Ultrastructure Organization of Collagen Fibrils and Proteoglycans of Stingray and Shark Corneal Stroma

Abstract

We report here the ultrastructural organization of collagen fibrils (CF) and proteoglycans (PGs) of the corneal stroma of both the stingray and the shark. Three corneas from three stingrays and three corneas from three sharks were processed for electron microscopy. Tissues were embedded in TAAB 031 resin. The corneal stroma of both the stingray and shark consisted of parallel running lamellae of CFs which were decorated with PGs. In the stingray, the mean area of PGs in the posterior stroma was significantly larger than the PGs of the anterior and middle stroma, whereas, in the shark, the mean area of PGs was similar throughout the stroma. The mean area of PGs of the stingray was significantly larger compared to the PGs, mean area of the shark corneal stroma. The CF diameter of the stingray was significantly smaller compared to the CF diameter in the shark. The ultrastructural features of the corneal stroma of both the stingray and the shark were similar to each other except for the CFs and PGs. The PGs in the stingray and shark might be composed of chondroitin sulfate (CS)/dermatan sulfate (DS) PGs and these PGs with sutures might contribute to the nonswelling properties of the cornea of the stingray and shark.

Figure 1

Light and electron micrograph of stingray cornea; (a) middle part of the stroma showing suture running obliquely to the parallel running lamellae; (b) part of the BW and anterior stroma showing BW fibrils blending into the most anterior stromal lamellae, thin lamellae, orthogonal CFs, and orthogonal suture CFs. BW: Bowman's layer, KR: keratocyte, L: lamellae, S: stroma, SU: suture, OCF: orthogonal running CFs, and SUC: orthogonal suture CFs.

Figure 2

Electron micrograph of collagen fibrils (CF) and proteoglycans (PGs) of stingray cornea; (a) part of the anterior stroma showing thin lamellae containing thin CF (20 nm); (b) part of the anterior stroma showing and orthogonal arrangement of the CFs (XCF); (c) part of the middle stroma showing longitudinally running CFs decorated with PGs; (d) part of the middle stroma showing cross section of uniformly distributed CF and PGs; (e) cross section of CF at high magnification showing microfibrils within CF; (f) posterior part of the stroma showing CF, PGs, and DM composed of microfibrils. BM: basement membrane, BW: Bowman's layer, DM: Descemet's membrane L: lamellae, CF: collagen fibrils, PG: proteoglycans, and XCF: orthogonal arrangement of the CFs.

Figure 3

Electron micrograph of stingray fish cornea; (a) part of the stroma showing the keratocyte containing large nucleus; (b) a single suture running obliquely between two lamellae; (c) the middle part of the suture showing presence of CFs and microfibrils. The microfibrils lacked the PGs whereas CFs were decorated with PGs. (d) Sagittal section of a suture showing the CFs surrounding the microfibrils; (e) part of the suture showing presence of PGs on the CFs of the suture; (f) part of suture showing absence of PGs on the microfibrils of suture. CF: collagen fibrils, DM: Descemet's membrane, L: lamellae, KR: keratocyte, MF: microfibrils, PG: proteoglycans, and SU: suture.

Figure 4

Electron micrograph of stingray cornea; (a) posterior part of a single suture containing randomly running CFs embedded within the lamella. (b) Basal part of multiple sutures containing randomly running CF which were embedded within the lamella. Group of microfibrils were present among the CF. (c) Posterior part of the suture showing presence of PGs on the CF but not on the microfibrils; (d) longitudinally running microfibrils of suture not surrounded by CF; (e) electron micrograph of longitudinally running CF of the middle stroma of shark; (f) electron micrograph of cross section of CF of the middle stroma of shark. CF: collagen fibrils, L: lamellae, MFL: microfilament, PG: proteoglycans, S: stroma, and SU: suture.

Figure 5

Electron micrograph and colour coded digital images of CF of the middle stroma of stingray and shark; (a) electron micrograph of middle stroma of stingray; (b) colour coded digital image of (a); (c) electron micrograph of middle stroma of stingray; (d) colour coded digital image of (c). Please note that, in stingray, most of the CFs were in the range from 15 to 25 nm (red and green) and few of them were in the range from 25 to 30 nm (blue). In shark very few CFs were in the range from 15 to 20 nm, and most of the CFs were in the range from 20 to 30 nm (green and blue). Blue: 25–30 nm; yellow: 30–35 nm; terracotta: 35–40 nm; pink: 40–45 nm.

Figure 6

Electron micrograph and colour coded digital images of the PGs of stingray. The electron micrograph was taken from section of corneal stroma that was not stained with uranyl acetate and lead citrate; (a) electron micrograph of PGs of the anterior stroma; (b) colour coded digital image of (a); (c) electron micrograph of PGs of the middle stroma; (d) colour coded digital image of (c); (e) electron micrograph of PGs of the posterior stroma. Please note that PGs are star shaped; (f) colour coded digital image of (e). Red: 50–257, green: 257–464, blue: 464–671, yellow: 671–878, terracotta: 878–1045, pink: 1085–1292, and brown: 1292–1500.

Figure 7

Electron micrograph and colour coded digital images of the PGs of shark. The electron micrograph was taken from section of corneal stroma that was not stained with uranyl acetate and lead citrate; (a) electron micrograph of PGs of the anterior stroma; (b) colour coded digital image of (a); (c) electron micrograph of PGs of the middle stroma; (d) colour coded digital image of (c); (e) electron micrograph of PGs of the posterior stroma; (f) colour coded digital image of (e). Red: 50–257, green: 257–464, blue: 464–671, yellow: 671–878, terracotta: 878–1045, pink: 1085–1292, and brown: 1292–1500.