DNA synthesis during the development of the chick cornea

Developmental Changes in Patterns of Distribution of Fibronectin and Tenascin-C in the Chicken Cornea: Evidence for Distinct and Independent Functions during Corneal Development and Morphogenesis

The cornea forms the tough and transparent anterior part of the eye and by accurate shaping forms the major refractive element for vision. Its largest component is the stroma, a dense collagenous connective tissue positioned between the epithelium and the endothelium. In chicken embryos, the stroma initially develops as the primary stroma secreted by the epithelium, which is then invaded by migratory neural crest cells. These cells secrete an organised multi-lamellar collagenous extracellular matrix (ECM), becoming keratocytes. Within individual lamellae, collagen fibrils are parallel and orientated approximately orthogonally in adjacent lamellae. In addition to collagens and associated small proteoglycans, the ECM contains the multifunctional adhesive glycoproteins fibronectin and tenascin-C. We show in embryonic chicken corneas that fibronectin is present but is essentially unstructured in the primary stroma before cell migration and develops as strands linking migrating cells as they enter, maintaining their relative positions as they populate the stroma. Fibronectin also becomes prominent in the epithelial basement membrane, from which fibronectin strings penetrate into the stromal lamellar ECM at right angles. These are present throughout embryonic development but are absent in adults. Stromal cells associate with the strings. Since the epithelial basement membrane is the anterior stromal boundary, strings may be used by stromal cells to determine their relative anterior-posterior positions. Tenascin-C is organised differently, initially as an amorphous layer above the endothelium and subsequently extending anteriorly and organising into a 3D mesh when the stromal cells arrive, enclosing them. It continues to shift anteriorly in development, disappearing posteriorly, and finally becoming prominent in Bowman's layer beneath the epithelium. The similarity of tenascin-C and collagen organisation suggests that it may link cells to collagen, allowing cells to control and organise the developing ECM architecture. Fibronectin and tenascin-C have complementary roles in cell migration, with the former being adhesive and the latter being antiadhesive and able to displace cells from their adhesion to fibronectin. Thus, in addition to the potential for associations between cells and the ECM, the two could be involved in controlling migration and adhesion and subsequent keratocyte differentiation. Despite the similarities in structure and binding capabilities of the two glycoproteins and the fact that they occupy similar regions of the developing stroma, there is little colocalisation, demonstrating their distinctive roles.

Insulin and IGF-2 support rat corneal endothelial cell growth and wound repair in the organ cultured tissue

The ability of insulin and IGF-2 to support wound repair in the organ-cultured rat corneal endothelium was investigated. Corneas given a circular transcorneal freeze injury, were explanted into organ cultures containing either insulin or IGF-2 and cultured up to72 h. Both factors increased [³H]-thymidine incorporation and mitotic levels compared to controls. Insulin's ability to mediate wound closure without serum was dependent on its continuous presence in the medium. PKC was also investigated in endothelial repair using the PKC promoter phorbol 12-myristate 13-acetate (PMA). Concentrations between 10^-6 and 10^-8 M, PMA failed to accelerate wound closure. When injured endothelia were cultured in the presence of insulin and the PKC inhibitor H-7, wound closure was also unaffected. These results indicate that insulin and IGF-2 stimulate cell growth in injured rat corneal endothelium and that insulin without the benefit of serum promotes wound closure in situ independent of the PKC pathway.

Proliferative capacity of corneal endothelial cells

The corneal endothelial monolayer helps maintain corneal transparency through its barrier and ionic "pump" functions. This transparency function can become compromised, resulting in a critical loss in endothelial cell density (ECD), corneal edema, bullous keratopathy, and loss of visual acuity. Although penetrating keratoplasty and various forms of endothelial keratoplasty are capable of restoring corneal clarity, they can also have complications requiring re-grafting or other treatments. With the increasing worldwide shortage of donor corneas to be used for keratoplasty, there is a greater need to find new therapies to restore corneal clarity that is lost due to endothelial dysfunction. As a result, researchers have been exploring alternative approaches that could result in the in vivo induction of transient corneal endothelial cell division or the in vitro expansion of healthy endothelial cells for corneal bioengineering as treatments to increase ECD and restore visual acuity. This review presents current information regarding the ability of human corneal endothelial cells (HCEC) to divide as a basis for the development of new therapies. Information will be presented on the positive and negative regulation of the cell cycle as background for the studies to be discussed. Results of studies exploring the proliferative capacity of HCEC will be presented and specific conditions that affect the ability of HCEC to divide will be discussed. Methods that have been tested to induce transient proliferation of HCEC will also be presented. This review will discuss the effect of donor age and endothelial topography on relative proliferative capacity of HCEC, as well as explore the role of nuclear oxidative DNA damage in decreasing the relative proliferative capacity of HCEC. Finally, potential new research directions will be discussed that could take advantage of and/or improve the proliferative capacity of these physiologically important cells in order to develop new treatments to restore corneal clarity.

The effects of soybean agglutinin binding on the corneal endothelium and the re-establishment of an intact monolayer following injury--a short review

This short review summarizes the localization and effects of the plant lectin soybean agglutinin (SBA) on the injured and non-injured organ-cultured rat corneal endothelium. Although the tissue exists as a non-cycling monolayer on the posterior corneal surface a circular freeze injury promotes wound repair as cells initiate DNA synthesis, mitosis and migration. As a result, by 24 h post-injury, endothelial cells express a surface protein that binds SBA in a diffuse punctate pattern, which by 48 h after injury, becomes confined to the cell periphery. As healing proceeds, SBA binding dramatically declines, such that, only scattered binding is observed by 72 h after wounding. In non-injured organ-cultured endothelia, weak SBA binding appears 24 h after explanation but becomes prominently detected around the cell periphery by 48 h. Incubating injured or non-injured endothelia in SBA leads to alterations in their cellular appearance due to the fact that lectin exposure results in the disruption of the actin cytoskeleton. Although this does not affect migration, treatment with either SBA or N-acetylgalactosamine (the SBA binding sugar) does interfere with the reestablishment of cell-cell contact. It is postulated that the surface protein that binds SBA is expressed during conditions that are stressful to the tissue. During organ-culture the protein's appearance suggests a cellular response to explantation in order to enhance or maintain monolayer integrity. In wound repair its appearance may serve to establish preliminary cell-cell contact during the restoration of the endothelial monolayer.

Cell cycle status in human corneal endothelium

Corneal endothelium is the single-cell layer that forms a physical barrier between the corneal stroma and aqueous humour. The barrier and ionic 'pump' functions of corneal endothelium help regulate stromal hydration. Loss of endothelial cells due to increasing age, trauma, disease, dystrophy, or previous corneal transplants can reduce the density of endothelial cells to a critical point below which the stroma becomes edematous and visual acuity is lost. Throughout life, division of endothelial cells either does not occur or occurs at a rate too slow to adequately replace dead cells. Thus, the major means of repairing the monolayer is by cell migration and/or enlargement. The basis for the lack of endothelial cell proliferation is not yet fully understood, although it is clear that cells do retain proliferative capacity. Previous studies from this laboratory have identified certain environmental conditions that may be responsible for maintaining these cells in a non-replicative state in vivo. In addition, corneal endothelial cells exhibit intrinsic, age-related differences in relative proliferative capacity. The studies described below provide evidence to support the hypothesis that, with age, an increasing number of HCEC enter a replicative senescence-like state in which they become increasingly refractive to mitogenic stimulation. This decreasing sensitivity to mitogens appears to be mediated, at least in part, by age-dependent alterations in the relative expression and activity of the cyclin-dependent kinase inhibitors, p27KIP1, p16INK4A, and p21CIP1.

Association of type XII collagen with regions of increased stability and keratocyte density in the cornea

The anterior avian cornea possesses several distinct cellular and extracellular regions including the epithelial basal lamina, Bowman's layer and the interfacial matrix that separates Bowman's layer from the stroma. These unique regions differ biochemically, physically and morphologically but all contain type XII collagen. Previously, the collagen fibrils of several of these interfacial regions were shown to be stable to thermal and enzymatic denaturation. We reasoned that type XII collagen, a fibril-associated collagen, would be a good candidate to confer such stabilizing properties. The studies described herein were performed to localize type XII collagen and to assess its role in the interfacial matrices (IM). Using antibodies that react with both the short and long type XII collagen isoforms and that react specifically with the long isoform, we demonstrate that it is the short isoform that is present in Bowman's layer and the associated interfacial matrix lying between Bowman's and the stroma proper. In situ hybridization analyses demonstrate that both the epithelial and endothelial cells synthesize type XII collagen. In vitro cell culture analyses, however, demonstrate that in addition to epithelial cell synthesis, the stromal fibroblasts are capable of synthesizing type XII collagen as well. Immunofluorescence analyses performed at elevated temperature demonstrate that type XII collagen is thermally stable in Bowman's layer, but not in the anterior interfacial matrix or Descemet's layer. In addition, we observed that the distribution of type XII collagen during the development of the anterior extracellular matrices correlates precisely with an elevated density of keratocytes populating the interfacial matrix just deep to Bowman's layer. We show that this cellular density is developmentally regulated and does not arise from a localized increase in cell proliferation. These data demonstrate that Bowman's layer and the anterior interfacial matrix have unique biochemical and morphologic properties. Type XII collagen is thermally stable in Bowman's layer and, as a surface component of type I collagen fibrils, may contribute to the stability of the fibrils in this region. Neither type XII nor type I collagen is stable in the adjacent interfacial matrix, suggesting that differences in the type I-XII collagen fibril organization may exist between Bowman's layer and IM.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

Lectin binding to injured corneal endothelium mimics patterns observed during development

Fluorochrome conjugated lectins were used to observe cell surface changes in the corneal endothelium during wound repair in the adult rat and during normal fetal development. Fluorescence microscopy of non-injured adult corneal endothelia incubated in wheat-germ agglutinin (WGA), Concanavalin A (Con A), and Ricinus communis agglutinin I (RCA), revealed that these lectins bound to cell surfaces. Conversely, binding was not observed for either Griffonia simplicifolia I (GS-I), soybean agglutinin (SBA) or Ulex europaeus agglutinin (UEA). Twenty-four hours after a circular freeze injury, endothelial cells surrounding the wound demonstrated decreased binding for WGA and Con A, whereas, RCA binding appeared reduced but centrally clustered on the apical cell surface. Furthermore, SBA now bound to endothelial cells adjacent to the wound area, but not to cells near the tissue periphery. Neither GS-I nor UEA exhibited any binding to injured tissue. By 48 h post-injury, the wound area repopulates and endothelial cells begin reestablishing the monolayer. These cells now exhibit increased binding for WGA, especially along regions of cell-to-cell contact, whereas, Con A, RCA and SBA binding patterns remain unchanged. Seventy-two hours after injury, the monolayer is well organized with WGA, Con A and RCA binding patterns becoming similar to those observed for non-injured tissue. However, at this time, SBA binding decreases dramatically. By 1 week post-injury, binding patterns for WGA, ConA and RCA closely resemble their non-injured counterparts while SBA continues to demonstrate low levels of binding. In early stages of its development, the endothelium actively proliferates and morphologically resembles adult tissue during wound repair.(ABSTRACT TRUNCATED AT 250 WORDS)

Keratan sulfate proteoglycan during embryonic development of the chicken cornea

Antibodies to corneal keratan sulfate proteoglycan (KSPG) were used to characterize the pattern of KSPG accumulation during differentiation of neural crest cells in the stroma of embryonic chick cornea. Immunohistochemistry with monoclonal antibody I22 to keratan sulfate found this KSPG antigen localized inside stromal cells at stage 29 (Day 6), ca. 12 hr after migration into the primary stroma. A 2- to 3-day lag then occurred before appearance of extracellular keratan sulfate, first seen on Day 9 (Stage 35) in the posterior stroma. Keratan sulfate antigen accumulated in a posterior to anterior direction during subsequent development. Uniform staining of the stroma for keratan sulfate did not occur until after Day 16. Among several tissues, only corneal stroma contained an extracellular matrix which stained for keratan sulfate, though intracellular staining of some cartilage cells was observed. Accumulation of KSPG antigens in developing cornea was measured in unfractionated guanidine extracts with a quantitative ELISA using three different antibodies against KSPG. Increases were first detected after Day 9 using monoclonal I22, and somewhat later with the other two antibodies. Assays with all three antibodies detected a sustained, exponential increase of KSPG throughout the 5 days prior to hatching. Keratan sulfate continued to accumulate after hatching, but an antibody with specificity to KSPG core protein, detected no relative increase in antigen after hatching. This suggests a modulation of KSPG primary structure late in development and after hatching. Overt differentiation of individual neural crest cells thus appears to begin ca. 12 hr after their arrival in the primary stroma; a lag of 2-3 days precedes active secretion of KSPG.

Change with age in murine corneal epithelial actin and myosin: immunofluorescent and ELISA analyses

The purpose of this study was to use immunofluorescence and ELISA immunoassay to determine whether the cellular distribution and concentration of corneal epithelial actin and myosin change with chronologic age. Diffuse anti-actin and anti-myosin indirect immunofluorescence was observed within the cytoplasm of the corneal epithelium from mice aged postnatal day (PND) 1-18 months. Additionally, highly fluorescent punctate foci were first observed in cortical cytoplasm consistently for both anti-actin and anti-myosin at PND 14. This fluorescent pattern remained relatively unchanged for the remaining ages examined. An enzyme-linked immunosorbent assay (ELISA) method was used to quantitate the amount of actin and myosin in corneal epithelium from mice aged PND 1 to 24 months. Corneal epithelial sheets were removed from whole eyes and processed for ELISA assay. Actin cellular concentration increased from PND 1-7 and decreased from PND 7-16. These results were statistically significant (p less than .005). No statistically significant difference in actin concentration was found for any of the remaining ages examined (PND 16-24 months). Myosin concentration increased from PND 1-7 and decreased until PND 14. These results also were statistically significant (p less than .005 and p less than .005, respectively). There was no significant change in myosin concentration for any of the remaining ages examined (PND 16-24 months).

Monoclonal antibodies against developmentally regulated corneal antigens

The chick cornea is comprised of three cellular layers, each associated with a discrete extracellular matrix. The absence of specific markers for these cellular and acellular components has made it difficult to investigate the cell-cell and cell-matrix interactions which occur during development of this organ. We have approached this problem by producing monoclonal antibodies to species-specific, developmentally regulated antigens of the chick cornea. By immunofluorescence staining patterns the antibodies fall into three distinct groups. One group is directed against the corneal extracellular matrix. At 9 days of embryonic development staining by these antibodies is detected at the endothelial surface (in Descemet's membrane), and in the posterior part of the stroma. During development it progresses anteriorly throughout the entire width of the corneal stroma and Bowman's membrane until, by 14 days, it is found in all three specialized extracellular matrices of the cornea. Throughout most of development these antibodies do not recognize any other ocular or nonocular tissue examined. Late in development they begin to lightly stain nerve bundles. A second group of antibodies is highly selective for the corneal epithelial cell layer. These begin to stain at 12 to 13 days of development and cause very bright fluorescence by 14 days. A third group stains the extracellular matrix of the cornea in a manner spatially and temporally identical to that of the first group, but in addition recognizes certain basement membranes. The possible relationship of the antigens recognized by these groups of antibodies to developmental events occurring at the time of their appearance, and the potential use of all three antibody groups in studying corneal development are discussed.

The association between prolyl hydroxylase metabolism and cell growth in cultured L-929 fibroblasts

Prolyl 4-hydroxylase (EC 1.14.11.2) is a key enzyme in collagen biosynthesis, its active form is a tetramer (alpha 2 beta 2). In L-929 fibroblasts in the log phase of culture there is a low level of active enzyme. When the cell culture reaches confluency, prolyl hydroxylase activity in cells increases by a process that requires de novo RNA and protein synthesis. The same result may be achieved by crowding the cells (replating log phase cells at the density of stationary phase cells). In the work reported here we further examined induction of the enzyme. RNA synthesis necessary for enzyme induction is complete 6 h after "crowding" while protein synthesis requires 12 h. Thymidine (0.2-0.5 mM) added to log phase cells will also cause enzyme induction to the level found in "crowded" or resting cells. We also looked at the decay of the enzyme activity after subculture. This occurs rapidly (enzyme half-life is 1-2 h) and is concurrent with the re-entry of resting cells into cell cycle; however, thymidine added at the time of subculture to block DNA synthesis does not prevent the loss of prolyl hydroxylase activity. These results suggest that when cells are not engaged in propagation, they begin to synthesize luxury proteins such as prolyl hydroxylase. However, the loss of prolyl hydroxylase during subculture is probably not a direct consequence of DNA synthesis.

Corneal morphogenesis in the Indian garden lizard, Calotes versicolor (agamidae)

The present study traces corneal morphogenesis in a reptile, the lizard Calotes versicolor, from the lens placode stage (stage 24) until hatching (stage 42), and in the adult. The corneal epithelium separates from the lens placode as a double layer of peridermal and basal cells and remains bilayered throughout development and in the adult. Between stages 32- and 33+, the corneal epithelium is apposed to the lens, and limbic mesodermal cells migrate between the basement membrane of the epithelium and the lens capsule to form a monolayered corneal endothelium. Soon thereafter a matrix of amorphous ground substance and fine collagen fibrils, the presumptive stroma, is seen between the epithelium and the endothelium. Just before stage 34 a new set of limbic mesodermal cells, the keratocytes, migrate into the presumptive stroma. Migrating limbic mesodermal cells, both endothelial cells and keratocytes, use the basement membrane of the epithelium as substratum. Keratocytes may form up to six cell layers at stage 37, but in the adult stroma they form only one or two cell layers. The keratocytes sysnthesize collagen, which aggregates as fibrils and fibers organized in lamellae. The lamellae become condensed as dense collagen layers subepithelially or become compactly organized into a feltwork structure in the rest of the stroma. The basement membrane of the endothelium is always thin. Thickness of the entire cornea increases up to stage 38 and decreases thereafter until stage 41. In the adult the cornea is again nearly as thick as at stage 38.