Developmental analysis of SV2 in the embryonic chicken corneal epithelium

Intraepithelial corneal nerves (ICNs) help protect the cornea as part of the blink reflex and by modulating tear production. ICNs are also thought to regulate the health and homeostasis of the cornea through the release of trophic factors. Disruption to these nerves can lead to vision loss. Despite their importance little is known about how corneal nerves function and even less is known about how the cornea is initially innervated during its embryonic development. Here, we investigated the innervation of the embryonic chicken cornea. Western blot and immunohistochemistry were used to characterize the localization of the synaptic vesicle marker SV2, a molecule thought to be involved in the release of trophic factors from sensory nerves. The data show that both SV2 and synaptotagmin co-localize to ICNs. Nerves in the conjunctiva also contained SV2 and synaptotagmin, but these were localized to below the basal layers of the conjunctiva epithelium. SV2 isolated from corneal epithelium migrates in western blot at a heavier weight than SV2 isolated from brain, which suggests a role in vesicle targeting, as the deglycosylating enzyme PnGase does not affect corneal SV2.

Embryonic chick corneal epithelium: a model system for exploring cell-matrix interactions

In her initial research, Elizabeth D. Hay studied amphibian limb regeneration, but later switched her focus, and for the remainder of her career addressed the role of the extracellular matrix (ECM) in regulating embryonic morphogenesis. Much of that work used the embryonic chick corneal epithelial model. This review highlights many of the discoveries that she made using this model. Hay was the first to show that embryonic corneal epithelial cells produce fibrillar collagen. Her lab was among the first to demonstrate that corneal epithelial cells respond to a collagenous substrate by increasing ECM production, and that purified ECM molecules, added to cultures of epithelial sheets, induce a reorganization of the actin cytoskeleton. These data led to the first theories of cell-matrix interactions, illustrated in a 'hands across the membrane' sketch drawn by Hay. Recent work with the epithelial sheet model system has elucidated many of the signal transduction pathways required for actin reorganization in response to the ECM. In all, this body of work has amply supported Hay's belief that the embryonic corneal epithelium is a powerful model system for exploring the role of the ECM in regulating the cytoskeleton, in directing cell migration, and in profoundly influencing cell growth and differentiation during development.

The requirement of pax6 for postnatal eye development: evidence from experimental mouse chimeras

PURPOSE

The small eye mouse mutant (Sey) is caused by a mutation of the Pax6 gene. Previous studies, in which aggregation chimeras were used, have demonstrated that Sey/Sey cells contribute poorly to the neural retina forming small clumps of cells restricted to the inner retina at embryonic day 16.5. In addition, Sey/+ cells are absent from the lens epithelium during this embryonic period and postnatally. This study was conducted to determine the fates of these Sey/Sey and Sey/+ cells with continued development in chimeric mouse eyes.

METHODS

Observations were made on heterozygous and homozygous Sey cells in chimeric eyes from postnatal day (P)0 to P10.

RESULTS

In Sey/Sey<-->wild-type (wt) chimeras, all Sey/Sey cells originating from retinal progenitor cells died at perinatal times. The only remaining Sey/Sey cells in the neural retina were associated with blood vessels, including vascular endothelial cells, pericytes, astrocytes, and microglia, which have extraretinal origins. In contrast, Sey/+ cells formed all retinal cell classes. As previously reported, Sey/Sey cells were absent from the lens and corneal epithelium. However, in contrast to previous reports, Sey/+ cells contributed to the lens epithelium as well as corneal tissues, and Sey/Sey cells were absent from the anterior retinal pigment epithelium.

CONCLUSIONS

All evidence showed that, when Pax6 is absent at the initial stages of the development, Sey/Sey cells that contribute to the neural retina die, even when wild-type cells are available to provide normal environmental cues.

The phenotype of limbal epithelial stem cells

PURPOSE

The purpose of this study was to identify phenotypic markers of human limbal stem cells in fetal and adult corneas.

METHODS

RNA from microscopically dissected superficial limbal and central fetal (18 weeks) corneas was amplified and used to generate P(32)-labeled, reverse-transcribed antisense RNA that was linearly amplified and hybridized to a focused stem cell cDNA microarray. Differential gene expression of fetal limbus was compared with the expression of central cornea. Microarray differential expression experiments were performed on P63-expressing primary cultured limbal epithelial cells (passage 1; Pa1) and primary cells passaged 5 times (Pa5). Semiquantitative RT-PCR assay and immunohistochemistry were performed on fetal and adult corneas and cultured primary limbal epithelial cells, to confirm the results of the microarray experiments. Slow-cycling (pulsed bromodeoxyuridine label-retaining) limbal epithelium in corneal organ culture was studied for the expression of four selected upregulated limbal genes.

RESULTS

Of the 266 genes tested, 33 were differentially overexpressed (more than twofold) in the fetal limbus (compared with central cornea) and primary cultured limbal epithelium compared with primary cells after 5 passages. Cytokeratin 15 (CK15) and cytokeratin 14 (CK14) are expressed in limbal basal epithelium and P-cadherin (CDH3) and Wnt-4 expression was restricted to basal and immediate parabasal limbal epithelium of both the adult and fetal corneas). Bromodeoxyuridine label retaining epithelium in corneal organ culture (slow-cycling cells) expressed the four selected limbal upregulated genes.

CONCLUSIONS

For the first time, a focused stem cell pathway microarray analysis has been performed on fetal cornea and cultured limbal explant epithelium. CK15, CK14, CDH3, and Wnt-4 are expressed in the basal limbal epithelial cells.

Deletion of JAM-A causes morphological defects in the corneal epithelium

Junctional adhesion molecule-A (JAM-A, JAM-1, F11R) is an Ig domain containing transmembrane protein that has been proposed to function in diverse processes including platelet activation and adhesion, leukocyte transmigration, angiogenesis, epithelial cell shape and endothelial cell migration although its function in vivo is less well established. In the mouse eye, JAM-A protein expression is first detected at 12.5 dpc in the blood vessels of the tunica vasculosa, while it is first detected in both the corneal epithelium and lens between 13.5 and 14.5 dpc. In the corneal epithelium, JAM-A levels remain appreciable throughout life, while JAM-A immunostaining becomes stronger in the lens as the animals age. Both the cornea and lens of mice lacking an intact JAM-A gene are transparent until at least a year of age, although the cells of the JAM-A null corneal epithelium are irregularly shaped. In wild-type mice, JAM-A protein is found at the leading edge of repairing corneal epithelial wounds, however, corneal epithelial wound repair was qualitatively normal in JAM-A null animals. In summary, JAM-A is expressed in the corneal epithelium where it appears to regulate cell shape.

Expression of keratin 12 and maturation of corneal epithelium during development and postnatal growth

PURPOSE

To determine the kinetics of corneal epithelial maturation during embryonic development and postnatal growth.

METHODS

Expression patterns of keratin (K)12 and K14 were determined in mouse embryos (embryonic days [E]15.5-19.5), corneas of postnatal day (P)0 to 10 months, and healing corneas after epithelial debridement in P30 and P90 mice. The expression of alkaline phosphatase (AP) was determined during postnatal growth and healing of epithelial debridement of Krt12(Cre/Cre)/ZAP bitransgenic mice.

RESULTS

During embryonic development, K12 expression by corneal peridermal epithelium commenced at E15.5. In the period from E15.5 to P10, the expression of K12 was restricted to the suprabasal and/or superficial cells of the corneal epithelium, whereas the K14 expression was restricted to the basal cells. After P30, K12 expression was sporadically detected in the basal corneal epithelium, and the number of K12-positive basal cells increased as the mice grew older. The number of K14-positive cells that coexpressed K12 increased with age and reached a plateau after P180. Healing of the debrided epithelium facilitated the increase in K14-positive cells that coexpressed K12. Many basal cells of Krt12(Cre/Cre)/ZAP mice remained undifferentiated and expressed LacZ at P15, and they then differentiated to express Cre, which leads to excision of LacZ and AP expression.

CONCLUSIONS

In the mouse, the corneal epithelium does not become fully mature until 3 to 6 months after birth, in that a significant number of corneal basal epithelial cells of young mice (<P30), which derive from embryonic surface ectoderm remain undifferentiated and serve as corneal epithelial progenitor cells. These progenitor cells may have some stem cell characteristics.

Developmentally regulated expression of Sp1 in the mouse cornea

PURPOSE

To examine the expression of transcription factor Sp1 in the cornea of the mouse eye throughout developmental stages. The environmental effect of light on Sp1 expression was also assessed.

METHODS

C57BL/6 mice were set up for timed mating. Embryos on embryonic day (E)10.5, E12.5, E15.5, and E18.5 and eyes from mice on postnatal day (P)0, P7, P11, P15, P30, and P60 were collected for immunohistochemical staining and in situ hybridization. One group of mice was bred strictly in the dark between E18.5 and P15, and the eyes were collected at P0, P7, P11, and P15 time points.

RESULTS

Sp1 expression was observed in the ectoderm and lens vesicle as early as E10.5. Both Sp1 protein and mRNA were abundant in the corneal basal epithelium and keratocytes until P11. Their levels were markedly reduced at P15, right after eyelid opening, and declined further between P15 and P60. In those mice bred in the dark, Sp1 was evident in the cornea at P0. The Sp1 level gradually increased until P11 and was decreased at P15. This expression pattern was nearly identical in mice bred either in a light/dark cycle or in the dark. The Sp1 level in the central lens epithelium was much lower than that in the cornea from E15.5 to late stages.

CONCLUSIONS

The present study indicates that Sp1 expression is developmentally regulated, providing a basis for further investigations on the regulation of the Sp1 gene during the course of corneal development and in diseases such as keratoconus.

Gene expression and immunolocalisation of a calcium-activated chloride channel during the stratification of cultivated and developing corneal epithelium

The spatial and temporal localisation of a calcium-activated chloride channel (CLCA) and its mRNA was investigated, during the in vivo and in vitro development of stratified epithelia, by fluorescence immunohistochemistry and quantitative polymerase chain reaction in embryonic chicken corneas and the expansion of excised human corneal stem cells on amniotic membrane. Single-layered human epithelial cultures on amniotic membrane and early day embryonic chicken corneas expressed relatively little human CLCA2 or its chicken homologue. However, as the epithelium in both models matured and the number of cell-layers increased, the gene expression level and protein staining intensity increased, primarily within the basal cells of both the cultured and embryonic tissues. These results demonstrate that human CLCA2 protein and mRNA expression are elevated during epithelial stratification, suggesting that this protein plays a role in the growth of multi-layered corneal epithelia during both natural development and tissue cultivation.

Minoranomalien der Hornhaut bei der murinen Trisomie�16

BACKGROUND

The prevalence of human Down's syndrome is about 1:700. Investigations using animal models are therefore of clinical relevance for understanding its etiopathogenesis. No corneal changes have been reported with transgenic murine trisomy 16.

METHODS

A total of 20 fetal mice (n=40 eyes) with experimentally induced trisomy 16 were investigated from day 18 of pregnancy in order to determine whether visible developmental disorders of the cornea occur. All specimen were investigated microscopically in serial sections.

RESULTS

In addition to disturbances in systemic development, the transgenic mouse fetuses showed high rates of malformation of the eyes. Developmental and differentiation disorders of the corneal epithelial cell layers and structural disturbances of the corneal parenchyma were found. Our findings are the first demonstration of developmental disorders of the cornea in mouse fetuses with trisomy 16. These minor anomalies of the cornea could well have resulted in keratoconus if the animals had survived.

CONCLUSIONS

Our findings in transgenic mouse fetuses with trisomy 16 correspond to the clinical pattern of Down's syndrome in humans. Disturbed development of lids and lenses have a high prevalence, whereas corneal hypoplasia is found less often.

ROCK inhibitor (Y27632) increases apoptosis and disrupts the actin cortical mat in embryonic avian corneal epithelium

The embryonic chicken corneal epithelium is a unique tissue that has been used as an in vitro epithelial sheet organ culture model for over 30 years (Hay and Revel [1969] Fine structure of the developing Avian cornea. Basel, Switzerland: S. Karger A.G.). This tissue was used to establish that epithelial cells could produce extracellular matrix (ECM) proteins such as collagen and proteoglycans (Dodson and Hay [1971] Exp Cell Res 65:215-220; Meier and Hay [1973] Dev Biol 35:318-331; Linsenmayer et al. [1977] Proc Natl Acad Sci U S A 74:39-43; Hendrix et al. [1982] Invest Ophthalmol Vis Sci 22:359-375). This historic model was also used to establish that ECM proteins could stimulate actin reorganization and increase collagen synthesis (Sugrue and Hay [1981] J Cell Biol 91:45-54; Sugrue and Hay [1982] Dev Biol 92:97-106; Sugrue and Hay [1986] J Cell Biol 102:1907-1916). Our laboratory has used the model to establish the signal transduction pathways involved in ECM-stimulated actin reorganization (Svoboda et al. [1999] Anat Rec 254:348-359; Chu et al. [2000] Invest Ophthalmol Vis Sci 41:3374-3382; Reenstra et al. [2002] Invest Ophthalmol Vis Sci 43:3181-3189). The goal of the current study was to investigate the role of ECM in epithelial cell survival and the role of Rho-associated kinase (p160 ROCK, ROCK-1, ROCK-2, referred to as ROCK), in ECM and lysophosphatidic acid (LPA) -mediated actin reorganization. Whole sheets of avian embryonic corneal epithelium were cultured in the presence of the ROCK inhibitor, Y27632 at 0, 0.03, 0.3, 3, or 10 microM before stimulating the cells with either collagen (COL) or LPA. Apoptosis was assessed by Caspase-3 activity assays and visualized with annexin V binding. The ROCK inhibitor blocked actin cortical mat reformation and disrupted the basal cell lateral membranes in a dose-dependent manner and increased the apoptosis marker annexin V. In addition, an in vitro caspase-3 activity assay was used to determine that caspase-3 activity was higher in epithelia treated with 10 microM Y-27632 than in those isolated without the basal lamina or epithelia stimulated with fibronectin, COL, or LPA. In conclusion, ECM molecules decreased apoptosis markers and inhibiting the ROCK pathway blocked ECM stimulated actin cortical mat reformation and increased apoptosis in embryonic corneal epithelial cells.

Analysis of mouse eye development with chimeras and mosaics

Analysis of experimental mouse chimeras (chimaeras) and mosaics provides a means of investigating patterning and differentiation within the developing mammalian eye. Chimeric and mosaic mice carry two or more genetically distinct cell populations and extend the repertoire of analytical tools available to the geneticist. Here we review the impact these techniques have had on our understanding of eye organogenesis. Chimeras and mosaics are routinely used to investigate cell lineages, patterns of growth and gene function, and provide a means to clear analytical hurdles that otherwise limit standard genetic approaches. In particular, chimeras are used to investigate the roles of genes in tissues that do not develop in conventional mutant or knock-out mice, to test whether genes act cell autonomously or non-autonomously in different tissues and to dissect tissue-tissue interactions in less tractable, complex systems. Chimeras, in which cells of different genetic composition are mixed at a fine-scale cellular level, may provide qualitatively different data from mosaic mice with conditional knockouts. The uses of chimeras, Cre-loxP mosaics and in vitro tissue recombination for study of ocular organogenesis are compared. Wider use of mosaics and chimeras should provide further insights into eye development.

Transdifferentiation of corneal epithelium: evidence for a linkage between the segregation of epidermal stem cells and the induction of hair follicles during embryogenesis

Corneal epithelium transdifferentiation into a hair-bearing epidermis provides a particularly useful system for studying the possibility that transient amplifying (TA) cells are able to activate different genetic programs in response to a change in their fibroblast environment, as well as to follow the different steps of rebuilding an epidermis from induced stem cells. Corneal stem and TA cells are found in different locations - stem cells at the periphery, in the limbus, and TA cells more central. Moreover, the TA cells already express the differentiating corneal-type keratin pair K3/K12, whereas the limbal keratinocytes express the basal keratin pair K5/K14. In contrast, suprabasal epidermal keratinocytes express keratin pair K1-2/K10, and basal keratinocytes the keratin pair K5/K14. The results of tissue recombination experiments show that adult central corneal cells are able to respond to specific information originating from embryonic dermis. First, the cells located at the base of the corneal epithelium show a decrease in expression of K12 keratin, followed by an increase in K5 expression; they then proliferate and form hair follicles. The first K10 expressing cells appear at the junction of the new hair follicles and the covering corneal epithelium. Their expansion finally gives rise to epidermal strata, which displace the corneal suprabasal keratinocytes. Corneal TA cells can thus be reprogrammed to form epidermal cells, first by reverting to a basal epithelial-type, then to hair pegs and probably concomitantly to hair stem cells. This confirms the role of the hair as the main reservoir of epidermal stem cells and raises the question of the nature of the dermal messages which are both involved in hair induction and stem cell specification.

Requirement for Pax6 in corneal morphogenesis: a role in adhesion

The Pax6 transcription factor functions early during embryogenesis to control key steps in brain, pancreas, olfactory and ocular system development. A requirement for Pax6 in proper formation of lens, iris and retina is well documented. By examining the corneas of heterozygous Small eye (SEY) mice, this report shows that Pax6 is also necessary for normal corneal morphogenesis. In particular, the epithelial component of the postnatal and adult SEY (+/-) cornea is thinner owing to a reduction in the number of cell layers, despite a tenfold increase in the proliferative index and no change in TUNEL labeling. Ultrastructural views revealed large gaps between corneal epithelial cells and a change in the appearance of desmosomes, suggesting that adhesion abnormalities contribute to the corneal phenotype of SEY (+/-) mice. Western blot analysis and immunofluorescence showed equivalent amounts and normal localization of E-cadherin in SEY (+/-) corneas, and the actin cytoskeleton appeared normal as judged by phalloidin staining. By contrast, the levels of desmoglein, beta-catenin and gamma-catenin were reduced in the SEY (+/-) cornea. In addition, the amount of keratin-12 mRNA and protein, the major intermediate filament, was reduced in SEY (+/-) corneal epithelium as shown by in situ hybridization and immunohistochemistry. Finally, the SEY (+/-) corneal epithelium adheres less well than wild-type when challenged with gentle rubbing using a microsponge. In conclusion, our results indicate that cellular adhesion is compromised in the SEY (+/-) corneal epithelium and suggests a role for Pax6 in the proper generation and maintenance of the adult cornea.

Expression of AP-1 (c-fos/c-jun) in developing mouse corneal epithelium

BACKGROUND

Activator protein-1 (AP-1) is a ubiquitous transcription factor which is believed to modulate cell behaviors such as proliferation and differentiation during wound healing and embryonic tissue morphogenesis. AP-1 consists of Fos family and Jun family proteins.

METHODS

We examined expression pattern of c-fos mRNA and c-Fos protein in developing mouse cornea. Expression of c-jun mRNA and c-Jun protein were also examined for comparison.

RESULTS

While no c-fos mRNAs were detected on embryonic day (E) 12.5, mRNA for c-fos was detected from E14.5 until postnatal day (P)14. The mRNAs for c-jun were also detected, although the temporal expression patterns differed. c-Fos-immunoreactive nuclei were present from E14.5 through P10 and c-Jun-immunoreactive nuclei were detected from E14.5 through P3.

CONCLUSION

These findings indicate that AP-1 ( c-fos/c-jun) transcription factor may play a role in the development and maturation of the corneal epithelium in mice.

ECM-stimulated signaling and actin reorganization in embryonic corneal epithelia are Rho dependent

PURPOSE

The goal of this study was to investigate the role of the small guanosine triphosphatase (GTPase), Rho, in the corneal epithelial response to extracellular matrix (ECM) molecules. The avian corneal epithelial model was used to establish that Rho is required for actin reorganization and tyrosine phosphorylation of integrin-mediated signal pathway proteins.

METHODS

Whole embryonic corneal epithelia were isolated without the basal lamina and either transfected with Rho-specific antisense oligonucleotides or treated with Clostridium botulinum C3 exoenzyme and then stimulated with fibronectin (FN) or collagen (COL). The epithelia were evaluated for actin reorganization and protein production including Rho protein levels and tyrosine phosphorylation with Western blot analysis.

RESULTS

After an overnight transient transfection with antisense oligonucleotides, Rho protein levels were decreased more than 80%, and tyrosine phosphorylation of all integrin-mediated signal transduction proteins was decreased compared with control epithelia. Intracellular Rho distribution did not change in the presence of antisense oligonucleotides; however, the amount of immunolabeled Rho decreased. Disrupting the signaling cascade with Rho antisense also blocked FN- and COL-stimulated actin cortical mat reformation. C. botulinum C3 exoenzyme, a pharmacologic agent that specifically causes adenosine diphosphate (ADP) ribosylation and inactivation of Rho, also blocked actin reorganization and tyrosine phosphorylation. In contrast, decreasing Raf protein levels did not change FN-mediated actin reorganization or tyrosine phosphorylation.

CONCLUSIONS

Decreasing Rho protein or blocking its function inhibited ECM-stimulated actin reorganization and signal transduction, as measured by tyrosine phosphorylation.

Clonal analysis of patterns of growth, stem cell activity, and cell movement during the development and maintenance of the murine corneal epithelium

Patterns of growth and cell movement in the developing and adult corneal epithelium were investigated by analysing clonal patches of LacZ-expressing cells in chimeric and X-inactivation mosaic mice. It was found that cell proliferation throughout the basal corneal epithelium during embryogenesis and early postnatal life creates a disordered mosaic pattern of LacZ(+) clones that contrasts with patterns of proliferation and striping produced during the later embryonic stages of retinal pigmented epithelium development. The early mosaic pattern in the corneal epithelium is replaced in the first 12 postnatal weeks by an ordered pattern of radial stripes or sectors that reflects migration without mixing of the progeny of clones of limbal stem cells. In contrast to previous assumptions, it was found that maturation of the activity of limbal stem cells and the pattern of migration of their progeny are delayed for several weeks postnatally. No evidence was found for immigration of the progeny of stem cells until the 5th postnatal week. There are approximately 100 clones of limbal stem cells initially, and clones are lost during postnatal life. Our studies provide a new assay for limbal and corneal defects in mutant mice.

Systematic analysis of E-, N- and P-cadherin expression in mouse eye development

Cadherins are a family of Ca(2+)-dependent cell adhesion molecules. Through their homophilic binding interactions, cadherins play important roles in tissue formation and maintenance during development. Here the authors compare the expression patterns of the three classical cadherins, E-, N- and P-cadherin, during mouse eye development from embryonic day 9.5 (E9.5) to adult. It was found that: (1) The expression patterns of N- and P-cadherin are mutually exclusive in most ocular tissues during development. N-cadherin mRNA is detected specifically in the lens placode during lens induction at E9.5, and is absent in the rest of the surface ectodermal tissues. In contrast, P-cadherin is expressed in the surface ectoderm but not in the lens vesicle. N-cadherin is expressed continuously in the lens pit, lens vesicle, and in the epithelial cells and newly differentiating fiber cells of the mature lens. P-cadherin is expressed in the epithelial cells of the cornea, eyelids and Harderian gland. Reciprocal expression patterns of N-and P-cadherins are also seen during retinal development. N-cadherin is initially expressed in both the inner and outer layers of the optic cup at E9.5. N-cadherin expression persists in the inner layer as it develops into neural retina, but is turned off in the outer layer where the cells differentiate into retinal pigment epithelial (RPE) cells and express P-cadherin. Reciprocal patterns of expression are also seen in the ciliary epithelium. N-cadherin is expressed in the inner layer and P-cadherin in the outer layer of the ciliary epithelium. (2) E- and P-cadherins are epithelial cadherins. Their expression patterns in the eye are not identical. Both cadherins are found in the epithelia of the cornea, eyelid and Harderian gland. In contrast, lens epithelial cells express E- but not P-cadherin, and RPE cells express P- but not E-cadherin. (3) In addition to its high expression in surface ectoderm-derived tissues, E-cadherin mRNA was also detected in some of the retinal ganglion neurons at postnatal day 14 (P14). E-cadherin expression in the neural retina has not been reported before. This study shows that cell fate determination in the eye occurs in conjunction with distinct changes in the patterns of cadherin gene expression.

Proliferation and apoptosis in the epithelium of the developing human cornea and conjunctiva

To determine the distribution of proliferating and apoptotic cells in the human cornea during prenatal and early postnatal development, we examined sections of the bulbar conjunctiva, the limbus as well as the central and peripheral cornea between 11 weeks of gestation and 6 months after birth. The objective was to localize dividing cells by proliferating cell nuclear antigen-like immunoreactivity (PCNA-LI) and apoptotic cells by terminal transferase-mediated nick-end labeling (TUNEL). Before the 17th gestational week, PCNA-LI was absent in all 4 regions examined, indicating negligible cell proliferation during early development. After 20 weeks, strong PCNA-labeling was observed in all regions examined suggestive of high proliferative activity not only in the limbus and the bulbar conjunctiva, but also in the central and peripheral cornea. This rise in proliferative activity was followed by a steady decline: after 28 weeks, anti-PCNA staining gradually disappeared in the central and peripheral cornea, so that, at 6 months after birth, it was confined to the limbus and the bulbar conjunctiva, resembling the picture described for the adult cornea. TUNEL-positive cells were virtually absent in all 4 regions examined before the 38th gestational week. Apoptotic cells only started to appear at 38 weeks; at this stage, they were confined to the bulbar conjunctival epithelium. At 6 months after birth, TUNEL-positive cells were observed in the bulbar conjunctival epithelium and the entire cornea; the limbus, however remained devoid of apoptotic cells throughout the entire prenatal and early postnatal period. The present study for the first time localizes proliferating and apoptotic cells in the epithelium of the developing human cornea. Three stages of development can be distinguished: Minimal proliferation (until 17th week), vigorous proliferation over the entire cornea including the limbus and the bulbar conjunctiva (until 28th week) and gradual decrease in proliferative activity (after 28th week) accompanied by the appearance of apoptotic cells.

Adult corneal epithelium basal cells possess the capacity to activate epidermal, pilosebaceous and sweat gland genetic programs in response to embryonic dermal stimuli

Recent work has shown remarkable plasticity between neural and hematopoeitic, as well as between hematopoeitic and muscle stem cells, depending on environmental stimuli (Fuchs, E. and Segre, J. A. (2000) Cell 100, 143–155). Stem cells give rise to a proliferative transient amplifying population (TA), which is generally considered to be irreversibly committed. Corneal epithelium provides a particularly useful system for studying the ability of TA cells to activate different genetic programs in response to a change in their fibroblast environment. Indeed, corneal stem and TA cells occupy different localities - stem cells at the periphery, and TA cells more central (Lehrer, M. S., Sun, T. T. and Lavker, R. M. (1998) J. Cell Sci. 111, 2867–2875) - and thus can be discretely dissected from each other. It is well known that pluristratified epithelia of cornea and skin display distinct programs of differentiation: corneal keratinocytes express keratin pair K3/K12 and epidermal keratinocytes keratin pair K1-2/K10; moreover, the epidermis forms cutaneous appendages, which express their own set of keratins. In our experiments, central adult rabbit corneal epithelium was thus associated either with a mouse embryonic dorsal, upper-lip or plantar dermis before grafting onto nude mice. Complementary experiments were performed using adult mouse corneal epithelium from the Rosa 26 strain. The origin of the differentiated structures were identified in the first case by Hoechst staining and in the second by the detection of beta-galactosidase activity. The results show that adult central corneal cells are able to respond to specific information originating from embryonic dermis. They give rise first to a new basal stratum, which does not express anymore corneal-type keratins, then to pilosebaceous units, or sweat glands, depending of the dermis, and finally to upper layers expressing epidermal-type keratins. Our results provide the first evidence that a distinct TA cell population can be reprogrammed.

Erk and PI-3 kinase are necessary for collagen binding and actin reorganization in corneal epithelia

PURPOSE

It was recently shown that phosphatidylinositol-(PI)3 kinase is upregulated in wounded rabbit corneal epithelia. Extracellular signal-regulated kinase (erk)-1 and -2 proteins and PI-3 kinase were activated in embryonic corneal epithelia after 1-hour stimulation by type I collagen. In the current investigation specific inhibitors of PI-3 kinase and mitogen-activated kinase-kinase (MEK-1 kinase) were used to determine the role of these signaling molecules in actin reorganization and collagen binding to isolated sheets of corneal epithelial tissue.

METHODS

Effects of specific PI-3 kinase and MEK-1 inhibitors (LY294002, PD98059, respectively) were investigated in embryonic corneal epithelial tissues. Avian embryonic corneal epithelia were isolated as tissue sheets, organ cultured in the presence of these specific inhibitors, and stimulated with type I collagen. The tissues were evaluated for collagen-stimulated actin reorganization, erk-1 and -2 and PI-3 kinase activity, total filamentous actin accumulation, and collagen binding.

RESULTS

The MEK-1 inhibitor PD98059 decreased erk-1 and -2 phosphorylation and blocked actin reorganization in a dose-dependent manner. The PI-3 kinase 85-kDa subunit was decreased 25% in LY294002-treated tissue, and collagen binding also decreased significantly in tissues treated with MEK-1 and PI-3 kinase inhibitors compared with control tissues. In addition, both inhibitors blocked actin cortical mat reorganization. CONCLUSIONS; PI-3 kinase and erk-1 and -2 signaling pathways are activated and necessary for collagen binding and integrin-mediated actin reorganization in embryonic avian corneal epithelium.

Spatiotemporal distribution of zonulae adherens and associated actin bundles in both epithelium and fiber cells during chicken lens development

Zonulae adherens and associated actin bundles (ZA/AB) are believed to play a major role in epithelial folding and invagination during morphogenesis of neural tube and other vesicular structures. The lens morphogenesis is associated with the formation of the lens vesicle in which ZA/AB would be needed during the formation process. However, the existence of ZA/AB in the lens has never been established. In this study we report for the first time the existence of ZA/AB in both lens epithelium and fiber cells during embryonic development of chicken lens from E4 to E20. Light microscopy revealed contacts between the lens epithelium and primary fiber cells, and between the lens epithelium and secondary fiber cells at E4 and E11, respectively. Thin-section electron microscopy consistently revealed ZA/AB near both the apical ends of lens epithelial cells and primary fiber cells at E4. This arrangement manifests as a parallel pair of belt-like ZA/AB along the epithelium-fiber interface. In semi-tangential sections, a continuous belt-like ZA/AB was also evidenced in individual epithelial cells and fiber cells. Furthermore, the same ZA/AB arrangement was observed near both the apical ends of epithelial cells and secondary fiber cells at E11. Besides ZA/AB, macular-type fasciae adherens were distributed regularly between epithelial cells, between primary fibers, between secondary fibers, and between epithelium and both primary and secondary fibers. Immunofluorescence strongly and preferentially labeled N-cadherin at both the apical ends of lens epithelium and primary or secondary fibers at the corresponding ages, suggesting a direct association with the zonulae adherens. Also punctate N-cadherin labeling was commonly seen along various regions of primary and secondary fiber cells at different ages, and to a larger extent in the mature fibers of older lenses. This study suggests that: (1) ZA/AB located at the apices of lens epithelial cells may play a crucial role in the early stages of lens morphogenesis (e.g. lens vesicle formation); (2) ZA/AB of primary and secondary fiber cells originate from the epithelial cells during their elongation and differentiation; (3) owing to the restricted distribution of ZA/AB, abundant fasciae adherens are needed to maintain the structural stability of the epithelium and fiber cells during development and maturation; and (4) N-cadherin is the principle adhesion protein for both the zonulae adherens and fasciae adherens in the lens.

Developmentally regulated appearance of spliced variants of type XII collagen in the cornea

PURPOSE

To determine whether temporal and spatial changes in the distribution of the long and short alternatively spliced variants of type XII collagen are associated with any specific morphogenetic events in pre- and postnatal development of the cornea and surrounding tissues.

METHODS

The distribution of alternatively spliced variants of type XII collagen in fetal and newborn rabbit tissues was analyzed immunohistochemically using monoclonal antibodies that recognize either only the long form or both the short and the long forms of type XII collagen.

RESULTS

During early fetal development of the cornea in rabbit (days 14 -17), the short form of type XII collagen was detected in the corneal stroma, the sclera, and the stroma in the rudimentary eyelid folds, whereas the long form was present only in the sclera. The long form was first evident in the cornea at day 24 but only in the posterior stroma. At later stages of prenatal development, the distribution of the long variant gradually extended toward the anterior stroma and in the newborn rabbit, the long variant was distributed throughout the entire stroma. However, in the eyelid, although the short form was present along the entire subepidermal regions both during fetal and neonatal development, the long form was transiently expressed between days 21 and 24 and was restricted to the subepidermal regions at the junction of the opposing eyelids. The long form of type XII collagen was first detectable in the basal epithelial cells and in its basement membrane (BM) at day 12 after birth, just before the opening of the eyelids. It continued to be present in the corneal BM zone in the adult rabbit but was not present in the limbal or conjunctival BM zone.

CONCLUSIONS

The expression and distribution of the alternatively spliced forms of type XII collagen are developmentally and differentially regulated in the cornea, the sclera, and the eyelid. Although the short form is expressed in the stromal matrices of the cornea and surrounding tissues from early stages of corneal development, the appearance and distribution of the long variant form of type XII collagen coincide with the pattern of stromal condensation. Its first appearance in the corneal epithelial BM precedes the eyelid opening by 1 to 2 days, possibly suggesting that it may be involved in the tighter anchoring of the corneal epithelium to the underlying tissue or in promoting stromal condensation to assist in the separation of the corneal epithelium from the juxtaposed palpebral conjunctival epithelium of the eyelid.

[Lectin-binding histochemical study on developing corneal epithelium and stroma of normal human being]

PURPOSE

To study glycoconjugates in developing cornea of normal human.

METHODS

Lectin-binding histochemical method with five biotin-coupled lectins (conA, LCA, PNA, WGA, RCA).

RESULTS

1. WGA mainly stained the membrane of epithelial cells, especially in surface cells. It also stained anterior stroma of embryo. 2. All the layers of epithelium could be stained by conA, uneven stain was seen in stroma of embryo corneas. 3. RCA, LCA receptor distributed in base of cornea epithelium. 4. PNA can only stain cornea epithelium of 16 embryo.

CONCLUSIONS

1. Glycoconjugates of binding conA, WGA, PNA, LCA, RCA are in the developing corneas of human. 2. The distribution of these lectin receptors may change with corneal developing. It may be related to the maturity of corneas.