Eye Cornea Stuns Physiologists
The Cornea: More complex than assumed a few weeks ago
As science progresses, evolution regresses
by Jerry Bergman, PhD
Almost weekly, new discoveries are made in science to support the intelligent design view of life. The research reviewed in this paper is no different.
It has long been believed that the eye’s protective cornea (i.e., its transparent outer covering) was devoid of any memory T cells because certain aggressive immune cells could damage the clear layer of corneal tissue and, as a result, significantly obstruct vision. In order to refract light properly, the cornea must remain transparent, free of obstructing particles. A high level of transparency of the most anterior layer of the cornea is critical for vision clarity. The cornea itself is a complex structure made of five main layers: the epithelium, Bowman’s membrane, the stroma, Descemet’s membrane, and the most inner layer, the endothelium (see Figure 1).
Figure 1. The 5 major layers of the cornea (not to scale). From Wikimedia Commons. Illustration by Parker Ludwig.
Structure of the cornea
The outer epithelium layer provides a critical barrier to prevent foreign material from entering the eye. It is composed of a single layer of basal cells and five cell layers of non-keratinized, stratified squamous epithelial cells held together by tight junctions. This layer forms an effective barrier reducing fluid loss and protecting the eye from pathogen penetration.[1]
Bowman’s membrane largely consists of collagen fibers that serve to structurally reinforce the cornea and ensure that its proper shape is maintained. Ninety percent of the corneal thickness consists of the stromal layer composed primarily of water and collagen to provide the cornea with the required flexibility and strength, as well as structural integrity. Pressure on the cornea, such as when you rub your eyes, is compensated for by the eye’s corneal elasticity and ability to retain its required shape.
The corneal endothelium consists of specialized, flattened, mitochondria-rich cells which line the posterior surface of the cornea. It regulates fluid and solute transport across the posterior surface of the cornea to maintain it in the slightly dehydrated state required for optical transparency.
Descemet’s membrane anchors the endothelium to the cornea, while simultaneously allowing nutrients and macromolecules to enter into the corneal stroma.[2] The corneoscleral junction, also known as the limbus, demarcates the cornea from the sclera, which consists of a spongy, fibroblastic connective tissue that supports the structural integrity of the eye.
The cornea receives nourishment from tears and the aqueous humor inside of the eye. Scarring of the cornea caused by a wide variety of infectious and inflammatory diseases leads to vision loss and, in severe cases, blindness.
This complex structure is adequate to protect the eye
Up till now, this complex cornea structure alone was assumed to achieve the pathogenic protection required to protect the eye from bacteria and virus assaults. Furthermore, because “aggressive immune cells could damage the clear layer of tissue and obstruct vision,” immune responses were known to be dampened to protect vision.[3] Researchers also assumed that specialized immune memory T cells did not normally reside in the transparent cornea, and it was not clear whether T cells persist and form immunological memory populations in the cornea.[4] Now it has been confirmed that specialized immune cells that circle the tissue are ready to attack pathogens. Furthermore, long-lived immune cells, the T-cells that swiftly attack pathogens they have previously encountered, are now known to produce ‘immune memory’ which persists after an infection.
The research methodology
Researchers at Johns Hopkins University used a powerful multiphoton microscope and intravital two-photon microscopy where two-photon fluorescence excitation is caused by two photons arriving within a femtosecond (one millionth of one billionth, or 10-15, of a second) of each other at a fluorophore (a fluorescent chemical compound that can re-emit light upon light excitation) to cause fluorescence (glowing that reveals the location of a molecule of concern). This technique was used to examine the living corneas of six healthy adult mice
whose eyes had been infected with herpes simplex virus. They saw that cytotoxic T-cells and T-helper cells — precursors for immune memory — had infiltrated the cornea and persisted for up to a month after the infection. Further investigations, including more intrusive microscopy techniques, revealed that the cytotoxic T-cells had developed into long-lived memory cells that resided in the cornea.[5]
The new research determined that long-lived memory T cells that “patrol and fight viral infections are present in the cornea, upending current thought that T-cells are not found in healthy corneas — expanding our understanding of the eye’s immune response to infections.”[6]
Figure 2. The vertebrate eye showing some of the main features.
This discovery shows that the outer eye environment is much more complex than previously believed.[7] The implications of this finding include a better understanding of diseases, such as chronic dry eye syndrome which can cause blindness if not treated, corneal-transplant rejection, and progressive corneal loss in persons suffering from certain autoimmune diseases. Corneal infections are more common in Africa and, fortunately, are relativity rare in the developed nations.[8] The main cause of corneal blindness include vitamin A deficiency, the aftereffects of bacterial, fungal, or viral infections, eye trauma, congenital disease, and traditional medicine or home remedies which often harm the eye rather than help relieve pain or improve eyesight.
Summary
New research has determined that pathogens on the eye cornea can trigger long-living memory T cells. This response has allowed us to better understand the complexity of the eye’s matrix and the level of protection built in to protect it. This finding adds to scientific knowledge of the complexity of the eye, illustrating how it can last a lifetime of healthy operation. No doubt more research will further enable scientists to learn how well-designed the eye truly is. The eye is a testimony to intelligent design and expert engineering.
References
[1] “Corneal Epithelium.” LifeMap Discovery. https://discovery.lifemapsc.com/in-vivo-development/eye/corneal-epithelium, 2022.
[2] Khurana, A.K., and Indu Khurana. Anatomy and Physiology of Eye, Third Edition. CBS Publishers & Distributors: New Delhi, India, 2017.
[3] Mallapaty, Smriti. A surprise in the eye: Long-lived T cells patrol the cornea. Nature; https://www.nature.com/articles/d41586-022-01578-2, 3 June 2022.
[4] Loi, J.K., et al., Corneal tissue-resident memory T cells form a unique immune compartment at the ocular surface. Cell Reports 39(8):110852, 24 May 2022.
[5] Mallapaty, 2022.
[6] The Peter Doherty Institute for Infection and Immunity. Cornea T cells protect eyes from viral infections, researchers discover. Science Daily; https://www.sciencedaily.com/releases/2022/05/220524110713.htm, 25 May 2022.
[7] Loi, et al., 2022.
[8] “Corneal Blindness.” See International; https://www.seeintl.org/corneal-blindness/.
Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.