Fine structure of bovine lens epithelial cells in vitro in relation to modifications induced by a retinal extract (EDGF)

Polygonal arrays of microfilaments in epithelial cells of the intact lens

Polygonal arrays of microfilaments have been discovered to line the inner apical plasma membrane of anterior epithelial cells of the intact rabbit lens. When tangential sections are studied with the electron microscope, the polygonal arrays are seen to consist of central vertices interconnected by rays of filaments. The rays near the cell periphery insert into the lateral plasma membrane. The vertices are spaced about 1 micron apart and appear to be attached to the apical plasma membrane. The polygonal arrays have little depth as judged by stereo-pairs and are incorporated within the dense band of microfilaments seen in cross-section at the epithelio-fiber junction. The diameter of the filaments and their similarity to actin-containing polygonal arrays described by other investigators in cultured cells suggest that these structures contain actin in lens epithelial cells. The function of the polygonal arrays in relation to maintenance of lens shape or to changes in lens shape in accommodation is discussed.

The role of growth factors in the embryogenesis and differentiation of the eye

The vertebrate eye is composed of a variety of tissues that, embryonically, have their derivation from surface ectoderm, neural ectoderm, neural crest, and mesodermal mesenchyme. During development, these different types of cells are subjected to complex processes of induction and suppressive interactions that bring about their final differentiation and arrangement in the fully formed eye. With the changing concept of ocular development, we present a new perspective on the control of morphogenesis at the cellular and molecular levels by growth factors that include fibroblast growth factors, epidermal growth factor, nerve growth factor, platelet-derived growth factor, transforming growth factors, mesodermal growth factors, transferrin, tumor necrosis factor, neuronotrophic factors, angiogenic factors, and antiangiogenic factors. Growth factors, especially transforming growth factor-beta, have a crucial role in directing the migration and developmental patterns of the cranial neural-crest cells that contribute extensively to the structures of the eye. Some growth factors also exert an effect on the developing ocular tissues by influencing the synthesis and degradation of the extracellular matrix. The mRNAs for the growth factors that are involved in the earliest aspects of the growth and differentiation of the fertilized egg are supplied from maternal sources until embryonic tissues are able to synthesize them. Subsequently, the developing eye tissues are exposed to both endogenous and exogenous growth factors that are derived from nonocular tissues as well as from embryonic fluids and the systemic circulation. The early interaction between the surface head ectoderm and the underlying chordamesoderm confers a lens-forming bias on the ectoderm; later, the optic vesicle elicits the final phase of determination and enhances differentiation by the lens. After the blood-ocular barrier is established, the internal milieu of the eye is controlled by the interactions among the intraocular tissues; only those growth factors that selectively cross the barrier or that are synthesized by the ocular tissues can influence further development and differentiation of the cells. An understanding of the tissue interactions that are regulated by growth factors could clarify the precise mechanism of normal and abnormal ocular development.

Development of actin polygonal arrays in rabbit lens epithelial cells

In searching for a clue to the role of actin filament bundles organized into polygonal arrays, or geodomes, in lens epithelial cells, we examined several physical events occurring in the young rabbit lens which may initiate their formation. We used NZW rabbits between the ages of 24 days gestation and 50 days postnatal. Data were obtained from TEM, SEM and fluorescence microscopy. Parameters measured were lens weights, apical surface areas of cells in epithelial whole mounts, epithelial cell thickness, and timing of eyelid opening, breakdown of the tunica vasculosa lentis (TVL) and formation of the ciliary zonules; these findings were correlated with the first signs of development of the arrays. Polygonal arrays formed slowly beginning at one to two days after birth, and with advancing time these thickened and made more numerous connections with the lateral plasma membranes. Development of the arrays was not correlated with onset of vision or disappearance of the TVL or a sudden increase in cell area, since these events occur postnatally at about 9-10 days, nor with the development of zonular fibers since these are already in place at 24 days of gestation. Only lens weights showed a dramatic increase between 24 days gestation and birth. It is surmised that the expanding lens mass may be involved in some way in signaling the organization of actin filaments into geodomes.

Appearance of alpha-smooth muscle actin in human eye lens cells of anterior capsular cataract and in cultured bovine lens-forming cells

Using light and electron-microscopic immunolocalization techniques, and gel electrophoresis combined with immunoblotting, we have examined the expression of cytoskeletal proteins in normal human fetal, child and adult lenses, in human anterior capsular cataract and in bovine lens cells in vivo and in vitro. In this report, we focus our observations on the pattern of actin-isoform expression during normal and pathological situations in vivo and culture conditions. We have noted that cells of developing and mature human lenses as well as bovine lens cells in situ contain only beta- and gamma-actins. In contrast, alpha-smooth muscle (alpha-sm) actin, an isoform typical of smooth muscle differentiation, was demonstrated in bovine lens cells at different times of culture. Moreover, the multilayered cells observed in the subcapsular zone of human anterior capsular cataract were characterized by the presence of alpha-sm actin. Thus, extensive changes in actin-isoform expression take place in lens cells growing in culture and may also occur during cataractogenesis. The biological meaning of the appearance of a marker of myoid differentiation in the ectodermally derived lens-forming cells is discussed.

Structural analysis of lens epithelial explants induced to differentiate into fibres by fibroblast growth factor (FGF)

Recently we identified fibroblast growth factor (FGF), which is present in significant amounts in neural retinas, as a potent inducer of lens fibre differentiation in our epithelial explant cultures. Fibre differentiation was assessed by synthesis of fibre specific, proteins, beta- and gamma-crystallins, and by cell elongation. However, to establish whether FGF induced the dramatic structural changes characteristic of fibre differentiation we carried out an ultrastructural analysis. In this study epithelial explants exposed to either the acidic or basic form of FGF were shown to undergo the structural changes characteristic of fibre differentiation in the intact lens. These include: (i) cell elongation, (ii) a reduction in cytoplasmic organelles, (iii) the formation of specialized cell-cell junctions, including finger-like processes and fingerprints, ball and socket junctions, tongue-like flaps and imprints, and gap junctions, and (iv) nuclear pyknosis. This shows that FGF faithfully reproduces structural events associated with fibre differentiation as well as the molecular events reported previously, thus providing further evidence that FGF in the eye is important for the control of normal lens fibre differentiation.

Comparative study of actin filament patterns in lens epithelial cells. Are these determined by the mechanisms of lens accommodation?

Actin filament patterns in lens epithelia from animals of various taxonomic groups were studied using rhodamine phalloidin fluorescence microscopy of epithelial whole mounts and transmission electron microscopy of tangential sections. The results were compared with the accommodative mechanism operating in each case as reported in the literature. Lenses that accommodate by deformation of the anterior surface, in squirrel, chipmunk, rabbit, monkey and human, showed polygonal arrays (PAs) at the apical end of the epithelial cells. Lenses that translate as a whole, in shark, bony fish and frog, showed stress fibers (SFs) at the basal or apical end of the cells. No specialized actin pattern was seen in turtle and bird, which have lenses that are squeezed into an anterior lenticonus; cat, where the lens is translated forward; or rat, cow and most mice, which have no defined accommodation. In exception, certain strains of laboratory mice did show sequestered actin bundles (SABs) and/or PAs. Based on our findings, we conclude that PAs, which resemble geodesic domes, do not take an active part in near-point accommodation; but like SFs, may serve to resist overextension by internal pressure of the fiber mass or by zonular tension.

Differential assemblage of the basal membrane-cytoskeleton complex in bovine epithelial lens cells

We have investigated the membrane-cytoskeleton complex involved in interactions between the epithelial cells and the capsule of the bovine eye lens. The organization of the molecular complex was determined from cell extraction, immunoprecipitation and immunoblotting experiments and from ultrastructural studies by scanning electron microscopy. We show that marked differences exist in the organization of this basal complex between the central epithelium (mitotic quiescent) and the peripheral epithelium which initiates lenticular differentiation into fibres. Our results support the view that: (a) the organization of several major membrane components in the peripheral epithelium differs from that of the central epithelium; (b) microfilaments and vimentin filaments are independent of each other in the peripheral epithelium, whereas microfilaments are involved in an inter-relationship with vimentin filaments in the central epithelium; (c) two surface proteins of 24 and 27 kD and two surface glycoproteins of 46 and 220 kD appear to be bound to vimentin filaments in the peripheral region, whereas the intermediate filaments appear to be solely in close association with the 46 and 220 kD glycoproteins in the central zone.

Immunogold-EM localization of actin and vimentin filaments in relation to polygonal arrays in lens epithelium in situ

An indirect immunogold technique for transmission electron microscopy was used for localizing two cytoskeletal proteins, actin and vimentin, in the epithelium of freshly removed rabbit lens, especially in relation to the polygonal array structures located at the apices of the epithelial cells. Antibody specificity was determined on semi-pure chicken breast muscle actin and bovine lens vimentin using Western blotting of these proteins and extracts of rabbit lens epithelium separated by SDS-PAGE. Whole lenses of rabbits were lightly fixed in glutaraldehyde and embedded in LR White resin. Tangential sections were taken at 70 to 80 nm and at 0.25 micron and used for single-labeling, and double-labeling with antibodies raised in different hosts and treated with appropriate second antibodies conjugated with non-overlapping sizes of gold particles. Routine and stereomicroscopy were used to analyze gold-label patterns. The study shows that the rays of the polygons project deeply into the cell from the vertices lying on the inner apical membrane. Actin is located on the filaments of rays, but vimentin is not associated with the polygons at the level in the cell that we studied. Vimentin filaments are found in deeper regions of the epithelial cell. Stereopairs were useful in differentiating where the gold-label was located and in fact, this technique demonstrated that most of the label is on the surface of sections where the filaments are exposed.

Actin filament patterns in mouse lens epithelium: a study of the effects of aging, injury, and genetics

Using mainly fluorescence microscopy after rhodamine-phalloidin staining, the F-actin distribution in the mouse lens epithelium was studied with regard to the effects of age, genetic strain, and mechanical injury. These studies have revealed that aside from its association with the plasma membrane the structural organization of F-actin in the mouse lens epithelium in situ is characterized by two major configurations: (1) a filamentous arrangement in such patterns as stress fibers, polygonal arrays (PAs), and meshworks, and (2) a highly concentrated structure called a sequestered actin bundle (SAB). The aging study indicated that the SAB is a consistent character in C57BL/6 mice from the age of 5 wk on, but not in CF1 mice. The size and shape of the SAB change gradually with age as inferred from two-dimensional measurements. The genetic study on the SAB character using hybrids and congenic strains showed that it is inherited as a Mendelian dominant, probably multigenic mode. Finally, the injury study revealed a structural modification in cells around the wound, including flattening of cells at the edge and extension of processes into the wound space. In the rest of the epithelium, injury amplified membrane infolding and fluorescence of polygonal arrays but diminished the size and fluorescence intensity of SABs. These changes are thought to be correlated with wound repair involving cell division and migration. These studies illustrate the variability in F-actin expression in situ in lens epithelial cells that can be induced by intrinsic and extrinsic factors.

Diabetic retinopathy. Pathogenesis and the role of retina-derived growth factor in angiogenesis

Diabetic retinopathy results from a combination of systemic and ocular abnormalities. Vasodilation, basement membrane pathology, microaneurysms, abnormal blood flow and tissue oxygenation, connective tissue abnormalities, and retinal ischemia are all components of early diabetic retinopathy. The pathogenesis of neovascularization is discussed with respect to the effects of vasodilation, vascular leakage, vitreous changes, and retinal ischemia. The evidence supporting Michaelson's hypothesis that a chemical messenger from the retina provides the stimulus for neovascularization is cited. The sequence of events involved in angiogenesis are cellular and basement membrane changes, endothelial cell migration, endothelial cell proliferation, and vessel formation. The experimental evidence in support of a role for retina-derived growth factor as a mediator of these cellular events is reviewed.

Actin in polygonal arrays of microfilaments and sequestered actin bundles (SABs) in lens epithelial cells of rabbits and mice

The pattern of localization of actin filaments was compared in whole mounts of lens epithelium of rabbit and mouse using the fluorescently-labeled actin-specific probe, rhodamine-phalloidin. In the adult rabbit lens, fluorescent polygonal arrays consisting of central vertices and interconnecting filaments were present in the apical end of each epithelial cell. Electron microscopy confirmed that these arrays lined the cytoplasmic side of the apical membrane. In the mature adult mouse, polygonal arrays were not seen either with fluorescence or electron microscopy. Instead, the actin was packaged in a single, elongated, often curved bundle near the epithelial cell apex, referred to as a "sequestered actin bundle" or SAB. The SAB often appeared attached to the plasma membrane and to approach the perinuclear basket of microfilaments. The significance of the differences in these two patterns of actin is discussed in terms of differences in the accommodative ability and static lens shape in these two animals.

In situ localization of S1-labeled actin filaments in chick lens epithelial cells

The actin filaments in the lens epithelial cells of three-day and eight-day post-hatched chicks have been labeled in situ with myosin subfragment 1 (S1). Labeling was accomplished by injuring the lens transcorneally with an ultramicroneedle 5 min or 24 hr before detergent treatment and incubation in S1. A band of filaments found at the epithelio-fiber junction in normal, uninjured chick lens is labeled in the 5 min and 24 hr injury. A subcapsular labeled band is found only in the 24 hr injury, and may be the result of a healing process.

Interaction of bovine epithelial lens (BEL) cells with extracellular matrix (ECM) and eye-derived growth factor (EDGF). III. Control of glycoprotein and proteoglycan synthesis

Bovine epithelial lens (BEL) cells, cultured in the presence or absence of an eye-derived growth factor (EDGF) on plastic surfaces or on extracellular matrix (ECM) laid down by corneal endothelial cells, were metabolically labelled with [3H] glucosamine and with [35S]O2+4. The newly formed glycoproteins and proteoglycans were determined and studied in the extracellular compartment. EDGF reduces radiosulfate incorporation and provides the formation of low molecular weight (LMW) proteoglycans and glycosaminoglycans. [3H]Glucosamine incorporation into the glycoprotein fractions is affected by EDGF and by ECM, but in a different manner. EDGF and ECM reduce the formation of the glycoproteins with an affinity towards the collagen, but the effect of EDGF and ECM on the other glycoproteins is different. It is suggested that the actions of EDGF and of ECM are at least partially different and mutually independent.

Interaction of bovine epithelial lens (BEL) cells with extracellular matrix (ECM) and eye-derived growth factor (EDGF). I. Effects on short-term adhesiveness and on long-term organization of the culture

A growth factor (EDGF) derived from the retina controls the proliferation and shape of adult bovine epithelial lens (BEL) cells in vitro as well as extracellular matrix (ECM) assembly. In order to analyse this mechanism and the specificity of the interactions between BEL cells and the extracellular matrix we have investigated the adhesion and growth of BEL cells on various substrata (fibronectin, laminin, ECM). BEL cells treated with EDGF adhered more slowly to plastic Petri dishes than untreated cells, in part due to EDGF inhibition of fibronectin deposition. The untreated BEL cells spread less well on ECM or laminin than on fibronectin-coated plastic. The preferential adhesiveness of BEL cells on fibronectin vs laminin was confirmed by attachment experiments performed on replicas of SDS-PAGE of these proteins. However, in long-term cultures, 8 days after seeding, BEL cells were very differently arranged on plastic or on ECM. ECM by itself did not increase the proliferation rate but helped to restore an organized cell monolayer. BEL cells stimulated to grow on ECM by treatment with EDGF exhibited at least transiently contact inhibition producing a perfectly organized epithelium similar to the one observed in vivo. These results suggest specific interactions between ECM or ECM components with BEL cell that restrain excessive cell spreading and restore an original polarized phenotype of the cells seen in vivo.

State of differentiation of bovine epithelial lens cells in vitro. Relationship between the variation of the cell shape and the synthesis of crystallins

Correlations between cell morphology and the expression of specific proteins (crystallins) have been investigated. Two different culture conditions have been chosen which keep bovine epithelial lens cells (BEL cells) in a monolayer of homogeneous epithelioid cells: (1) bovine retinal extract (EDGF) supplemented medium; (2) extracellular matrix (ECM) provided by corneal endothelial cells in standard medium has been compared to previous results obtained with BEL cells cultivated on plastic (Simonneau et al., 1983). Variations of the cell shape had no effect upon crystallin synthesis.

Ultrastructure of trypan blue induced ocular defects: I. Retina and lens

The histological and cytological basis of trypan blue-induced ocular defects were studied using scanning and transmission electron microscopy. Microphthalmic and anophthalmic eyes of 16-day rat fetuses were utilized from dams exposed to a teratogenic dose of trypan blue. Retinal and lenticular anlagen were specifically examined for architectural and cellular changes. Nearly all severely abnormal eyes showed no evidence of retina development: Of 41 such eyes, only two retinal rudiments were observed. Those eyes with mild microphthalmia always demonstrated retinae although architectural changes were present. In every abnormal eye, some degree of lenticular morphogenesis was always present. Lenses were small, displaced in the eye field, and arrested at the lens vesicle stage. Lens cells were markedly undifferentiated and thus lacked most of the cytological features normally present at this developmental stage. Neither retinal nor lenticular rudiments were necrotic despite major architectural and cytological disturbances. The data offer three conclusions: First, the absence of necrosis suggests that trypan blue causes developmental arrest in this eye model; second, absence of retinae is most likely due to primary failure of optic vesicle development; third, lack of lens differentiation is attributed to absence of the retina, the primary lens inducer.