Ocular pathology in trisomy 18. A histopathological report of three cases

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

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.

Congenital optic nerve pit in trisomy 18

The authors report the first case of trisomy 18 associated with a clinically detectable optic nerve pit. A female infant with a birth weight of 2,150 g was born by cesarean section to a healthy 40-year-old woman at 38 weeks of gestation. Trisomy 18 had been diagnosed by prenatal genetic testing. Ophthalmologic examination was remarkable for bilateral narrowed palpebral fissures with punctal agenesis, corectopic pupils without reaction to light, bilateral inferior peripapillary retinochoroidal hypopigmentation, and significant optic nerve cupping in the left eye with associated temporal optic nerve pit. It has generally been accepted that optic nerve pits are a congenital anomaly. However, the pathophysiological background of optic nerve pits remains unclear and controversial. This is the first clinical and photographic documentation of an optic nerve pit in a neonate and in Edwards syndrome.

Bowman's layer structure and function: critical or dispensable to corneal function? A hypothesis

PURPOSE

The purpose of this article is to review available information regarding development, structure, and function of Bowman's layer in the cornea. Disease-related abnormalities of Bowman's layer are described. A hypothesis is advanced to explain the development and maintenance of Bowman's layer.

METHODS

Literature review and hypothesis formulation based on previous studies.

RESULTS

Information is presented that supports the hypothesis that Bowman's layer forms as a result of cytokine-mediated interactions occurring between corneal epithelial cells and keratocytes that include chemotactic and apoptotic effects on the keratocytes. This hypothesis suggests that Bowman's layer results from such interactions beginning in early development and continuing into adulthood in humans and other animals, such as chickens.

CONCLUSIONS

Bowman's layer may be a visible indicator of ongoing stromal-epithelial interactions in the human and have no critical function in corneal physiology. Bowman's layer is commonly destroyed in diseases such as advanced bullous keratopathy where stromal-epithelial interactions may be interrupted. Bowman's-like layers often form in response to epithelium, for example when epithelial plugs extend into the stroma in corneas with radial keratotomy incisions.