND

This study was initiated after observation of some intriguing epithelial growth properties of contact lenses used as a bandage for patients after pterygium surgery.

AIM

To determine the efficacy of culturing human ocular surface epithelial cells on therapeutic contact lenses in autologous serum with a view of using this system to transfer epithelial cells to patients with persistent corneal or limbal defects.

METHODS

Excess graft tissue resected from patients undergoing pterygium surgery (n = 3) consisting of limbal epithelium was placed on siloxane-hydrogel contact lenses (lotrafilcon A and balafilcon A). Limbal explants were cultured in media with 10% autologous serum. Morphology, proliferative capacity and cytokeratin profile were determined by phase contrast, light and electron microscopy, and immunohistochemical analysis.

RESULTS

Lotrafilcon A contact lenses sustained proliferation and migration from limbal tissue. Cells became confluent after 10-14 days and consisted of 2-3 layers with a corneal phenotype (CK3(+)/CK12(+)/CK19(-)) and a propensity to proliferate (p63(+)). Electron microscopy showed microvilli on the apical surface with adhesive projections, indicating that these cells were stable and likely to survive for a long term. Growth was not observed from limbal explants cultured on balafilcon A contact lenses.

CONCLUSION

A method for culturing human ocular surface epithelium on contact lenses that may facilitate expansion and transfer of autologous limbal epithelial cells while avoiding the risks associated with transplanting allogeneic tissue has been developed. This technique may be potentially useful for the treatment of patients with limbal stem cell deficiency.

Human Limbal Progenitor Cell Characteristics are Maintained in Tissue Culture

Introduction: Acute central serous chorioretinopathy (CSCR) afflicts young middle-aged males in the Western population. We aimed to analyse patient demographics and to determine the angiographic characteristics of acute CSCR in an Asian population. Materials and Methods: This is a retrospective study of all patients presenting with acute CSCR who had fundal fluorescein angiograms performed within a 4-year period (between 1 January 1998 and 31 December 2001). Results: The fluorescein angiograms of 128 patients were analysed. The majority were male (109/128) with a male-to-female ratio of 5.7:1. The age range of patients was 26 to 60 years, with a mean age of 41 years. The majority of patients (84%) were aged 30 to 50 years. With regard to racial distribution, 83% were Chinese, 6% were Malays and 11% were Indians or of other races. Unilateral disease was found in 74 patients (58%) and 52 had bilateral disease. The macula was the most common site of fluorescein leakage and was found in 97 patients (76%). Almost half the patients (44%) had more than one site of disease involvement (i.e., multifocal). The inkblot leakage pattern was found in 103 patients (80%). Conclusions: The patient demographics of acute CSCR in our population were compared to that reported in the West. The gender ratio was similar, with males being afflicted 6 to 10 times more compared to females. There was no racial predilection found for acute CSCR in the local population. We also found a significant proportion of patients with bilateral and multifocal disease compared to the West. The inkblot pattern of leakage was the most common pattern seen on angiography. There were a significant number of cases with bilateral and multifocal involvement, exceeding those reported in non-Asian populations. Key words: Cell culture, Feeder layer, Limbal epithelial cells, Progenitor cells, Stem cells

Localization of candidate stem and progenitor cell markers within the human cornea, limbus, and bulbar conjunctiva in vivo and in cell culture

Corneal diseases are some of the most prevalent causes of blindness worldwide. While the most common treatment for corneal blindness is the transplantation of cadaver corneas, expanded limbal stem cells are finding recent application. Unknown, however, is the identity of the actual repopulating stem cell fraction utilized in both treatments and the critical factors governing successful engraftment and repopulation. In order to localize potential stem cell populations in vivo, we have immunohistochemically mapped a battery of candidate stem and progenitor cell markers including c-Kit and other growth factor receptors, nuclear markers including DeltaNp63, as well as adhesion factors across the cornea and distal sclera. Cell populations that differentially and specifically stained for some of these markers include the basal and superficial limbal/conjunctival epithelium and scattered cells within the substantia propria of the bulbar conjunctiva. We have also determined that the culture of differentiated cornea epithelial cells as dissociated and explant cultures induces the expression of several markers previously characterized as candidate limbal stem cell markers. This study provides a foundation to explore candidate corneal stem cell populations. As well, we show that expression of traditional stem cell markers may not be reliable indicator of stem cell content during limbal stem cell expansion in vitro and could contribute to the variable success rates of corneal stem cell transplantation.

Expression of cell cycle-regulatory proteins, MIB-1, p16, p53, and p63, in squamous cell carcinoma of conjunctiva: not associated with human papillomavirus infection

Squamous cell carcinoma (SCC), the most common primary malignant tumor of the conjunctiva, has a variable clinical presentation and immunohistochemical profile. Abundant cell cycles exist, including MIB-1 (Ki67 antigen), p16, p53, and p63, within the conjunctiva SCC. This investigation first reports the expressions of cell cycle markers in SCC. A retrospective study was conducted between December 1976 and June 2004, comprising 13 consecutive patients with conjunctiva SCC who were treated with surgical excision. Detailed clinical parameters were also reviewed. Overexpression of MIB-1, p16, p53, and p63 genes were studied by immunohistochemistry. Genechip containing 39 subtypes was used to elucidate human papillomavirus (HPV). The study group contained 13 (100%) men, with a mean age of 68+/-18 years and follow-up period of 20+/-17 months. The sample included four (33%) SCC located in the left eye and two (17%) recurrent SCC. Overexpression of the p53 and p63 was considerably higher than that of the p16 (P<0.01). HPV DNA was not detected in any of the 13 cases. This work first examined the immunohistochemical overexpression of cell cycle (MIB-1, p16, p53, and p63) in SCC. This investigation then showed that the expression of cell cycles in SCC was associated with key tumor clinicopathological features. This approach can help distinguish the potential roles of cell cycle in the development of SCC.

Factors Modulating p63 Expression in Cultured Limbal Epithelial Cells

PURPOSE

The expression pattern of p63, a homologue of the transcription factor p53, and whether it can be used as a corneal epithelial stem cell specific marker remain controversial. We investigated the p63 expression pattern in cultured limbal epithelial cells at different time points in culture, in sparse and confluent cultures, after growth factor starvation, and in single-cell-derived colonies.

METHODS

Harvested limbal epithelial cells were plated at 2.5 (sparse) or 5 (dense) x 10 cells/cm and evaluated for p63 expression at day 1, day 4, day 7, after starvation for 72 hours, or in colonies derived from single cells. Expression of corneal lineage specific differentiation marker keratin 3 (K3) was correlated with p63 expression. Results were compared by 1-way ANOVA.

RESULTS

More than 85% (85%-90%) of cells expressed p63 on day 1 regardless of cell plating density. On day 4, sparsely plated cultures were subconfluent and demonstrated high p63 expression (87.4%), whereas densely plated cells were confluent and had markedly reduced p63 expression (16.9%). Starvation of subconfluent cultures arrested cell division but did not decrease p63 expression. High-p63-expressing cultures expressed low levels of K3, and this trend was reversed in confluent cultures. Most cells in all colonies derived from single cells expressed p63.

CONCLUSIONS

The majority of corneal limbal epithelial cells express p63 in colonies derived from single cells and in subconfluent cultures regardless of time in culture or continuance of cell division. This suggests that p63 expression in culture cannot be used as a marker for stem cells. Significantly reduced number of cells express p63 in confluent cultures, associated with increased cell-cell contact. It is notable that these cells continue to express p63 amid areas of increased cell-cell contact several days after cultures have attained full confluency. This may represent a unique subpopulation of cells that retain proliferative potential in a confluent culture and may be analogous to a subpopulation of stem cells present in vivo.

Optimised two-dimensional electrophoresis procedures for the protein characterisation of structural tissues

The protein analysis of structural tissues is typically highly problematic. Amniotic membrane displays unique wound healing and anti-scarring properties; however, little is known concerning its active protein content. The structural nature of amniotic membrane necessitated development and extensive optimisation of the entire two-dimensional (2-D) workflow. Proteins were extracted using powerful solubilisation buffers and analysis carried out using 2-D electrophoresis followed by mass spectrometry (MS) identification. Preservation and processing resulted in prefractionation of soluble from structural and membrane-associated proteins. Enhanced protein solubility was achieved by cysteine blocking using both N,N-dimethylacrylamide (DMA) alkylation and bis(2-hydroxyethyl) disulphide (HED); an alternative procedure for the effective application of HED is demonstrated. The benefits of precipitation and cup-loading versus in-gel rehydration were also assessed, with procedures for the employment of HED with the latter described. Following optimisation, a representative sample 21 proteins were identified from amniotic membrane using MS verify procedures were MS-compatible. Our results demonstrate that techniques for the reproducible separation of proteins from a proteinaceous structural tissue have been optimised. Briefly, proteins are extracted using a thiourea/urea extraction buffer containing carrier ampholytes, dithiothreitol (DTT), and 3-(cyclohexylamino)-1-propanesulfonic acid (CHAPS). After DMA alkylation, proteins were precipitated (using the 2-D clean-up kit from Amersham Biosciences) and resolubilised in extraction buffer containing a lower concentration of DTT. Samples were either cup-loaded onto rehydrated HED-containing strips or rebuffered into HED-containing buffer followed by in-gel rehydration.

Selective growth and expansion of human corneal epithelial basal stem cells in a three-dimensional-organ culture

We report on a three dimensional (3D)-organotypic culture in vitro for selective growth and expansion of human corneal epithelial stem cells. Limbal corneal explants were cultured on porous collagen sponges submerged in Epilife medium containing 10% fetal bovine serum. The fragments were analyzed by immunohistochemistry for the expression and distribution of a spectrum of corneal epithelium markers: p63, CK-19, CK-3, Ki-67, pan-cytokeratins and vimentin. Early in culture the epithelium began to exfoliate losing its differentiated high-zone layers into the medium, maintaining only basal and few parabasal cells (mostly both p63 and CK-19 positive), which had remained attached to the specimen. After 14 days a new epithelium was formed displaying an increasing prominence of basal and suprabasal cells that, sliding onto the whole explant, showed the tendency to underlay stromal tissue and infiltrate into the underlaying sponge. After 21 days, sponge and fragments were incubated with trypsin-EDTA and dispersed epithelial cells were pipetted on a feeder monolayer of mitomycin-c-treated murine NIH.3T3 fibroblasts. Colonies of undifferentiated epithelial cells (p63, CK-19 and Ki-67 positive, CK-3 negative) were obtained: their cells, if seeded onto a collagen matrix containing embedded primary human corneal fibroblasts as feeder, provided the basic building blocks for reconstructing in vitro a 3D-multilayered corneal epithelium.

P63 expression levels in side population and low light scattering ocular surface epithelial cells

PURPOSE

Because stem cells exhibit high self-renewal capacity, slow cycling, and high proliferative potential, and one of many markers postulated for epithelial stem cells, p63, is challenged by widespread expression within stem cell-free regions, we examined p63 expression in these stem cell-associated cohorts compared with their controls.

METHODS

Rabbit limbocorneal cryosections, cytospun cell-sorted (by fluorescence-activated cell sorter) side population (SP) and low side scatter (LSSC) cells, and limbal epithelial cells over feeders were stained for p63 by indirect immunofluorescence. Clones were fixed and stained daily for 7 days. Image analysis measured p63 intensity, plotting it against colony size.

RESULTS

All basal limbal cells were positive for p63, yet only 5% to 7% expressed high p63 intensities, 40% intermediate, and the majority low. Side population cells were less than 1% of total cells. The average intensity of SP staining was three times that of controls. Subpopulations displaying stemlike features exhibited highest p63 expression. Replication rates of isolated cells differed. Day 5 colonies contained 256 (16 hours/cycle) to two (96 hours/cycle) cells. Whereas all cells were positive for p63, intensity in slow-cycling cells was three to four times that in rapidly proliferating congeners. Increased cell doublings did not decrease fluorescence.

CONCLUSIONS

Results suggest that p63 concentration is maximal in stem cells and decreases with differentiation. High p63 levels seem to correlate with cells of the SP and LSSC phenotypes, indicating high cell stemness. With identification of stem cells, further studies can elucidate their use in supporting ocular surface health.