Functional morphology of the cornea of the Little Penguin Eudyptula minor (Aves)

Abstract

The cornea is a specialized component of the vertebrate eye that provides protection, refractive power, transparency for optical imaging and mechanical support. However, the corneas of birds have received little attention with no comprehensive study of their functional morphology. Using light microscopy and both scanning and transmission electron microscopy, the first description of the ultrastructure of all of the main components of the cornea in two different-sized individuals of the Little Penguin Eudyptula minor is presented. Two types of microprojections protrude from the surface of the cornea with a predominance of microridges and microvilli found in central (flattened) and peripheral regions, respectively. Epithelial cell density is higher in peripheral cornea, especially in the larger (older) individual, while there is a reduction of epithelial cell density with age. The cornea comprises a thick epithelium uniquely attached to the basement membrane with numerous incursions rather than anchoring fibres and anchoring plaques as is found in other vertebrate corneas. Posterior to Bowman's layer, the orthogonally-arranged collagen fibril lamellae in the stroma form extensive branches and anastomoses. Desçemet's membrane is well-developed with an anterior or foetal portion with long banding. However, the thickness of Desçemet's membrane is larger in the older individual with the inclusion of an additional irregular pale-staining posterior portion. Polygonal endothelial cells extend across the cornea as a monolayer with often tortuous cell junctions. Endothelial cell density increases towards the periphery, but decreases with age. Primary cilia are observed protruding through the central region of some endothelial cells into the anterior segment but subsurface structures resembling cilia suggest that these features may be more common. The ultrastructure of the corneal components reveals a range of functional adaptations that reflect the amphibious lifestyle of this seabird.

Keywords: Desçemet's membrane; cornea; endothelium; epithelium; microridges; microvilli; penguin; stroma.

FIGURE 1

The eyes and cornea of the Little Penguin, Eudyptula minor. (a) Frontal view of the Little Penguin E. minor showing the two eyes subtending a binocular overlap in front of the beak. (b) Close up of the eye. Asterix depicts the medial canthus of the right eye. (c) Light micrograph of the cornea in transverse section. B, location of Bowman's layer; en, endothelium; ep, epithelium; s, stroma. Scale bars: 10 mm (a); 10 mm (b) and 70 µm (c)

FIGURE 2

Corneal (outer) surface ultrastructure. (a) Scanning electron micrograph of the central region of the cornea of Eudyptula minor showing the variable size of the polygonal epithelial cells covered with microridges. (b) Scanning electron micrograph of the peripheral region of the cornea showing the epithelial cells covered with microvilli. (c, d) Close up of the surface ultrastructure of the microridges (c) and microvilli (d) in the central and peripheral regions respectively. (e, f) Transmission electron micrographs of a mixture of microridges (arrowheads in e) and microvilli (arrows in f) protruding from the surface of the epithelium. The inset in (e) shows a complex of microridges viewed in tangential section (cut parallel to the epithelial surface). Scale bars: 5 µm (a, b); 1 µm (c, d); 0.5 µm (e, f); 0.5 µm (Inset in e)

FIGURE 3

Ultrastructure of the corneal epithelium and stroma. (a) Transmission electron micrograph of the corneal epithelium of Eudyptula minor. Note the multilayered epithelium with columnar basal cells and flattened squamous cells near the surface. (b) Low‐power electron micrograph showing the posterior stroma, Desçemet's membrane and the endothelium in the smaller of the two individuals examined. (c) Higher power of the basement membrane with numerous incursions into the basal epithelial cells (arrows). (d) Bowman's layer showing the irregular arrangement of collagen fibrils underlying the basement membrane. (e) Close up of the branching and anastomosing (asterisks) collagen lamellae within the corneal stroma. bm, basement membrane; Bow, Bowmans layer; cl, collagen lamella; De, Desçemet's membrane; en, endothelium; ep, epithelium; k, keratocyte; n, epithelial cell nucleus. Scale bars: 5 µm (a); 2 µm (b); 1 µm (c–e)

FIGURE 4

Ultrastructure of the posterior cornea. (a–d) High‐power transmission electron micrographs of the posterior cornea in Eudyptula minor in transverse section. A. Posterior cornea in the small (younger) individual, which possesses a well‐developed Desçemet's membrane (foetal portion) closely opposing the endothelium. Note the inclusion of a number of lipid globules within the endothelium. (b–d) Posterior cornea in the large (older) individual, where there is an additional irregular pale‐staining posterior portion of Desçemet's membrane (asterisks) apposing an endothelium with inter‐cell junctions that are extremely tortuous, with projections that often rise above the surface as ruffled margins (arrows). cl, collagen lamellae; De, Desçemet's membrane; en, endothelium; k, keratocyte; l, lipid globule; n, nucleus. Scale bars: 1 µm (a–d)

FIGURE 5

Corneal (inner) surface ultrastructure. (a, b) Scanning electron micrographs of the central corneal endothelium in the large (a) and small (b) penguins. Note the endothelium consists of mainly hexagonal and pentagonal cells with rounded edges and tortuous frilled or ruffled margins, some with centrally located primary cilia. (c, d) Surface ultrastructure of the interdigitating edges of adjacent endothelial cells, which protrude above the surface (see Figure 4c). Inset in (c) shows a scanning electron micrograph of a primary cilium. Inset in d shows a transmission electron micrograph of a transverse section of a developing cilium just below the surface of the endothelium. Scale bars: 10 µm (a, b); 1 µm (c, d) and 0.5 µm (insets in c and d)