Defining compartmentalized stem cell populations with distinct cell division dynamics in the ocular surface epithelium.

Adult tissues contain label-retaining cells (LRCs), which are relatively slow-cycling and considered to represent a property of tissue stem cells (SCs). In the ocular surface epithelium, LRCs are present in the limbus and conjunctival fornix; however, the character of these LRCs remains unclear, owing to lack of appropriate molecular markers. Using three CreER transgenic mouse lines, we demonstrate that the ocular surface epithelium accommodates spatially distinct populations with different cell division dynamics. In the limbus, long-lived Slc1a3CreER-labeled SCs either migrate centripetally toward the central cornea or slowly expand their clones laterally within the limbal region. In the central cornea, non-LRCs labeled with Dlx1CreER and K14CreER behave as short-lived progenitor cells. The conjunctival epithelium in the bulbar, fornix and palpebral compartment is regenerated by regionally unique SC populations. Severe damage to the cornea leads to the cancellation of SC compartments and conjunctivalization, whereas milder limbal injury induces a rapid increase of laterally expanding clones in the limbus. Taken together, our work defines compartmentalized multiple SC/progenitor populations of the mouse eye in homeostasis and their behavioral changes in response to injury.

Can Human Oral Mucosa Stem Cells Differentiate to Corneal Epithelia?

Human oral mucosa stem cells (hOMSCs) arise from the neural crest, they can self-renew, proliferate, and differentiate to several cell lines and could represent a good source for application in tissue engineering. Because of their anatomical location, hOMSCs are easy to isolate, have multilineage differentiation capacity and express embryonic stem cells markers such as-Sox2, Oct3/4 and Nanog. We have used SHEM (supplemented hormonal epithelial medium) media and cultured hOMSCs over human amniotic membrane and determined the cell's capacity to differentiate to an epithelial-like phenotype and to express corneal specific epithelial markers-CK3, CK12, CK19, Pan-cadherin and E-cadherin. Our results showed that hOMSCs possess the capacity to attach to the amniotic membrane and express CK3, CK19, Pan-Cadherin and E-Cadherin without induction with SHEM media and expressed CK12 or changed the expression pattern of E-Cadherin to a punctual-like feature when treated with SHEM media. The results observed in this study show that hOMSCs possess the potential to differentiate toward epithelial cells. In conclusion, our results revealed that hOMSCs readily express markers for corneal determination and could provide the ophthalmology field with a therapeutic alternative for tissue engineering to achieve corneal replacement when compared with other techniques. Nevertheless, further studies are needed to develop a predictable therapeutic alternative for cornea replacement.

In Vitro Compatibility of Contact Lenses With Corneal Epithelial Cells.

PURPOSE: To determine the interaction of contact lenses of different materials with corneal epithelial cells grown in tissue culture. METHODS: Two different corneal epithelial cell lines were grown to confluence in culture media. Two hydrogel contact lenses with and without polyvinylpyrrolidone (PVP) {1-DAY ACUVUE MOIST (1-Day ACUVUE [hydrogel lenses]) and a silicone hydrogel contact lens, AIR OPTIX NIGHT & DAY} were removed from their blister packs, washed in phosphate-buffered saline, and applied to the cells. After exposure for 24 hr at 37°C, lenses were removed, and the corneal cells and supernatants processed. Supernatants from the cell assays were used to quantify the amount of 17 different cytokines that were produced using a multiplex bead assay. Cells were stained to assess amount of cell death (apoptosis or necrosis) or stained to determine the level of mitochondrial activity. Stimulants of necrotic death (latex) or apoptotic death (sorbitol) were used as positive controls. RESULTS: Cells produced cytokines during normal growth. Exposure of cells to the hydrogel lenses resulted in only minimal changes to normal production of cytokines, but latex or sorbitol produced the most change. Exposure of the cells to all three lenses caused 4% to 23% reduction in mitochondrial activity, whereas exposure to the positive controls caused 71% to 98% reduction in mitochondrial activity. Exposure of the corneal epithelial cells to contact lenses produced minimal morphological changes, whereas exposure to latex or sorbitol produced significant changes to the human corneal epithelial cell line. CONCLUSIONS: Exposure of corneal epithelial cells to contact lenses had minimal impact on their physiology. There was no difference in epithelial cell responses to hydrogel with or without PVP compared with the silicone hydrogel contact lens.

Limited versus total epithelial debridement ocular surface injury: Live fluorescence imaging of hemangiogenesis and lymphangiogenesis in Prox1-GFP/Flk1::Myr-mCherry mice.

BACKGROUND: Immunohistochemical staining experiments have shown that both hemangiogenesis and lymphangiogenesis occur following severe corneal and conjunctival injury and that the neovascularization of the cornea often has severe visual consequences. To better understand how hemangiogenesis and lymphangiogenesis are induced by different degrees of ocular injury, we investigated patterns of injury-induced corneal neovascularization in live Prox1-GFP/Flk1::myr-mCherry mice, in which blood and lymphatic vessels can be imaged simultaneously in vivo. METHODS: The eyes of Prox1-GFP/Flk1::myr-mCherry mice were injured according to four models based on epithelial debridement of the: A) central cornea (a 1.5-mm-diameter circle of tissue over the corneal apex), B) total cornea, C) bulbar conjunctiva, and D) cornea+bulbar conjunctiva. Corneal blood and lymphatic vessels were imaged on days 0, 3, 7, and 10 post-injury, and the percentages of the cornea containing blood and lymphatic vessels were calculated. RESULTS: Neither central corneal nor bulbar conjunctival debridement resulted in significant vessel growth in the mouse cornea, whereas total corneal and corneal+bulbar conjunctival debridement did. On day 10 in the central cornea, total cornea, bulbar conjunctiva, and corneal+bulbar conjunctival epithelial debridement models, the percentage of the corneal surface that was occupied by blood vessels (hemangiogenesis) was 1.9±0.8%, 7.14±2.4%, 2.29±1%, and 15.05±2.14%, respectively, and the percentage of the corneal surface that was occupied by lymphatic vessels (lymphangiogenesis) was 2.45±1.51%, 4.85±0.95%, 2.95±1.27%, and 4.15±3.85%, respectively. CONCLUSIONS: Substantial corneal debridement was required to induce corneal neovascularization in the mouse cornea, and the corneal epithelium may therefore be partially responsible for maintaining corneal avascularity. GENERAL SIGNIFICANCE: Our study demonstrates that GFP/Flk1::myr-mCherry mice are a useful model for studying coordinated hemangiogenic and lymphangiogenic responses.

Corneal epithelial cells function as surrogate Schwann cells for their sensory nerves.

The eye is innervated by neurons derived from both the central nervous system and peripheral nervous system (PNS). While much is known about retinal neurobiology and phototransduction, less attention has been paid to the innervation of the eye by the PNS and the roles it plays in maintaining a functioning visual system. The ophthalmic branch of the trigeminal ganglion contains somas of neurons that innervate the cornea. These nerves provide sensory functions for the cornea and are referred to as intraepithelial corneal nerves (ICNs) consisting of subbasal nerves and their associated intraepithelial nerve terminals. ICNs project for several millimeters within the corneal epithelium without Schwann cell support. Here, we present evidence for the hypothesis that corneal epithelial cells function as glial cells to support the ICNs. Much of the data supporting this hypothesis is derived from studies of corneal development and the reinnervation of the ICNs in the rodent and rabbit cornea after superficial wounds. Corneal epithelial cells activate in response to injury via mechanisms similar to those induced in Schwann cells during Wallerian Degeneration. Corneal epithelial cells phagocytize distal axon fragments within hours of ICN crush wounds. During aging, the proteins, lipids, and mitochondria within the ICNs become damaged in a process exacerbated by UV light. We propose that ICNs shed their aged and damaged termini and continuously elongate to maintain their density. Available evidence points to new unexpected roles for corneal epithelial cells functioning as surrogate Schwann cells for the ICNs during homeostasis and in response to injury. GLIA 2017;65:851-863.

Expression of pluripotency factors in larval epithelia of the frog Xenopus: evidence for the presence of cornea epithelial stem cells.

Understanding the biology of somatic stem cells in self renewing tissues represents an exciting field of study, especially given the potential to harness these cells for tissue regeneration and repair in treating injury and disease. The mammalian cornea contains a population of basal epithelial stem cells involved in cornea homeostasis and repair. Research has been restricted to mammalian systems and little is known about the presence or function of these stem cells in other vertebrates. Therefore, we carried out studies to characterize frog cornea epithelium. Careful examination shows that the Xenopus larval cornea epithelium consists of three distinct layers that include an outer epithelial layer and underlying basal epithelium, in addition to a deeper fibrous layer that contains the main sensory nerve trunks that give rise to numerous branches that extend into these epithelia. These nerves convey sensory and presumably also autonomic innervation to those tissues. The sensory nerves are all derived as branches of the trigeminal nerve/ganglion similar to the situation encountered in mammals, though there appear to be some potentially interesting differences, which are detailed in this paper. We show further that numerous pluripotency genes are expressed by cells in the cornea epithelium, including: sox2, p63, various oct4 homologs, c-myc, klf4 and many others. Antibody localization revealed that p63, a well known mammalian epithelial stem cell marker, was localized strictly to all cells in the basal cornea epithelium. c-myc, was visualized in a smaller subset of basal epithelial cells and adjacent stromal tissue predominately at the periphery of the cornea (limbal zone). Finally, sox2 protein was found to be present throughout all cells of both the outer and basal epithelia, but was much more intensely expressed in a distinct subset of cells that appeared to be either multinucleate or possessed multi-lobed nuclei that are normally located at the periphery of the cornea. Using a thymidine analog (EdU), we were able to label mitotically active cells, which revealed that cell proliferation takes place throughout the cornea epithelium, predominantly in the basal epithelial layer. Species of Xenopus and one other amphibian are unique in their ability to replace a missing lens from cells derived from the basal cornea epithelium. Using EdU we show, as others have previously, that proliferating cells within the cornea epithelium do contribute to the formation of these regenerated lenses. Furthermore, using qPCR we determined that representatives of various pluripotency genes (i.e., sox2, p63 and oct60) are upregulated early during the process of lens regeneration. Antibody labeling showed that the number of sox2 expressing cells increased dramatically within 4 h following lens removal and these cells were scattered throughout the basal layer of the cornea epithelium. Historically, the process of lens regeneration in Xenopus had been described as one involving transdifferentiation of cornea epithelial cells (i.e., one involving cellular dedifferentiation followed by redifferentiation). Our combined observations provide evidence that a population of stem cells exists within the Xenopus cornea. We hypothesize that the basal epithelium contains oligopotent epithelial stem cells that also represent the source of regenerated lenses in the frog. Future studies will be required to clearly identify the source of these lenses.

The corneal epithelium and lens develop independently from a common pool of precursors.

BACKGROUND: The corneal epithelium (CE) overlays a stroma, which is derived from neural crest cells, and appears to be committed during chick development, but appears still labile in adult rabbit. Its specification was hitherto regarded as resolved and dependent upon the lens, although without experimental support. Here, we challenged CE fate by changing its environment at different stages. RESULTS: Recombination with a dermis showed that CE commitment is linked to stroma formation, which results in Pax6 stabilization in both species. Surgical ablation shows that CE specification has already taken place when the lens placode invaginates, while removal of the early lens placode led to lens renewal. To block lens formation, bone morphogenetic protein (BMP) signaling, one of its last inducing factors, was inhibited by over-expression of Gremlin in the ocular ectoderm. This resulted in lens-less embryos which formed a corneal epithelium if they survived 2 weeks. CONCLUSION: The corneal epithelium and lens share a common pool of precursors. The adoption of the CE fate might be dependent on the loss of a lens placode favoring environment. The corneal fate is definitively stabilized by the migration of Gremlin-expressing neural crest cells in the lens peripheral ectoderm.

An investigation of donor and culture parameters which influence epithelial outgrowths from cultured human cadaveric limbal explants.

Limbal stem cell deficiency is a blinding disease which affects the cornea at the front of the eye. The definitive cure involves replacing the corneal epithelial (limbal) stem cells, for example by transplanting cultured limbal epithelial cells. One method of performing cultures is to grow a sheet of epithelial cells from a limbal explant on human amniotic membrane. The growth of limbal tissue can be variable. The aim of this study is to investigate how different donor and culture factors influence the ex vivo growth of cadaveric limbal explants. Limbal explant cultures were established from 10 different cadaveric organ cultured corneo-scleral discs. The growth rate and the time taken for growth to be established were determined. Statistical analysis was performed to assess correlation between these factors and donor variables including donor age, sex, time from donor death to enucleation, time from enucleation to organ culture storage and duration in organ culture. Growth curves consistently showed a lag phase followed by a steeper linear growth phase. Donor age, time between death and enucleation, and time between enucleation and organ culture were not correlated to the lag time or the growth rate. Time in organ culture had a significant correlation with the duration of lag time (P = 0.003), but no relationship with the linear growth rate. This study shows that an important factor correlating with growth variation is the duration of corneo-scleral tissue in organ culture. Interestingly, donor age was not correlated with limbal explant growth.

Hydrogel membranes based on genipin-cross-linked chitosan blends for corneal epithelium tissue engineering.

Novel polymeric hydrogel scaffolds for corneal epithelium cell culturing based on blends of chitosan with some other biopolymers such as hydroxypropylcellulose, collagen and elastin crosslinked with genipin, a natural substance, were prepared. Physicochemical and biomechanical properties of these materials were determined. The in vitro cell culture experiments with corneal epithelium cells have indicated that a membrane prepared from chitosan-collagen blend (Ch-Col) provided the regular stratified growth of the epithelium cells, good surface covering and increased number of the cell layers. Ch-Col membranes are therefore the most promising material among those studied. The performance of Ch-Col membranes is comparable with that of the amniotic membrane which is currently recommended for clinical applications.

Bovine corneal epithelial primary cultures as an in vitro model for ophthalmic drugs studies.

In the recent years a significant development of investigations with regard to bioavailability of ocular drugs has been noticed. The corneal epithelial barrier is the main pathway for ocular penetration of topically applied ophthalmic drugs into the anterior chamber. To work out an in vitro model of bovine corneal epithelial primary cultures and exercise it for permeability research with lipophilic and hydrophilic markers, permeability coefficients estimation of the 6-carboxyfluorescein and rhodamin B was made. The corneal epithelial cultures of the 3th or 4th passage were chosen for layered culture with inserts based on the liquid-liquid interface (for the first week) and the air-liquid interface (for the two following weeks). On the 7th, 12th, 18th, 21st experiment day TER values, and on the 21st day drug permeability coefficients, were determined. The mean TER values of the 7th, 12th, 18th, 21st day of corneal epithelial culture were: 122.14, 155.14, 198.43 and; 247.43 Wcm2, respectively. The mean values of permeability coefficients on the 21st day of culture for 6-carboxyfluorescein and rhodamin B were 3.87 +/-0.10 x 10(-6)cm/s and 3.65 +/- 0.06 x 10(-6)cm/s, respectively. We state that the in vitro bovine corneal epithelial primary culture model is useful for ocular studies.

Corneal Repair Models in Mice: Epithelial/Mechanical Versus Stromal/Chemical Injuries.

The tissue response to injury is a complex process. The cornea is an excellent model for studying wound repair processes because of its simple anatomy, easy accessibility, and normal avascular state. Here, we describe two corneal repair models in mice: an epithelial/mechanical injury model and a stromal/chemical injury model. The two models induce different repair responses, and consequently enable the study of independent repair processes. Here, we describe how these two wound models may be used to study basic cellular and molecular mechanisms of corneal repair.

Topical Adoptive Transfer of Plasmacytoid Dendritic Cells for Corneal Wound Healing.

Plasmacytoid dendritic cells (pDCs) are crucial for corneal homeostasis through secretion of various anti-angiogenic molecules and growth factors. Due to its avascular nature, only a limited number of adoptively transferred cells home to the cornea, when administered systemically. In addition, local adoptive transfer of cells poses several challenges and the clinical application of commonly used techniques is limited. Herein, we detail a novel approach for local adoptive transfer of pDCs to the cornea for the treatment of corneal wounds. This approach utilizes a commonly used fibrin sealant as a means of transferring previously isolated cells locally on the cornea. The technique is simple, reproducible, and is accompanied with successful transfer and integration of a substantial number of the cells to the cornea. Application of this approach to transfer pDCs promotes corneal wound healing. Furthermore, this technique can be applied for adoptive transfer of any cell of interest to the cornea.

Conditional deletion of Cited2 results in defective corneal epithelial morphogenesis and maintenance.

Cited2 is an important transcriptional cofactor involved in multiple organ development. Gene profile analysis has identified Cited2 as one of the transcription factors expressed at high levels in adult mouse cornea. To address the function of Cited2 in corneal morphogenesis, we deleted Cited2 in surface ectoderm derived ocular structures including cornea by crossing Cited2-floxed mice with Le-Cre transgenic mice. Cited2(flox/flox);Le-Cre(+) eyes invariably displayed corneal opacity and developed spontaneous corneal neovascularization at older age. Fewer layers of corneal epithelial cells and the absence of cytokeratin 12 (K12) expression featured Cited2 deficient postnatal and adult eyes. Cited2 deficient cornea exhibited impaired healing in response to corneal epithelial debridement by manifesting abnormal histology, lack of K12 expression and corneal neovascularization. Moreover, mechanistic studies suggest that Cited2 may play a role in corneal morphogenesis in part through modulating the expression of Pax6 and Klf4. Collectively, these findings demonstrate a novel function of Cited2 in postnatal corneal morphogenesis and maintenance. Our study will help better understand the molecular mechanisms involved in corneal biology, and more importantly, it may provide a valuable animal model for testing therapeutics in the treatment of corneal disorders, especially blindness as a result of corneal epithelial cell deficiency.

Expression of semaphorin 3A and its receptors during mouse corneal development.

Semaphorin 3A (Sema3A) functions to guide the growth of neurons during development, with its effects being mediated by receptor complexes composed of neuropilin (Npn) and plexin (Plx) proteins. We have now examined the expression of Sema3A and its receptor components Npn1 and PlxA during development of the mouse cornea. Sema3A and Npn1 showed similar patterns of expression by immunohistofluorescence analysis, with such expression being prominent in the corneal epithelium during both embryonic and postnatal development. In contrast, PlxA was not expressed in the corneal epithelium until after eye opening between postnatal days 12 and 14. Laser capture microdissection followed by reverse transcription and polymerase chain reaction analysis also showed that the abundance of PlxA mRNA in corneal epithelial cells increased significantly during postnatal development, again in association with eye opening. Given that atmospheric oxygen is thought to play a role in corneal epithelial differentiation and maintenance, our results suggest that the up-regulation of PlxA expression in the corneal epithelium during postnatal development is triggered by exposure of the cornea to the atmosphere. Furthermore, the newly expressed PlxA may contribute to the differentiation of corneal epithelial cells by mediating Sema3A signaling.

Proliferative and migratory aptitude in pterygium.

Pterygium is a chronic fibrovascular overgrowth on the corneal surface and is often associated with inflammation, astigmatism and obstructed vision. The common treatment is surgical removal but post-operative recurrences with more aggressive behavior are common. However, there is a controversy in the pathogenesis of primary pterygium between limbal stem cell failure versus proliferation. In this study, we explore the proliferative and migratory aptitude in pterygium by characterizing the growth and migration pattern of pterygial cells in the head (on the cornea), the neck (over the focal limbus), and the body (on the conjunctiva) epithelia of 12 full-length primary pterygia. Immunofluorescence and quantification analyses showed a spatial expression pattern of markers for stem cells, cell growth, and matrix metalloproteinases. Beside the basal epithelia in all three regions, p63α(strong) cells were located in suprabasal layers in head, weak in the body and absent in neck. Pertinent cell proliferation in head than body epithelia was revealed by its higher colony-forming efficiency. ATP-binding cassette transporter glycoprotein family member-2 and cytokeratin-15 were found mainly in the body basal epithelia, similar to that in normal conjunctiva. Much fewer proliferating stem-like cells in the neck region supported the limbal failure as a cause of pterygium formation. Pax6, matrix metalloproteinase-2 and -9 were more expressed in the head than in the other two regions. Our results indicate the importance of pterygium head in tissue growth and invasion and its likely involvement in post-operation recurrence.

Reconstruction of a tissue-engineered cornea with porcine corneal acellular matrix as the scaffold.

Interest in developing tissue-engineered cornea has increased with the decrease in the supply of donor tissue. The aim of the present study was to investigate the feasibility and method of reconstructing corneal equivalents with porcine corneal acellular matrix as the scaffold in a dynamic culturing system. Applying the detergent Triton X-100 (1%) and a freeze-drying process, porcine corneas were decellularized and prepared as a scaffold, and hematoxylin-eosin staining and scanning electron microscopy showed no cells in the decellularized stroma. In order to measure the in vivo biocompatibility, part of the scaffold was transplanted into a pocket of rabbit corneal stroma and observed for 3 months. No sign of rejection were observed, and the acellular matrix gradually integrated in the rabbit cornea, indicating that the scaffold had good biocompatibility. To reconstruct a tissue-engineered cornea, cultured rabbit keratocytes were seeded into the scaffold. After 1 week of culture in a culturing vessel, rabbit epithelial and endothelial cells were seeded on both sides of the stroma, respectively. The reconstructed cornea consisted of three layers in histological structure: the epithelium, stoma and endothelium. Stratified epithelial cells formed on the surface, which were cytokeratin 3 positive in the cytoplasm; endothelial cell monolayers were located on the inner side, and pump-related aquaporin 1 was found in the cells. These results confirmed that the corneal acellular matrix can be used as a scaffold for tissue-engineered cornea, and a biological corneal equivalent can be reconstructed in a dynamic culturing system.

Evaluation of Megacell MEM as a storage medium for corneas destined for transplantation.

PURPOSE: Gibco's Minimum Essential Medium with Earle's salts and HEPES supplemented with glutamine, antibiotics (EB MEM) and 2% foetal calf serum (FCS) is used in European eye banks to store corneas. Although FCS is important to endothelial cell survival in this medium, it is a potential biohazard. Megacell MEM, formulated to reduce the FCS requirement of cells by a factor of 5, has therefore been evaluated as a corneal storage medium. METHODS: Corneal stromal and epithelial cells were incubated in Megacell MEM (serum-free or 2% FCS) to assess their viability in these media. Endothelial cell densities of paired corneas held in either EB MEM 2% FCS or Megacell MEM (serum-free or 2% FCS) were measured over 5 weeks. Discs subsequently punched from the centre of these corneas were weighed, dried and reweighed to determine hydration levels. RESULTS: Both corneal stromal and epithelial cells proliferated in Megacell MEM 2% FCS. Relative to EB MEM, 2% FCS Megacell MEM prolonged the viability of corneal endothelial cells and improved their morphological appearance, irrespective of whether it contained FCS or not. This was independent of corneal swelling. CONCLUSION: Serum-free Megacell MEM is a better storage medium than EB MEM 2% FCS for corneas destined for transplantation.

Case report of restoration of the corneal epithelium in a patient with atopic keratoconjunctivitis resulting in amelioration of ocular allergic inflammation.

BACKGROUND: Atopic and vernal keratoconjunctivitis are severe types of ocular allergic disease characterized not only by conjunctival inflammation but also by corneal involvement. In vitro studies have suggested that breakdown of corneal epithelial barrier function and subsequent activation of stromal fibroblasts may amplify ocular allergic inflammation. CASE SUMMARY: A 27-year-old man with atopic dermatitis developed atopic keratoconjunctivitis including corneal ulcer with plaque deposition in his right eye. Conjunctival inflammation in the right eye was resistant to topical steroid therapy. Surgical removal of corneal plaque and administration of autologous fibronectin eyedrops resulted not only in resurfacing of the corneal epithelium but also in amelioration of conjunctival inflammation. DISCUSSION: This case suggests that loss of corneal epithelial integrity likely exacerbates conjunctival allergic inflammation and that restoration or maintenance of the barrier function of the corneal epithelium may be one of the important targets for the treatment of severe ocular allergic diseases.

Primary Culture of Cornea-Limbal Epithelial Cells In Vitro.

The cultivation of corneal-limbal cells in vitro represents an excellent means to generate models to study cornea function and disease processes. These in vitro expanded cornea-limbal epithelial cell cultures are rich in stem cells for cornea, and hence can be used as a cell therapy for cornea-limbal deficiency. This chapter details the primary culture of these cornea-limbal cells, which can be used as model for further studies of the cornea surface.

Optimization of Human Limbal Stem Cell Culture by Replating a Single Limbal Explant.

Cultured limbal epithelial stem cell transplantation is a clinical procedure used to regenerate the corneal epithelium in patients with limbal stem cell deficiency. The protocols used to expand limbal epithelial cells in vitro need to be optimized, since the scarcity of human ocular tissue donors is limiting the potential use of this procedure. Here, we describe a method to consecutively expand a single human limbal explant. With this method it is possible to obtain up to three limbal epithelial primary cultures from the same explant, thus increasing the efficiency of the in vitro cell culture.