Monoclonal antibodies against developmentally regulated corneal antigens

Phosphorylation regulates the ferritoid-ferritin interaction and nuclear transport

Ferritin is an iron-sequestering protein that is generally cytoplasmic; however, our previous studies have shown that in avian corneal epithelial (CE) cells ferritin is nuclear. We have also observed that this nuclear localization involves a tissue-specific nuclear transporter that we have termed ferritoid, and that nuclear ferritin protects DNA from oxidative damage. Recently we have determined that ferritoid functions not only as a nuclear transporter, but also, within the nucleus, it remains associated with ferritin as a heteropolymeric complex. This ferritoid-ferritin complex has unique properties such as being half the size of a typical ferritin molecule and showing preferential binding to DNA. It is likely that the association between ferritoid and ferritin is involved both in the nuclear transport of ferritin and in determining certain of the properties of the complex; therefore, we have been examining the mechanisms involved in regulating the association of these two components. As the ferritoid sequence contains six putative phosphorylation sites, we have examined here whether phosphorylation is one such mechanism. We have determined that ferritoid in the nuclear ferritoid-ferritin complexes is phosphorylated, and that inhibition of this phosphorylation, using inhibitors of PKC, prevents its interaction with ferritin. Furthermore, in an experimental model system in which the nuclear transport of ferritin normally occurs (i.e., the co-transfection of COS-1 cells with full length constructs for ferritin and ferritoid), when phosphorylation sites in ferritoid are mutated, the interaction between ferritoid and ferritin is inhibited, as is the nuclear transport of ferritin.

Developmental regulation of the nuclear ferritoid-ferritin complex of avian corneal epithelial cells: roles of systemic factors and thyroxine

Previously we observed that avian corneal epithelial cells protect their DNA from oxidative damage by having the iron-sequestering molecule ferritin - normally cytoplasmic - in a nuclear location. This localization involves a developmentally-regulated ferritin-like protein - ferritoid - that initially serves as the nuclear transporter, and then as a component of a ferritoid-ferritin complex that is half the size of a typical ferritin and binds to DNA. We also observed that developmentally, the synthesis of ferritin and ferritoid are regulated coordinately - with ferritin being predominantly translational and ferritoid transcriptional. In the present study we examined whether the mechanism(s) involved in this regulation reside within the cornea itself, or alternatively involve a systemic factor(s). For this, we explanted embryonic corneas of one age to the chorioallantoic membrane (CAM) of host embryos of a different age - all prior to the initiation of ferritin synthesis. Consistent with systemic regulation, the explants initiated the synthesis of both ferritin and ferritoid in concert with that of the host. We then examined whether this systemic regulation might involve thyroxine - a hormone with broad developmental effects. Employing corneal organ cultures, we observed that thyroxine initiated the synthesis of both components in a manner similar to that which occurs in vivo (i.e. ferritin was translational and ferritoid transcriptional).

Nuclear ferritin: a ferritoid-ferritin complex in corneal epithelial cells

PURPOSE

Ferritin is an iron storage protein that is generally cytoplasmic. However, in embryonic avian corneal epithelial (CE) cells, the authors previously observed that the ferritin was predominantly nuclear. They also obtained evidence that this ferritin protects DNA from oxidative damage by UV light and hydrogen peroxide and that the nuclear localization involves a tissue-specific nuclear transporter, termed ferritoid. In the present investigation, the authors have determined additional properties of the nuclear ferritoid-ferritin complexes.

METHODS

For biochemical characterization, a combination of molecular sieve chromatography, immunoblotting, and nuclear-cytoplasmic fractionation was used; DNA binding was analyzed by electrophoretic mobility shift assay.

RESULTS

The CE nuclear ferritin complex has characteristics that differentiate it from a "typical" cytoplasmic ferritin, including the presence of ferritin and ferritoid subunits; a molecular weight of approximately 260 kDa, which is approximately half that of cytoplasmic ferritin; its iron content, which is below our limits of detection; and its ability to bind to DNA.

CONCLUSIONS

Within CE cell nuclei, ferritin and ferritoid are coassembled into stable complex(es) present in embryonic and adult corneas. Thus, ferritoid not only serves transiently as a nuclear transporter for ferritin, it remains as a component of a unique ferritoid-ferritin nuclear complex.

Corneal epithelial nuclear ferritin: developmental regulation of ferritin and its nuclear transporter ferritoid

The corneal epithelium is exposed to reactive oxygen species that are potentially deleterious to nuclear DNA. However, our previous studies show that corneal epithelial cells have a novel, developmentally regulated mechanism for protection from such damage that involves having the iron-sequestering molecule, ferritin, in the nucleus. Nuclear localization of ferritin is achieved through the action of a tissue-specific nuclear transporter, ferritoid, which is itself a ferritin family member. Here, we show that during development ferritoid appears before ferritin. At this time, ferritoid is cytoplasmic, suggesting that its nuclear transport function requires an interaction with ferritin. To examine the developmental regulation of these two interacting components, cultured corneas were treated with the iron chelator deferoxamine. The results show that, while iron-mediated translational regulation is involved in the synthesis of both molecules, ferritoid is also transcriptionally regulated, demonstrating that these family members--whose functions depend upon one another--are regulated differently.

Nuclear ferritin in corneal epithelial cells: tissue-specific nuclear transport and protection from UV-damage

We have identified the heavy chain of ferritin as a developmentally regulated nuclear protein of embryonic chicken corneal epithelial cells. The nuclear ferritin is assembled into a supramolecular form that is indistinguishable from the cytoplasmic form of ferritin found in other cell types. Thus it most likely has iron-sequestering capabilities. Free iron, via the Fenton reaction, is known to exacerbate UV-induced and other oxidative damage to cellular components, including DNA. Since corneal epithelial cells are constantly exposed to UV light, we hypothesized that the nuclear ferritin might protect the DNA of these cells from free radical damage. To test this possibility, primary cultures of cells from corneal epithelium and other tissues were UV irradiated, and damage to DNA was detected by an in situ 3'-end labeling assay. Consistent with the hypothesis, corneal epithelial cells with nuclear ferritin had significantly less DNA breakage than the other cells types examined. However, when the expression of nuclear ferritin was inhibited the cells now became much more susceptible to UV-induced DNA damage. Since ferritin is normally cytoplasmic, corneal epithelial cells must have a mechanism that effects its nuclear localization. We have determined that this involves a nuclear transport molecule which binds to ferritin and carries it into the nucleus. This transporter, which we have termed ferritoid for its similarity to ferritin, has at least two domains. One domain is ferritin-like and is responsible for binding the ferritin; the other domain contains a nuclear localization signal that is responsible for effecting the nuclear transport. Therefore, it seems that corneal epithelial cells have evolved a novel, nuclear ferritin-based mechanism for protecting their DNA against UV damage. In addition, since ferritoid is structurally similar to ferritin, it may represent an example of a nuclear transporter that evolved from the molecule it transports (i.e., ferritin).

Ferritoid, a tissue-specific nuclear transport protein for ferritin in corneal epithelial cells

Previously we reported that ferritin in corneal epithelial (CE) cells is a nuclear protein that protects DNA from UV damage. Since ferritin is normally cytoplasmic, in CE cells, a mechanism must exist that effects its nuclear localization. We have now determined that this involves a nuclear transport molecule we have termed ferritoid. Ferritoid is specific for CE cells and is developmentally regulated. Structurally, ferritoid contains multiple domains, including a functional SV40-type nuclear localization signal and a ferritin-like region of approximately 50% similarity to ferritin itself. This latter domain is likely responsible for the interaction between ferritoid and ferritin detected by co-immunoprecipitation analysis. To test functionally whether ferritoid is capable of transporting ferritin into the nucleus, we performed cotransfections of COS-1 cells with constructs for ferritoid and ferritin. Consistent with the proposed nuclear transport function for ferritoid, co-transfections with full-length constructs for ferritoid and ferritin resulted in a preferential nuclear localization of both molecules; this was not observed when the nuclear localization signal of ferritoid was deleted. Moreover, since ferritoid is structurally similar to ferritin, it may be an example of a nuclear transporter that evolved from the molecule it transports (ferritin).

Clinical and ultrastructural features of a novel hereditary anterior segment dysgenesis

OBJECTIVE

To describe the clinical, histopathologic, and hereditary features of a novel familial anterior segment dysgenesis.

DESIGN

Prospective, observational case series and interventional case report.

PARTICIPANTS

Ten individuals from three generations of a single family with iris and corneal abnormalities associated with congenital cataracts.

MAIN OUTCOME MEASURES

An ophthalmic evaluation including slit-lamp examination, corneal topography, pachymetry, and specular biomicroscopy of all family members, and histopathologic and ultrastructural evaluation of one excised corneal button.

RESULTS

The proband was an 81-year-old man with bilateral aphakia and diffuse corneal haze, and thinning associated with corneal guttae. His pupils were small, mildly eccentric, and difficult to dilate. Pachymeter readings were 335 microm (right eye) and 330 microm (left eye). Topography confirmed advanced steepening of both corneas. Light microscopic and transmission electron microscopic examinations of the corneal button revealed an attenuated endothelium with prominent intracellular random aggregates of small-diameter filaments staining positively for cytokeratin. Descemet's membrane was thickened and had marked posterior nodularity. Various-sized polymorphic vacuoles containing layered electron-dense material were present within and between collagen lamellae and within keratocytes throughout the stroma and Bowman's membrane. Secondary bullous changes of the epithelium with thickening of the basement membrane were also observed. The family pedigree demonstrated an autosomal dominant inheritance pattern.

CONCLUSIONS

This constellation of autosomal dominantly inherited corneal endothelial and stromal disorder, with congenital cataracts and iris abnormalities, represents a novel anterior segment disorder. Its etiology may involve an abnormal migration of the secondary mesenchyme.

Nuclear translocation of ferritin in corneal epithelial cells

Our previous studies have shown that ferritin within developing avian corneal epithelial cells is predominantly a nuclear protein and that one function of the molecule in this location is to protect DNA from UV damage. To elucidate the mechanism for this tissue-specific nuclear translocation, cultured corneal epithelial cells and corneal fibroblasts were transfected with a series of deletion constructs for the heavy chain of ferritin, ferritin-H, tagged with a human c-myc epitope. The subcellular localization of the ferritin was determined by immunofluorescence for the myc-tag. For the corneal epithelial cells, the first 10 or the last 30 amino acids of ferritin-H could be deleted without affecting the nuclear localization. However, larger deletions of these areas, or deletions along the length of the body of the molecule, resulted largely in retention of the truncated proteins within the cytoplasm. Thus, it seems that no specific region functions as an NLS. Immunoblotting analysis of SDS-PAGE-separated extracts suggests that assembly of the supramolecular form of ferritin is not necessary for successful nuclear translocation, because one deletion construct that failed to undergo supramolecular assembly showed nuclear localization. In transfected fibroblasts, the endogenous ferritin remained predominantly in the cytoplasm, as did that synthesized from transfected full-length ferritin constructs and from two deletion constructs encoding truncated chains that could still assemble into the supramolecular form of ferritin. However, those truncated chains that were unable to participate in supramolecular assembly generally showed both nuclear and cytoplasmic localization, indicating that, in this cell type, supramolecular assembly is involved in restricting ferritin to the cytoplasm. These data suggest that for corneal epithelial cells, the nuclear localization of ferritin most likely involves a tissue-specific mechanism that facilitates transport into the nucleus, whereas, in fibroblasts, the cytoplasmic retention involves supramolecular assembly that prevents passive diffusion into the nucleus.

The lens organizes the anterior segment: specification of neural crest cell differentiation in the avian eye

During the development of the anterior segment of the eye, neural crest mesenchyme cells migrate between the lens and the corneal epithelium. These cells contribute to the structures lining the anterior chamber: the corneal endothelium and stroma, iris stroma, and trabecular meshwork. In the present study, removal of the lens or replacement of the lens with a cellulose bead led to the formation a disorganized aggregate of mesenchymal cells beneath the corneal epithelium. No recognizable corneal endothelium, corneal stroma, iris stroma, or anterior chamber was found in these eyes. When the lens was replaced immediately after removal, a disorganized mass of mesenchymal cells again formed beneath the corneal epithelium. However, 2 days after surgery, the corneal endothelium and the anterior chamber formed adjacent to the lens. When the lens was removed and replaced such that only a portion of its anterior epithelial cells faced the cornea, mesenchyme cells adjacent to the lens epithelium differentiated into corneal endothelium. Mesenchyme cells adjacent to lens fibers did not form an endothelial layer. The cell adhesion molecule, N-cadherin, is expressed by corneal endothelial cells. When the lens was removed the mesenchyme cells that accumulated beneath the corneal epithelium did not express N-cadherin. Replacement of the lens immediately after removal led to the formation of an endothelial layer that expressed N-cadherin. Implantation of lens epithelia from older embryos showed that the lens epithelium maintained the ability to support the expression of N-cadherin and the formation of the corneal endothelium until E15. This ability was lost by E18. These studies provide evidence that N-cadherin expression and the formation of the corneal endothelium are regulated by signals from the lens. N-cadherin may be important for the mesenchymal-to-epithelial transformation that accompanies the formation of the corneal endothelium.

Nuclear ferritin protects DNA from UV damage in corneal epithelial cells

Previously, we identified the heavy chain of ferritin as a developmentally regulated nuclear protein of embryonic chicken corneal epithelial cells. The nuclear ferritin is assembled into a supramolecular form indistinguishable from the cytoplasmic form of ferritin found in other cell types and thus most likely has iron-sequestering capabilities. Free iron, via the Fenton reaction, is known to exacerbate UV-induced and other oxidative damage to cellular components, including DNA. Since corneal epithelial cells are constantly exposed to UV light, we hypothesized that the nuclear ferritin might protect the DNA of these cells from free radical damage. To test this possibility, primary cultures of cells from corneal epithelium and stroma, and from skin epithelium and stroma, were UV irradiated, and DNA strand breaks were detected by an in situ 3'-end labeling method. Corneal epithelial cells without nuclear ferritin were also examined. We observed that the corneal epithelial cells with nuclear ferritin had significantly less DNA breakage than other cell types examined. Furthermore, increasing the iron concentration of the culture medium exacerbated the generation of UV-induced DNA strand breaks in corneal and skin fibroblasts, but not in the corneal epithelial cells. Most convincingly, corneal epithelial cells in which the expression of nuclear ferritin was inhibited became much more susceptible to UV-induced DNA damage. Therefore, it seems that corneal epithelial cells have evolved a novel, nuclear ferritin-based mechanism for protecting their DNA against UV damage.

Ferritin is a developmentally regulated nuclear protein of avian corneal epithelial cells

Previously, we generated monoclonal antibodies against chicken corneal cells (Zak, N. B., and Linsenmayer, T. F. (1983) Dev. Biol. 99, 373). We have now observed that one group of these antibodies reacts with a developmentally regulated component of corneal epithelial cell nuclei. This component is the heavy chain of ferritin, as determined by analyses of immunoisolated cDNA clones and immunoblotting of the protein. Immunoblotting also suggests that the nuclear ferritin may be in a supramolecular form that is similar to the iron-binding ferritin complex found in the cytoplasm of many cells. In vitro cultures and transfection studies show that the nuclear localization depends predominantly on cell type but can be altered by the in vitro environment. The appearance of nuclear ferritin is at least partially under translational regulation, as is known to be true for the cytoplasmic form of the molecule. The tissue and developmental distributions of the mRNA for the molecule are much more extensive than the protein itself, and the removal of iron from cultures of corneal epithelial cells with the iron chelator deferoxamine prevents the appearance of nuclear ferritin. At present the functional role(s) of nuclear ferritin remain unknown, but previous studies on cytoplasmic ferritin raise the possibility that it prevents damage due to free radical generation ("oxidative stress") by sequestering iron. Although it remains to be tested whether nuclear ferritin prevents oxidative damage, we find this an attractive possibility. Since the corneal epithelium is transparent and is constantly exposed to free radical-generating UV light, it is possible that the cells of this tissue have evolved a specialized mechanism to prevent oxidative damage to their nuclear components.

Temporal expression of types XII and XIV collagen mRNA and protein during avian corneal development

Using immunohistochemistry and competitive PCR for collagen types XII and XIV, we have followed the expression of these fibril-associated molecules during development of the avian cornea. By immunofluorescence histochemistry, both molecules are found in the acellular primary stroma and are therefore presumably of epithelial origin. During formation and development of the secondary corneal stroma, which is populated by mesenchymal cells, the molecules generally appear to be spatially segregated from each other. Type XIV collagen is found throughout most of the stroma, and therefore is predominantly a product of stromal fibroblasts. During subsequent compaction of the cornea, an event necessary for corneal transparency, the collagen XIV mRNA level increases dramatically, suggesting that this molecule may play a role in this event. Type XII collagen is more localized, occurring mainly in regions of the secondary stroma where matrices interface, such as where Bowman's membrane and Descemet's membrane abut the orthogonally layered collagen fibrils of the stromal matrix. These interfacial regions are highly stable areas of the cornea as determined previously by protease digestion and thermal denaturation studies. Type XII collagen may be involved in this stabilization.

Cellular invasion and collagen type IX in the primary corneal stroma in vitro

During different stages in the development of the avian cornea, various collagen types have been shown to participate in matrix formation and have been implicated in morphogenesis. One of these is the fibril-associated collagen type IX. This molecule is present when the primary corneal stroma is in a compact state, but rapidly disappears just prior to stromal swelling and its invasion by mesenchymal cells. The temporospatial pattern of the disappearance of type IX collagen in the developing cornea suggests that this molecule may be involved in stabilizing the primary corneal stromal matrix by interacting either with other type IX collagen molecules or with other matrix components. To explore further whether the removal of type IX collagen is involved in stromal swelling, we have employed an in vitro culture system in which swelling of the primary stroma and mesenchymal cell invasion can be experimentally manipulated by culturing chick corneal explants on a Nuclepore filter support in the presence or absence of an associated lens. We have also examined the effect of exogenously added human recombinant tissue inhibitor of metalloproteinases (TIMP-1) on the presence of type IX collagen and cellular invasion. When stage 25-26+ corneal explants were cultured with an associated lens, the primary stroma did not swell; immunohistochemically detectable type IX collagen was still present, and mesenchymal cell invasion failed to occur. Conversely, when the same stages of corneal explants were cultured without an associated lens, the primary stroma swelled; type IX collagen disappeared, and mesenchymal cell migration occurred. Under both conditions, however, the type II collagen of the stroma, which is known to be a component of the striated fibrils, remained clearly detectable and with time even seemed to increase in amount. This result is consistent with the proposition that type IX collagen is one factor involved in maintaining the primary stroma as a compact matrix, possibly by functioning as a bridging/stabilizing factor. When TIMP was added to cultures of corneal explants, type IX collagen remained detectable in focal regions, suggesting that one or more metalloproteinases are involved in the removal of the type IX collagen. In addition, some of these type IX-containing regions contained mesenchymal cells, suggesting that in addition to type IX collagen other factors are likely to be involved in regulating mesenchymal cell migration.

Acquisition of type IX collagen by the developing avian primary corneal stroma and vitreous

Previous investigations from our laboratory and others have demonstrated that type II collagen, once thought to be a cartilage-specific molecule, is also a component of both the primary corneal stroma and the vitreous of embryonic chickens. In the present immunohistochemical study we have examined the expression in these embryonic matrices of another "cartilage-specific" collagen, type IX, along with type II. In the cornea, type IX collagen is in the primary stroma, but is not detectable in the mature, secondary stroma. Even within the primary stroma this collagen has a brief, transitory existence. It first appears in the peripheral stroma at the time the endothelial cells begin to migrate along its posterior surface, and spreads throughout the stroma during the following 24-36 hr. The epitopes on type IX collagen then suddenly become undetectable just before this matrix swells and becomes populated by the periocular mesenchymal cells (future keratocytes). In comparison, collagen type II (along with type I) is present in the stroma before and long after these events. Deposition of immunodetectable type IX collagen in the developing corneal stroma thus seems to be independent of type II. In the vitreous, we observed type IX collagen along with type II as soon as authentic vitreous could be identified and at all subsequent stages of development. In this tissue, therefore, the expression of collagen types IX and II appears to be coordinate.

Presumed autoimmune corneal endotheliopathy

I reviewed 20 previously published cases of presumed autoimmune corneal endotheliopathy. The disease appeared clinically with stromal edema and a slowly migrating line of keratic precipitates. All patients had acute stromal edema and keratic precipitates, and decreased visual acuity. Anterior chamber cells were noted in 11 patients. Inflammatory processes, such as pars planitis and iritis, and intraocular lens implantation were present in 13 patients.

Suprabasal expression of a 64-kilodalton keratin (no. 3) in developing human corneal epithelium

We have previously shown that a basic 64-kilodalton (no. 3 in the catalog of Moll et al.) and an acidic 55-kilodalton (no. 12) keratin are characteristic of suprabasal cell layers in cultured rabbit corneal epithelial colonies, and therefore may be regarded as markers for an advanced stage of corneal epithelial differentiation. Moreover, using an AE5 mouse monoclonal antibody, we showed that the 64-kilodalton keratin marker is expressed suprabasally in limbal epithelium but uniformly (basal layer included) in central corneal epithelium, suggesting that corneal basal cells are in a more differentiated state than limbal basal cells. In conjunction with previous data implicating the centripetal migration of corneal epithelial cells, our data support a model of corneal epithelial maturation in which corneal epithelial stem cells are located in the limbus, the transitional zone between the cornea and conjunctiva. In the present study, we analyzed the expression of the 64-kilodalton keratin in developing human corneal epithelium by immunohistochemical staining. At 8 weeks of gestation, the presumptive corneal epithelium is composed of a single layer of cuboidal cells with an overlying periderm; neither of these cell layers is AE5 positive. At 12-13 weeks of gestation, some superficial cells of the three- to four-layered epithelium become AE5 positive, providing the earliest sign of overt corneal epithelial differentiation. At 36 weeks, although the epithelium is morphologically mature (four to six layers), AE5 produces a suprabasal staining pattern, this being in contrast to the adult epithelium which exhibits uniform staining.(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.

Analysis of corneal development with monoclonal antibodies. II. Tissue autonomy in cornea-skin recombinants

Developmental autonomy of corneal epithelial and stromal components was assessed by their subsequent differentiation after recombination with feather-forming thigh dermis and epidermis, respectively. Work by others has shown that feather-forming dermis exhibits strong inductive ability when used in such epithelial-mesenchymal recombinations. After culture of the recombinants on the chorioallantoic membrane (CAM) of host embryos, differentiation as "cornea" was assessed immunohistochemically using the anti-corneal stromal matrix and anti-corneal epithelial antibodies described previously (Zak and Linsenmayer, Dev. Biol. 99, 373-381, 1983). Feather initiation and outgrowth and keratin synthesis served as markers for differentiation as skin. It has been found that corneal epithelia from 5-day embryos, when grown in association with feather-forming dermis from the thigh, will participate in feather formation. In such recombinants, when the corneal epithelium became incorporated into feathers it failed to express the corneal epithelial antigen, but in regions of the recombinant where feathers did not form, de novo expression of the antigen was sometimes detected. The limited liability of the epithelium is not present in corneal epithelia taken from embryos a day or two older. When such epithelia were used for making the recombinants, no feathers were formed and the corneal epithelial antigen was extensively produced. Thus epithelial determination occurs long before the epithelium would begin to overtly differentiate and express the epithelial antigen in vivo (about 12 days of development). In reciprocal recombinations of corneal stromas with feather-forming epidermis, the stromas proceeded to express the corneal stromal matrix specific antigen de novo after culture on the CAM. They did not, however, redirect differentiation of the epidermis which never expressed the corneal epithelial antigen and in some cases went on to keratinize. These results indicate that development of both the corneal epithelial and stromal components becomes autonomous at least several days before these tissues overtly differentiate. This suggests that the component tissues of the cornea may not interact in a manner typical of those of other organs which, in general, are thought to require continual interaction of their epithelial and mesenchymal components for normal development.

Analysis of corneal development with monoclonal antibodies. I. Differentiation in isolated corneas

Monoclonal antibodies highly selective for developmentally regulated antigens present in the cornea (Zak and Linsenmayer, Dev. Biol. 99, 373-381, 1983) have been used to immunohistochemically evaluate differentiation in intact chick corneas cultured on the chorioallantoic membrane (CAM) of host embryos. One antibody is directed against the epithelial cell layer and the other is against the corneal stromal matrix. It has been established that both antigens recognized by the antibodies are expressed de novo in young explanted corneas and that the stromal matrix antigen is a product of the corneal fibroblasts. Thus expression of the antigens can be used as criteria for overt differentiation of the respective cell types. The antibodies have been employed to assess when the corneal epithelial and stromal cells become capable of autonomous differentiation within isolated corneas. To accomplish this, corneas of various ages were explanted with and without adjacent pericorneal tissues. The results indicate that, under the culture conditions employed, corneal stromal differentiation is dependent on the presence of the lens until stage 28 (51/2-6 days of development), which is the time when invasion of the stroma by pericorneal mesenchymal cells is initiated. After stage 28, the stromal matrix antigen was expressed by isolated corneas irrespective of the presence of the lens. Possibly the lens acts by maintaining the integrity of the corneal endothelial monolayer and thus promoting normal migration of pericorneal mesenchymal cells into the primary corneal stroma, where they undergo differentiation. Conversely, differentiation of the corneal epithelium was independent of any pericorneal structure from the earliest stage examined (41/2-5 days of development). It was even independent of overt stromal differentiation, thus suggesting an early and strong determination for this tissue.

In vitro analysis of corneal development with monoclonal antibodies

Anti-corneal monoclonal antibodies, highly selective for corneal stromal and corneal epithelial components, were used to immunohistochemically evaluate differentiation of intact embryonic chick corneas grown in organ culture in the presence or absence of an associated lens. It was observed that both epithelial and stromal components of 5-day corneas initiated expression of their antigens, irrespective of the presence of a lens. This was unlike previous results obtained when 5-day lens-less corneas were explanted to the chorioallantoic membrane, a condition under which epithelial differentiation ensued but stromal differentiation did not. Possibly, in organ culture, the filter support may replace the lens as a substratum for cell migration of neural crest-derived pericorneal mesenchymal cells into the primary corneal stroma. In 5-day organ cultures with lenses, cellular migration into the primary corneal stroma seems to be largely inhibited (also unlike previous results on the chorioallantoic membrane), but mesenchymal cells which had accumulated at the periphery of the eye did express the differentiation antigen.