Chick Cornea Research Articles

A tribute to Elizabeth D. Hay, 1927–2007

A tribute to Elizabeth D. Hay, 1927–2007

Differential expression of the keratan sulphate proteoglycan, keratocan, during chick corneal embryogenesis

Keratan sulphate (KS) proteoglycans (PGs) are key molecules in the connective tissue matrix of the cornea of the eye, where they are believed to have functional roles in tissue organisation and transparency. Keratocan, is one of the three KS PGs expressed in cornea, and is the only one that is primarily cornea-specific. Work with the developing chick has shown that mRNA for keratocan is present in early corneal embryogenesis, but there is no evidence of protein synthesis and matrix deposition. Here, we investigate the tissue distribution of keratocan in the developing chick cornea as it becomes compacted and transparent in the later stages of development. Indirect immunofluorescence using a new monoclonal antibody (KER-1) which recognises a protein epitope on the keratocan core protein demonstrated that keratocan was present at all stages investigated (E10-E18), with distinct differences in localisation and organisation observed between early and later stages. Until E13, keratocan appeared both cell-associated and in the stromal extracellular matrix, and was particularly concentrated in superficial tissue regions. By E14 when the cornea begins to become transparent, keratocan was located in elongate arrays, presumably associated along collagen fibrils in the stroma. This fibrillar label was still concentrated in the anterior stroma, and persisted through E15-E18. Presumptive Bowman's layer was evident as an unlabelled subepithelial zone at all stages. Thus, in embryonic chick cornea, keratocan, in common with sulphated KS chains in the E12-E14 developmental period, exhibits a preferential distribution in the anterior stroma. It undergoes a striking reorganisation of structure and distribution consistent with a role in relation to stromal compaction and corneal transparency.

Collagen organization in the chicken cornea and structural alterations in the retinopathy, globe enlarged ( rge) phenotype—An X-ray diffraction study

An investigation into the collagenous structure of the mature avian cornea is presented. Wide-angle X-ray diffraction is employed to assess collagen organization in 9-month-old chicken corneas. The central 2–4 mm corneal region features a preponderance of fibrils directed along the superior–inferior and nasal-temporal orthogonal meridians. More peripherally the orientation of fibrils alters in favor of a predominantly tangential arrangement. The chicken cornea appears to be circumscribed by an annulus of fibrils that extends into the limbus. The natural arrangement of collagen in the chicken cornea is discussed in relation to corneal shape and the mechanical requirements of avian corneal accommodation. Equivalent data are also presented from age-matched blind chickens affected with the retinopathy, globe enlarged ( rge) mutation, characterized by an abnormally thick and flat cornea. The data indicate considerable realignment and redistribution of collagen lamellae in the peripheral rge cornea. In contrast to normal chickens, no obvious tangential collagen alignment was evident in the periphery of rge corneas. In mammals, the presence of a limbal fibril annulus is believed to be important in corneal shape preservation. We postulate that corneal flattening in rge chickens may be related to biomechanical changes brought about by an alteration in collagen arrangement at the corneal periphery.

Roles of MMP/TIMP in regulating matrix swelling and cell migration during chick corneal development

Tissue remodeling is central to embryonic development. Here, we used immunohistochemistry, Western blotting, and RT-PCR analysis to investigate the roles of matrix metalloproteinases (MMPs) and the related "a disintegrin and metalloproteinase" (ADAM) family proteinases in chick corneal development. While MMP-13 was expressed in developing chick corneas from embryonic day (ED) 5 to ED 10, its inhibitor, tissue inhibitors of metalloproteinase-1 (TIMP-1), was expressed from ED 18 to 2 days post-hatching (P2). Early MMP-13 activity may be associated with degradation of type IX collagen from the primary stroma, which loosens the collagen fibrils and facilitates neural crest (NC) cell migration. The membrane-bound and secreted forms of ADAM10 were both detected throughout corneal development, and active ADAM10 formed a cleavage complex with CD44v6, a CD44 splice variant that is a major cell surface adhesion molecule for hyaluronic acid (HA) and has been implicated in cell migration. Both CD44v6 and its ectodomain cleavage products were detected from ED 5 to ED 14, and a broad-spectrum MMP inhibitor blocked ectodomain cleavage in cultured stromal cells. These findings suggest that ADAM10 mediates CD44v6 cleavage in the developing cornea, facilitating NC cell-derived mesenchymal cell migration. Finally, we identified high levels of active membrane-type 3-MMP (MT3-MMP) in developing corneas at ED 7, ED 14, and ED 18. MT3-MMP takes part in MMP-2 activation and possibly also CD44v6 shedding, suggesting that this pathway may be involved in cell migration. These findings collectively show for the first time that multiple MMPs, ADAMs, and TIMPs appear to functionally interact during corneal development.

Expression of Type XXIII Collagen mRNA and Protein

Collagen XXIII is a member of the transmembranous subfamily of collagens containing a cytoplasmic domain, a membrane-spanning hydrophobic domain, and three extracellular triple helical collagenous domains interspersed with non-collagenous domains. We cloned mouse, chicken, and humanalpha1(XXIII) collagen cDNAs and showed that this non-abundant collagen has a limited tissue distribution in non-tumor tissues. Lung, cornea, brain, skin, tendon, and kidney are the major sites of expression. In contrast, five transformed cell lines were tested for collagen XXIII expression, and all expressed the mRNA. In vivo the alpha1(XXIII) mRNA is found in mature and developing organs, the latter demonstrated using stages of embryonic chick cornea and mouse embryos. Polyclonal antibodies were generated in guinea pig and rabbit and showed that collagen XXIII has a transmembranous form and a shed form. Comparison of collagen XXIII with its closest relatives in the transmembranous subfamily of collagens, types XIII and XXV, which have the same number of triple helical and non-collagenous regions, showed that there is a discontinuity in the alignment of domains but that striking similarities remain despite this.

Basement Membrane and Collagen Deposition After Laser Subepithelial Keratomileusis and Photorefractive Keratectomy in the Leghorn Chick Eye

To evaluate corneal scar formation and new collagen deposition after laser subepithelial keratomileusis (LASEK) compared with photorefractive keratectomy (PRK) in the leghorn chick corneal model. Leghorn chick corneas treated with LASEK surgery (using 20% ethanol for 30 seconds) or PRK were evaluated by indirect confocal immunofluorescence and transmission electron microscopy at 1, 2, 7, 14, and 28 days after surgery. New collagen deposition was determined by dichlorotriazinylaminofluorescein staining 2 and 4 weeks after surgery. Laminin was detected around the basal layers during the immediate postoperative period and 4 weeks after LASEK surgery, and from day 2 onwards after PRK. Collagen III deposition in the cornea was about 3 times greater with PRK than with LASEK. The thickness of new collagen deposition at 4 weeks was 34 microm +/- 2.5 microm in the PRK group, which was significantly greater than that of the LASEK group (11 microm +/- 1 microm; P<.001). Reduced subepithelial stromal tissue deposition was observed in LASEK-treated eyes compared with PRK-treated eyes. Postoperative preservation of the epithelial basement membrane and survival of epithelial cells in LASEK and possibly in epithelial laser in situ keratomileusis may contribute to this phenomenon. An advantage of LASEK over PRK is the reduction of postoperative haze.

Determination of Keratan Sulfate and Chondroitin /Dermatan Sulfate Disaccharides during Chick Corneal Development by Electrospray Tandem Mass Spectrometry

In cornea, keratan sulfate (KS) and chondroitin/dermatan sulfate (CS/DS) are the two major glycosaminoglycans (GAGs). In this study, electrospray ionization tandem mass spectrometry (ESI-MS/MS) was employed to quantify changes in KS and CS/DS sulfated disaccharides in the developing chick cornea. The concentration of KS monosulfated disaccharide (MSD) Gal-â-1,4-GlcNAc(6S) in embryonic Day 8 (E8) cornea equals that at E20, falls to its lowest level by E10, rises to a second peak by E14, falls to a second low by E18, peaks again by E20, and remains high in adult corneas. A similar concentration profile is observed for KS disulfated disaccharide (DSD) Gal(6S)-â-1,4-GlcNAc(6S), and thus also for total sulfated KS disaccharides. The molar percent of DSD is higher than that of MSD from E8-E18, equivalent at E20, and less than that of MSD in adult corneas. In contrast, total concentration of CS/DS Ddi-4S plus Ddi-6S decreases as development progresses and is lowest in adult corneas. Concentration and molar percent of Ädi-6S is highest at E8, then decreases through development as the concentration and molar percent of Ddi-4S increases from E8 and exceeds that of Ddi-6S after E14. New rapid, direct chemical analysis of extracellular matrix components obtained from sections from embryonic and adult chick corneas reveals heretofore undetected changes in sulfation characteristics of KS and CS/DS disaccharides during corneal development.

Thyroxine Affects Expression of KSPG-Related Genes, the Carbonic Anhydrase II Gene, and KS Sulfation in the Embryonic Chicken Cornea

Opaque chick corneas become thin and transparent from embryonic day (E)9 to E20 of incubation. Thyroxine (T4) injected in ovo on E9 induces precocious transparency by E12. The present study was conducted to determine whether corneal cells differentially express genes for T4 regulation, keratan sulfate proteoglycan (KSPG) synthesis, crystallins, and endothelial cell ion transporters during transparency development and whether these expressions are altered when E9 embryos are treated with T4. E9 eggs received T4 or buffer; corneas were dissected on E12. Corneal transparency was measured digitally and thickness was determined from cryostat cross sections. mRNA expressions were determined by real-time PCR using cDNA synthesized from whole-cell RNA, cells expressing T4 receptor mRNAs assessed by in situ hybridization, and KS disaccharide sulfation measured by electrospray ionization tandem mass spectrometry (ESI-MS/MS). All corneal layers expressed T4 receptor alpha (THRA) mRNA; keratocytes and endothelial cells expressed T4 receptor beta (THRB) mRNA. During normal development, THRB expression increased 20-fold from E12 to E20; THRA expression remained constant. Expressions of most genes involved in KS synthesis increased from E9 to E16, and then decreased from E16 to E20. From E9 to E20, expressions of crystallin genes increased; T4/3-deiodinase DIII (DIO3) increased 10-fold; and sodium-potassium ATPase transporter (ATP1A1), sodium-bicarbonate transporter (NBC), and carbonic anhydrase II (CA2) increased 5- to 10-fold. E9 T4 administration decreased corneal thickness by E12; increased DIO3, THRB, and CA2 expressions 5- to 20-fold; decreased KSPG core protein genes and galactose sulfotransferase CHST1 expressions 2-fold; and reduced KS disulfated/monosulfated disaccharide (DSD/MSD) ratios. Thyroxine modifies expressions of KSPG synthesis and carbonic anhydrase genes.

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.

Detection and Quantification of Sulfated Disaccharides from Keratan Sulfate and Chondroitin/Dermatan Sulfate during Chick Corneal Development by ESI-MS/MS

To identify and quantify changes in keratan sulfate (KS) and chondroitin/dermatan sulfate (CS/DS) sulfated disaccharides in the developing chick cornea using electrospray ionization tandem mass spectrometry (ESI-MS/MS). Cryostat sections of fresh nonfixed corneas were obtained from White Leghorn embryonic day (E)8 to E20 chicks, and from 4- and 70-week-old chickens. Tissue sections on glass slides were incubated with selected glycosidase enzymes. Digest solutions were analyzed directly by ESI-MS/MS. The concentration of KS monosulfated disaccharide (MSD) Gal-beta-1,4-GlcNAc(6S) in E8 cornea equaled that at E20, declined to its lowest level by E10, increased to a second peak by E14, decreased to a second low by E18, peaked again by E20, and remained high in adult corneas. A similar concentration profile was observed for KS disulfated disaccharide (DSD) Gal(6S)-beta-1,4-GlcNAc(6S), and thus also for total sulfated KS disaccharides. The molar percent of DSD was higher than that of MSD from E8 to E18, equivalent at E20, and less than that of MSD in adult corneas. In contrast, total concentration of CS/DS Deltadi-4S plus Deltadi-6S decreases as development progresses and is lowest in adult corneas. Concentration and molar percent of Deltadi-6S is highest at E8, then decreases through development as the concentration and molar percent of Deltadi-4S increases from E8 and exceeds that of Deltadi-6S after E14. New, rapid, direct chemical analysis of extracellular matrix components obtained from sections from embryonic and adult chick corneas reveals heretofore undetected changes in sulfation characteristics of KS and CS/DS disaccharides during corneal development.

Spatial and temporal alterations in the collagen fibrillar array during the onset of transparency in the avian cornea

In the latter stages of development, the embryonic avian cornea undergoes significant changes in structure, composition and transparency. The rearrangement of stromal collagen fibrils at this time is important because it is believed to play a key role in the acquisition of corneal transparency. Here, we investigate spatial alterations in the internal fine structure of the avian cornea during development. Chicken corneas at developmental days 14, 16 and 18 were examined by transmission electron microscopy and quantitative image analysis. For anterior and posterior regions we determined fibril number densities, two-dimensional distribution functions, and, where appropriate, radial distribution functions. Stromal collagen fibrils became more closely spaced over the developmental range studied here. Changes in fibril number density indicated that fibrils became compacted first in the anterior stroma, and later (i.e. after day 16) in the posterior stroma. By day 18 collagen fibril number densities were essentially the same in superficial and deep tissue regions. At day 14, two-dimensional distribution functions of collagen fibrils in the posterior stroma pointed to a fibrillar array that was unlike that in the anterior stroma because there was no clear radial symmetry. Rather, in the deep stroma at day 14 there was evidence of different nearest neighbour spacings in two orthogonal directions. By day 18, fibril distributions in the anterior and posterior stroma were spatially homogeneous and radially symmetric, with radial distribution functions typical of those ordinarily found in mature cornea. Corneal transparency requires the stromal matrix to have some degree of regularity in the arrangement of its uniformly thin collagen fibrils. The chicken cornea becomes progressively transparent between days 14 and 18 of development as the stroma dehydrates and thins. We show that over this time period collagen fibrils in the anterior stroma become configured in advance of fibrils in deeper stromal regions, leading to questions over the potential roles of sulphated proteoglycans in different regions of the corneal stroma during morphogenesis.

A new paradigm for corneal wound healing research: The white leghorn chicken (Gallus gallus domesticus)

Purpose. To evaluate the chicken cornea as a model for corneal wound healing research. Methods. We conducted studies on normal chicken corneas and on corneas following mechanical debridement and photorefractive keratectomy (PRK). Results. The chicken cornea possesses six distinct layers that resemble the layers of the human cornea, including a substantial Bowman’s layer measuring 5.2 ± 0.3µm thick. Reepithelialization time was 44.8 ± 1.1 hours with a sliding rate of 75.3 ± 3.2µm/hour following mechanical debridement and 54.4 ± 2.8 hours with a sliding rate of 63.3 ± 3.2µm/hour for PRK-treated corneas. Biomicroscopic haze post-PRK peaked at 4–6 weeks and regressed until 20 weeks post-PRK. Histologic and clinical observations of wound healing strongly paralleled that noted in humans. Conclusion. The chicken cornea’s morphology and wound healing response render it a useful model for corneal wound healing investigations.

Collagen organization in the secondary chick cornea during development.

The latter stages of morphogenesis in the embryonic chick cornea are instrumental in the establishment of a properly formed corneal stroma. This study was designed to provide better appreciation of collagen reorganization in the avian corneal stroma during the latter stages of embryogenesis. High-angle synchrotron x-ray diffraction patterns were obtained from 47 developing chick corneas daily at developmental days 13 through 18 (n = 7 or 8 at each time point) and analyzed to establish collagen molecular spacing and fibril orientation. Collagen intermolecular x-ray reflections were of approximately constant intensity between days 13 and 15 of development, but thereafter became progressively more intense, suggesting that extra collagen is deposited in embryonic chick corneas after day 16 of development. At all times, the mean collagen intermolecular spacing measured approximately 1.43 nm. X-ray intensity was not uniform around the intermolecular x-ray reflections at earlier time points. Rather, a fourfold symmetry was evident, indicative of an orthogonal array of collagen fibrils. An index of this symmetry was essentially unchanged between developmental days 13 and 15, but thereafter diminished considerably. The lateral spacing of fibril-forming collagen molecules does not change as the chick cornea develops between days 13 and 18. An orthogonal array of collagen fibrils is present in the corneas of developmental day-13 to -18 chicks, but starting at developmental day 16, additional collagen is deposited in a less well-oriented manner and thus acts to obscure the overall orthogonality, with implications for the biomechanical strength and shape of the cornea.

A Wide-Angle X-ray Diffraction Study of the Developing Embryonic Chicken Cornea

In terrestrial vertebrates the cornea is the main refractive component of the eye. Its remarkable mechanical toughness and almost 100% light-transparency are largely a consequence of the unique collagenous architecture of the corneal stroma. We have used WAXS methods to investigate stromal remodelling in the embryonic chicken cornea in the latter stages of development. Collagen organisation at day 13-15 of embryogenesis is dominated by a four-fold orthogonal arrangement of fibrils. Thereafter this preferential alignment recedes, seemingly because further collagen is deposited in a more isotropic manner, masking the initial orthogonal template. In contrast, the mean lateral spacing of fibril-forming collagen molecules remains unaltered over this developmental period. Our observations have important implications for the biomechanical strength and shape of the cornea.

Identification and Characterization of β- d-Galactosyl-transferase in Chick Corneas

Purpose: To purify β-galactosyltransferase, which is involved in the biosynthesis of keratan sulfate, from 2-day-old chick corneas. Methods: The activity was assayed using pyridylaminated GlcNAcβ1-3Galβ1-4Glc as acceptor substrate. Results: The β-galactosyltransferase did not bind to several ion exchange and affinity columns. In particular, it did not bind completely to an α-lactalbumin-agarose column. The partially purified enzyme showed an optimum pH at 7.0 [( N-[2-hydroxyethyl]piperazine- N′-[2-ethanesulfonic acid]) (HEPES) buffer]. Pyridylaminated GlcNAcβ1-3Galβ1-4GlcNAc, as well as the pyridylaminated GlcNAcβ1-3Galβ1-4Glc, served as the acceptor substrate of the enzyme, but not p-nitrophenyl-β-GlcNAc. The crude extract from chick corneas contained a high activity of the β-galactosyltransferase, which transfers Gal to pyridylaminated 6-sulfo-GlcNAcβ1-3Gal, but the activity was almost all lost during the purification procedures. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the enzyme fraction showed many bands. When each of the five main band proteins was partially analyzed for the amino acid sequence, none showed homology with the recently reported chick β-galactosyltransferases 1 and 2. Conclusions: β-Galactosyltransferase, which transfers Gal to 6-sulfo-GlcNAc end, was identified in chick corneas. The chick corneal β-galactosyltransferase(s) may be a novel one.

Changes in distribution of the long form of type XII collagen during chicken corneal development.

The expression and distribution of the long form of Type XII collagen were investigated histochemically during chicken corneal development using a monoclonal antibody (P3D11) raised against the N-terminal domain of chicken Type XII collagen. Specificity of the antibody was confirmed by immunoprecipitation before and after bacterial collagenase digestion. Immunofluorescent microscopic studies showed that during chicken cornea formation, the long form of Type XII collagen is initially detected on Day 3 embryo (stage 19) in the sub-epithelial matrix of the corneal periphery and in the matrix around the optic cup. On Day 5 embryo (stage 27) the long form was expressed in the primary stroma. Thereafter, as the secondary stroma was formed, the long form localized in the sub-epithelial and sub-endothelial matrices and in the anterior region of the limbus (corneoscleral junction) before the formation of Descemet's and Bowman's membranes. After hatching, the immunoreactivity decreased predominantly in the sub-epithelial and sub-endothelial matrices but remained at the anterior region of the limbus. Immunoelectron microscopic examination demonstrated that the long form localizes in the Descemet's and Bowman's membranes and along the collagen fibrils in the stroma with a periodic repeat. Based on the distribution of the long form of Type XII collagen in the sub-epithelial and sub-endothelial matrices and limbus, it was suggested that the long form of Type XII collagen is involved in formation of the Descemet's and Bowman's membranes and in stabilization of the limbus.

Purification and characterization of chick corneal beta-D-glucuronyltransferase involved in chondroitin sulfate biosynthesis.

Beta-D-Glucuronyltransferase, which transfers D-glucuronic acid (GlcA) from UDP-GlcA to N-acetyl-D-galactosamine (GalNAc) at the nonreducing end of chondro-pentasaccharide-PA (pyridylamino-), GalNAcbeta1-(4GlcAbeta1-3GalNAcbeta1)2-PA, was purified 339-fold with an 11.0% yield from 2-d-old chick corneas by chromatography on DEAE-Sepharose, WGA-agarose, heparin-Sepharose, and 1st and 2nd UDP-GlcA-agarose (in the presence of Gal) columns. The activity was detected by fluorescence of PA residues of the product. The purified enzyme has an optimum pH of 7.0 (Mes buffer), and much higher activity toward chondro-heptasaccharide-PA than toward the chondro-pentasaccharide-PA, but no activity toward p-nitrophenyl-beta-GalNAc. The enzyme activity was almost completely inhibited by GalNAc (20 mm). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the purified enzyme fraction showed one band of 38 kDa with many other bands. The amino acid sequence was determined for the tryptic digests of the 38 kDa band protein. The sequences determined showed no homology to those of several beta-glucuronyltransferases reported previously. It seems that the enzyme is involved in the elongation of chondroitin sulfate chains in vivo.

Purification and Characterization of N-Acetylglucosaminyl Sulfotransferase from Chick Corneas

N-Acetylglucosaminyl(GlcNAc) sulfotransferase, which transfers sulfate from adenosine 3′-phosphate 5′-phosphosulfate to GlcNAc at the nonreducing end of oligosaccharides, was purified 887-fold with a 8.4% yield from 2-day-old chick corneas by chromatography on CM-Sepharose, WGA-agarose, GlcNAc-agarose, and 3′,5′-ADP-agarose columns. The purified enzyme has an optimum pH of 6.0 (Mes buffer) and specifically transfers a sulfate to GlcNAc at the nonreducing end but not to internal GlcNAc. The enzyme was stimulated by protamine and Mn2+. SDS–polyacrylamide gel electrophoresis of the purified enzyme still showed two main bands (66 and 55 kDa) with some minor bands. It appears that this enzyme competes with β-galactosyltransferase in binding to the nonreducing GlcNAc residue on KS synthesis; this suggests that the sulfation of the GlcNAc residue is coupled with the elongation of the KS chain.

Corneal surface area: an index of anterior segment growth.

Corneal surface area and perimeter were assessed as novel indices to monitor anterior segment growth, using chicks reared under different photoperiods. We obtained central and mid-peripheral corneal curvatures using photokeratometry. Anatomical tracings of the anterior corneal surface also were made from freeze-dried non-fixed preparations of the anterior segments of the same eyes. Using either photokeratometry or anatomical data, the profile of the anterior corneal surface was fit to a general equation for conical sections; corneal surface area was estimated from surfaces of revolution. Optical techniques modeled the chick cornea as a circle or as an ellipse closely resembling a circle. The anatomical technique, in contrast, modeled the chick corneal profile as a hyperbola. Potential explanations of this discrepancy are discussed. Regardless of which model is evaluated, the corneal surface area and perimeter of two-week-old chicks are affected by the photoperiod of rearing. Corneal surface area in particular proved more sensitive than conventional measurements in identifying anterior segment effects of rearing under different photoperiods. Analysis of corneal area may prove useful in evaluating the mechanisms governing anterior segment growth.

Corneal surface area: an index of anterior segment growth

Corneal surface area and perimeter were assessed as novel indices to monitor anterior segment growth, using chicks reared under different photoperiods. We obtained central and mid-peripheral corneal curvatures using photokeratometry. Anatomical tracings of the anterior corneal surface also were made from freeze-dried non-fixed preparations of the anterior segments of the same eyes. Using either photokeratometry or anatomical data, the profile of the anterior corneal surface was fit to a general equation for conical sections; corneal surface area was estimated from surfaces of revolution. Optical techniques modeled the chick cornea as a circle or as an ellipse closely resembling a circle. The anatomical technique, in contrast, modeled the chick corneal profile as a hyperbola. Potential explanations of this discrepancy are discussed. Regardless of which model is evaluated, the corneal surface area and perimeter of two-week-old chicks are affected by the photoperiod of rearing. Corneal surface area in particular proved more sensitive than conventional measurements in identifying anterior segment effects of rearing under different photoperiods. Analysis of corneal area may prove useful in evaluating the mechanisms governing anterior segment growth.