How Do You Build a Transparent Cornea Out of Cells and Proteins? - Evolution News & Views

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How Do You Build a Transparent Cornea Out of Cells and Proteins?


A glimpse into cornea construction, a "poorly understood" process, has been provided by UK scientists publishing in PNAS. They examined chick embryos. In the film Flight: The Genius of Birds, Dr. Ann Gauger of Biologic Institute calls embryogenesis an "elaborate dance," and viewers get just the 30,000-foot overview of the performance. Dr. Timothy Standish adds, "What you're seeing is a mechanism at work: information being translated into a physical product at each step along the way: machines doing jobs -- it's absolutely incredible." But as Gauger says, "You gotta pay attention to the details."

Let's examine some of the details in just one organ in the chick embryo: the cornea, which is visible in the video clip from Flight. The UK team, using volume scanning electron microscopy, mapped some of the ordered layers of collagen, keratocyte cells and fluid-filled matrix that comprise the stroma, about 90 percent of the thickness of the cornea. They watched the progress of the cornea from day 10 of the 21-day egg incubation period; all systems were completed by day 18. One new discovery was of long fibrils extending out from the cells:

The results show that corneal keratocytes occupy a significantly greater tissue volume than was previously thought, and there is a clear orthogonality in cell and matrix organization, quantifiable by Fourier analysis. Three-dimensional reconstructions reveal actin-associated tubular cell protrusions, reminiscent of filopodia, but extending more than 30 μm into the extracellular space. The highly extended network of these membrane-bound structures mirrors the alignment of collagen bundles and emergent lamellae and, we propose, plays a fundamental role in dictating the orientation of collagen in the developing cornea.

Critical to the transparency of the cornea is the orderly arrangement of material. The keratocytes are distributed throughout the matrix, where they can regulate the layout of collagen with these protrusions (which they dubbed keratopodia). The collagen fibrils are highly regulated in their layout. Such things don't just happen. You can detect some astonishment in their description:

The unique transparent quality of the cornea arises from its remarkably ordered architecture of aligned and regularly spaced fibrils with a small, consistent diameter (∼30 nm), which are arranged, not into fibers or fascicles as in most other tissues but in superimposed, flattened layers, or lamellae. Lamellae and their component collagen fibrils exhibit preferential orientations throughout the corneal thickness, which appear to be closely related to the biomechanical loads to which the tissue is subjected. In adult vertebrates, lamellae traverse the full diameter of the cornea for most of its thickness, and in the avian eye -- the subject of most developmental studies -- undergo a gradual rotation in their orientation with depth. Individual collagen fibrils within midstromal lamellae also appear to traverse the entire diameter of the cornea, a distance of ∼11 mm in adult human eyes. The extraordinary level of order in matrix organization within a hierarchy of fibril, lamella, and stroma overall appears to reflect a considerable level of regulatory influence presumably involving both cell activity and intermolecular interactions.

The organization of these components thus serves at least two functions: it preserves transparency, and it creates a fibrillar network that can tolerate the biomechanical load. (Think about that when you rub your eyes.)

Their micrographs show the collagen laid out in intersecting sheets at nearly right angles, flat and layered, with the cells embedded within them. Connecting it all are the keratopodia extending from the cells, their actin filaments providing stretch and tension like the webbing in a hammock to tolerate stress while maintaining the proper shape. To maintain transparency, the fibers must remain within a certain size range and spacing.

Such exquisite detail at the cellular level, involving "information being translated into a physical product... machines doing jobs," truly is incredible. And it all comes together in a circular organ that is transparent and endowed with just the right curvature and index of refraction for focusing light on the retina -- all in just 8 days.

After cataract surgery, patients can focus light fairly well with the cornea alone. Indeed, most of the focusing is performed at the cornea, with the crystalline lens (another wonder of cellular engineering) providing fine focusing.

It's amazing enough that cells can build this transparent stroma layer, with its lamellae of collagen at the right orientation, its keratocytes evenly spaced, and the keratopodia helping to hold it all together. It's another level of amazement to have the entire structure conform to an ideal optical shape for focusing a motion picture on a distant screen, the retina.

And remember -- this is living tissue, served by a transparent fluid, the aqueous humor, that delivers nutrients and transmits waste to the blood vessels at the rim. We know from the pain of a blow to the eye that the cornea is a very sensitive organ. That's because of a dense network of nerves that make the cornea one of the most sensitive tissues in the body, 20-40 times more sensitive than tooth pulp and 300-600 times more sensitive than skin (Wikipedia).

Details, details. Such exquisite organization of multiple independent parts, all regulated by information systems and executed by machines, challenges materialistic explanations of its origin. Naturalism, by definition, is incapable of planning for functional goals. When we see an integrated system regulated by software and machinery, we know from uniform experience that a mind played a role in its construction.

We've been discussing design just in the cornea -- one little organ in a developing bird embryo. Now extend that thought to the rest of the eye, the brain, and the whole bird that will hatch out of the egg in a few days.

Image credit: Illustra Media.