Bowman's layer in the cornea- structure and function and regeneration

Abstract

Bowman's layer lies immediately posterior to the epithelial basement membrane (EBM) and anterior to the stroma proper in humans, chickens, quail, zebra fish, deer, giraffe, antelope, California sea lions, guinea pig and several other species. It is not found in dog, wolf, cat, tiger, lions, rabbit, pigs, cows, goats, or horses. Developmental anomalies of Bowman's layer are rare, but acquired damage to Bowman's layer, or even complete destruction, is frequently seen in advanced bullous keratopathy or Fuchs' endothelial dystrophy. No detrimental effects of removal of Bowman's layer over the central 6-7 mm of central cornea have been noted in millions of patients who've had photorefractive keratectomy (PRK). Recent studies have suggested the randomly-oriented collagen fibrils that make up Bowman's layer do not have a significant barrier function in modulating the passage of moderate- to large-sized proteins. It is hypothesized that Bowman's layer develops in the corneas of those species that have one because of cytokine-mediated interactions occurring between corneal epithelial cells and underlying keratocytes, including negative chemotactic and apoptotic effects on the keratocytes by low levels of cytokines such as interleukin-1α that are gradually released as epithelial cells die and slough during their normal development. A "Bowman's like layer" can generate around stromal epithelial plugs after radial keratotomy, and possibly beneath the central corneal epithelial basement membrane many years after PRK.

Keywords: Bowman's layer; Chemotaxis; Cornea; Histopathology; Wound healing.

Fig. 1.

Hematoxylin and eosin staining of a human cornea (A) with Bowman’s layer (arrowheads) and a rabbit cornea without Bowman’s layer. Mag. 200X.

Fig. 2.

Hematoxylin and eosin staining (A, 200X) and transmission electron microscopy (B) of a mouse cornea. There is no Bowman’s layer in this C57 Bl/6 strain of mice. Images graciously provided by Paul FitzGerald, PhD, Dept. of Cell Biology and Human Anatomy, UC Davis School of Medicine.

Fig. 3.

Destruction of Bowman’s layer in bullous keratopathy. A human cornea with advanced bullous keratopathy stained with hematoxylin and eosin. Ectopic fibrosis has extended through Bowman’s layer, into the epithelium (large arrow), and immediately beneath the epithelium (arrowheads). Mag 200X.

Fig. 4.

Immunohistochemistry for 470 kilodalton protein perlecan (an epithelial basement membrane component) in a normal human unwounded cornea (A–C) or a normal human cornea at 30 minutes after epithelial debridement with removal of the epithelial basement membrane (D–F) in eyes enucleated for choroidal melanoma. It can be seen that perlecan penetrates the full thickness of Bowman’s layer (area between the two superior and one inferior arrowheads in each panel) either from keratocytes that have upregulated perlecan production (seen throughout stroma in E and F) or from the scraped epithelial cells as they were removed, or both. As a part of the wound healing response, some stromal cells have moved anterior where they are nearer or even in Bowman’s layer in the scraped cornea (F). Blue is DAPI staining of cell nuclei in all panels. e is epithelium. Mag 400x. Republished with permission from Torricelli et al. Exp Eye Res., 2015;134:33–8.

Fig. 5.

Evidence for negative chemotactic effects of epithelium on keratocytes/corneal fibroblasts in the cornea. When 40 ng in one microliter of mouse interleukin-1 alpha (IL-1a) is microinjected into a normal BALB/c mouse corneas (a strain of mice without Bowman’s layer visible on transmission electron microscopy²⁸), many keratocytes at the injection undergo apoptosis.²⁸ More peripheral keratocytes, that survive, redistribute in the stroma. Those anterior to the injection move more anterior. At 4 hours after the injection, staining with H&E (A) or propidium iodide (B), shows redistribution of stromal cells. The injection occurred inferior to the large arrows, which indicate the direction of redistribution of cells that were anterior to the depot of IL-1α. The small arrows in A and B indicate lines of stromal cells that have left an area of the most anterior stroma (between the asterisks) free of stromal cells. Mag 400X. Republished with permission from Wilson SE et al. Exp Eye Res 1996;62:325–8.