Does the cornea have 5 or 6 layers?

In a recent issue of Optometry Times (July 2013), it was reported that researchers at the University of Nottingham, UK, had discovered a new corneal layer, and the observation was published in Ophthalmology.¹ In the same issue of this journal, a dissenting interpretation of this observation was presented.²

Dua et al¹  reported that using the Big Bubble (BB) technique they were able to separate the five to eight most posterior lamellae from the rest of the stroma and that these posterior lamellae were acellular.¹ The thickness of this postulated layer was on average 10 microns and contained the same collagenous components as the rest of the stroma. Dua and coauthors felt that this new observation “will have considerable impact on posterior corneal surgery.”

The authors of the alternative interpretation² pointed out that this posterior portion of the stroma is not acellular and showed pictures of keratocytes within five microns of the posterior limiting lamina (eponymous name: Descemet’s membrane). A review of literature shows that it has been known for 150 years that there is a difference between anterior and posterior stroma with regards to its biomechanical properties. For years, many surgeons have avoided the BB procedure in posterior corneal surgery because corneal microperforation can easily occur. Trefination with blunt dissection is a safer alternative. The extensive and intricate interwoven layout of anterior lamellae as opposed to the more-simple layering of posterior lamellae on top of each other explains the biomechanical change the stroma undergoes from anterior to posterior. Indeed, over the past couple of decades my own dissections of human cadaver corneas confirm this mechanical difference. A current and annually updated textbook on ocular anatomy has for years contained text and images pointing out the differences between anterior and posterior stromal architecture.³

Therefore, the BB technique, which is after all a highly non-physiological approach, has not demonstrated anything about which we did not already know. In addition, the proposed corneal layer does not contain anything we do not find in the rest of the stroma. The dissenting view article² acknowledges Dua et al’s¹ work as novel in demonstrating the biomechanical property of the extreme posterior stroma, but it was argued that their work has not isolated a new corneal layer. However, future research will determine the final outcome of this difference of opinion among scientists,

An additional guest editorial in the same issue of Ophthalmology made the point that that science is self correcting but not self congratulating.⁴ The latter is a reference to the fact that Dua et al¹ proposed that we call this new corneal structure “Dua’s layer.” Historically it is not the discoverer who adds his or her name to the anatomical nomenclature but subsequent followers may wish to promote such an acknowledgement. A classic example of this more respectful approach is Sir William Bowman, who 1847 proposed that the layer he was first to describe and sandwiched between the epithelium and the stroma be called the anterior elastic lamina. However, modern morphologists are trying to get away from the confusing anatomy of eponyms and replace these names with a more descriptive terminology. This is not a new idea but a 150-year-old effort, which most recently was published in 1998 as Terminologica Anatomica.⁵

In summary, my last count of the layers forming the cornea ended with five layers, and that is what I will teach my students.ODT

References

1. Dua HS, Faraj LA, Said DG, et al. Human Corneal Anatomy Redefined: a novel pre-Descemet’s layer (Dua’s Layer). Ophthalmology. 2013 Sep;120(9):1778-85.

2. Jester JV, Murphy, CJ, Winkler M, et al. Lessons in Corneal Structure and Mechanics to Guide the Corneal Surgeon. Ophthalmology. 2013 Sep;120(9):1715-7.

3. Bergmanson, JPG. Clinical Ocular Anatomy and Physiology. 20^th Edition. Texas Eye Research and Technology Center, Houston, Texas, 2013.

4. Schwab I. Who’s on First. Ophthalmology. 2013 Sep;120(9):1718-9.

5.Terminologica Anatomica. Federative Committee on Anatomical Terminology (FCAT). Stuttgart, Thieme, 1998.