Characterization of corneal epithelium and endothelium with scanning electron microscopy in three avian species

Imagine your eye as a high-tech camera lens, but instead of glass, the front window is made of living tissue called the cornea. This clear, dome-shaped shield is the star of the show when it comes to focusing light so you can see the world in sharp detail. But for this "window" to stay crystal clear, it needs a very special team of workers on its surface and on its backside. Think of the front layer (the epithelium) as a bustling city sidewalk, covered in tiny, finger-like projections that help grab nutrients and keep the tear film stable, like a non-slip mat on a wet floor. On the flip side, the back layer (the endothelium) acts like a one-way pump and a structural support beam, keeping the cornea from getting waterlogged and cloudy. Scientists have spent years studying these cellular "tiles" in humans and some animals, but there are still huge gaps in our knowledge about birds. With over 11,000 species of birds zooming around the sky, we know surprisingly little about how their eye windows are built, especially for the orders Columbiformes (pigeons) and Passeriformes (songbirds). Understanding these tiny structures helps us appreciate the evolutionary engineering behind flight and vision, and it gives us a baseline to spot what happens when things go wrong.

This study decided to zoom in—way in—on three very different birds to see what their corneal "tiles" look like under a super-powerful microscope called a scanning electron microscope (SEM). The researchers picked a blue-and-white swallow, a common pigeon, and a rufous-bellied thrush. They didn't just take a quick peek; they counted the cells, measured their sizes, and looked at the tiny bumps and holes on their surfaces. It's like taking a census of a microscopic city to see how many houses there are, how big they are, and whether the roofs are covered in shingles or spikes.

Here is what they found. The front layer of the cornea in all three birds is a mosaic of differently shaped cells, and every single one is densely packed with tiny, hair-like spikes called microvilli. It's like a forest of microscopic grass covering the surface. In the blue-and-white swallow, the edges of these cells are raised up, forming little walls, but in the pigeon and the thrush, the borders are a bit blurry and hard to define. When they counted the population, the swallow had about 8,333 cells per square millimeter, the pigeon had roughly 8,667, and the thrush was the most crowded with 9,000 cells per square millimeter.

On the back side of the cornea, the story is a bit more uniform. The endothelial cells in all three species are mostly shaped like hexagons, looking like a perfect honeycomb. However, the surface texture varies. The swallow and the pigeon have a mix of tiny spikes (microvilli) and little pits (microholes) on their cells, while the thrush's cells are covered mostly in short spikes. The researchers couldn't count the swallow's back-layer cells because some of their photos got lost, but for the others, the numbers were high: the pigeon had about 9,074 cells per square millimeter, and the thrush had about 9,537.

The study also measured the size of the honeycomb tiles. For the pigeon, the average cell area was 96.84 ± 10.72 µm², and for the thrush, it was 86.25 ± 7.10 µm². These are quite small compared to what has been seen in other birds. The cells were also very regular in shape; in both the pigeon and the thrush, exactly 75% of the cells were perfect hexagons. The variation in cell size (a measure called the coefficient of variation) was low, at 0.11 for the pigeon and 0.08 for the thrush, suggesting these are healthy, well-organized cellular communities.

One interesting thing the paper notes is that while some other birds have a tiny, single hair-like structure called a cilium in the middle of their endothelial cells, the researchers didn't see any in these three species. They suggest this might just be because the structure retracts or disappears depending on the cell's state at the moment of capture, rather than meaning these birds lack them entirely.

Ultimately, this paper is the first to describe these specific ultrastructural details for the blue-and-white swallow, the common pigeon, and the rufous-bellied thrush. It confirms that while these birds share some general features with other species—like the honeycomb shape of the back cells and the micro-spike coverage on the front—there are unique variations in cell borders and surface textures. The authors suggest that the small size of the endothelial cells might be a special trait for these bird orders, but they are careful to say that more studies with larger groups of birds are needed to confirm if this is a rule or just a coincidence. For now, we have a clearer picture of the microscopic architecture that keeps these birds' eyes clear and their vision sharp.