Functional morphology of the cornea of the Little PenguinEudyptula 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.

The Functional Anatomy of the Cornea and Anterior Chamber in Lampreys: Insights From the Pouched Lamprey, Geotria australis (Geotriidae, Agnatha)

“…It has a structure with an anterior (embryonic) banded portion with a periodicity of between 110 and 120 nm ( Murphy et al, 1984 ; Gipson, 1994 ) and a less dense, non-banded portion. A similar structure has been reported in the aquatic Little penguin ( Eudyptula minor ) ( Collin and Collin, 2021b ) and in numerous mammals, including, the rat (3.5 μm thick with a periodicity of 120 nm), the mouse (ddY strain) (2.5 μm thick with a periodicity of 120 nm), the mouse (C3H strain) (5 μm thick with a periodicity of 120 nm), the guinea pig (13 μm thick with a periodicity of 120 nm), the cat (periodicity of 100 nm), the rabbit (5.6 μm thick) and various species of cattle (20 μm thick, periodicity 120 nm) ( Hayaski et al, 2020 ).…”

“…Between 20 and 50 examples of each corneal feature were measured (± standard deviation) and dimensions were compared using a two-tailed t-test for independent variables. Measurements were performed on both left ( n = 3) and right ( n = 4) eyes of seven individual downstream migrants, but all features were not found to be significantly different, as has been found for corneal features in a range of other vertebrate eyes ( Nam et al, 2006 ; Werther et al, 2017 ; Collin and Collin, 2021b ). Therefore, the data for left and right eyes were pooled.…”

“…No cells, i.e., keratocytes or inflammatory cells, are present within the stroma of the scleral cornea. No branching and anastomosing of collagen lamellae or any vertical sutures are present in the scleral cornea ( Collin and Collin, 2021b ).…”

“…Animals were kept in aquaria in a temperature-controlled room that was maintained at 10–14°C with a 12 h:12 h light:dark cycle. While maintained in aquaria, the head and eyes of both downstream and upstream migrants were photographed using a Nikon digital camera (D5600) ( Collin and Collin, 2021b ).…”

Confocal and Electron Microscopic Structure of the Cornea from Three Species of Penguin

“…However, the shape of the penguin cornea does vary, with the smaller penguins having smaller, more steeply curved corneas [ 3 ]. The most recent transmission (TEM) and scanning (SEM) electron microscopic study of the penguin cornea was performed on what was described as a little penguin Eudyptula minor [ 4 ], although we note that these specimens were obtained in Western Australia while ours came from Auckland, New Zealand and recent evidence suggests that they are different species although of the same genus [ 5 , 6 , 7 , 8 , 9 ]. The cornea of both the former penguin and the Magellanic penguin Spheniscus magellanicus have also been the subject of previous electron micrographic studies [ 2 , 10 , 11 , 12 ].…”