Bowman’s layer

Microscopically, the cornea shows a rather simple and multilayered structure that can be divided into six layers the epithelium, basement membrane, Bowman s layer, stroma, Descemet s membrane, and endothelium. The corneal tissue consists of three different cell types epithelial cells, keratocytes (corneal fibroblasts), and endothelial cells. The outermost corneal surface is covered with the preocular tear film, which is functionally associated with the cornea. The epithelial surface must be kept moist and smooth, a role played by the tear film in conjunction with a spreading function of the eyelids during blinking motions. Furthermore, the tear film provides a protection against infectious agents that may gain access into the eye. 

As another extracellular component in the cornea, the Bowman s layer is an acellular and amorphous band between the corneal epithelium and stroma. The layer is about 8-12 [im thick and consists of randomly arranged collagen fibers (types I and III) and proteoglycans. The physiological function of Bowman s layer is not yet completely understood, since not all animal species exhibit this membrane in the corneal structures, but an important role in the maintenance of the corneal epithelial structure is expected or probable, since a damaged Bowman s membrane usually results in scarring during wound repair [16],... 

Merindano MD, Costa J, Canals M, Potau JM, Ruano D. A comparative study of Bowman s layer in some mammals Relationships with other constituent corneal structures. Eur J Anat 6 133-139 (2002). 

FIGURE 24.2 Cornea. EPITH, epithelium ENDOT, endothelium BM, basement membrane BWN, Bowman s layer ST, stroma DSM, Descemet s membrane ZO, zonulae occludentes. 

Bowman s layer is the modified anterior border of stroma in humans.This layer is 8 to 14 mm thick and is composed of clear randomly oriented collagen fibrils surrounded by mucoprotein ground substance. Numerous pores in the inner structure allow the passage of terminal branches of comeal nerves from the stroma into epithelium.The surface of Bowman s layer adjoins the stmcturally distinct epithelial... 

Histologic cross-section of the cornea reveals five identifiable layers epithelinm. Bowman s layer, stroma, Descemet s membrane, and endothelium. Fluid surrounds the cornea in the forms of the tear film in front and the aqneons behind. The various corneal layers combine to form a structure that is approximately 633 mcm thick at the inferior periphery, 673 mcm at the superior periphery, and 515 mcm thick centrally. The adult corneal diameter measmes 11 to 12 mm horizontally and 9 to 11 mm vertically, creating a horizontally oriented ellipse. The radius of ciuvature of the central 3-mm optical zone ranges between 7.5 and 8.0 mm. 

Bowman s layer is a thin homogeneous sheet of acellular randomly arranged collagen fibers lying between the epithelial basement membrane and the stroma. Bowman s layer is relatively tough and provides substantial resistance to corneal injury or infection. Because it cannot regenerate, scarring results when it is disrupted. 

Corneal dystrophy of Bowman s layer type 1 Reis-Biicklers TGBFI AD Comeal surface appears rough and irregular with accumulation of opacities at Bowman s layer in annular, crescent, polygonal, or map-like formations. Opacities are confined to central and mid-peripheral cornea, whereas the extreme periphery remains transparent. RCE common with surgery often required in second or third decades due to severe vision loss. 

Corneal dystrophy of Bowman s layer type 11 Thiel-Behnke TGFBI AD Characteristic superficial opacification in a honeycomb pattern. RCE common, though symptoms and opacification not as severe as in Bowman s type I. 

Figure 26-9 (A) Vertical striae (B) Fleischer s ring (C) scarring at Bowman s layer. (Courtesy of Pat Caroline.)...

If fluid enters the cornea at a rate fester than it is removed by the endothelial cells, edema results. Fluid accumulates in the epitheUiun as well as the stroma and causes the epitheliiun to separate from Bowman s layer. Clinically, these areas of separation between Bowman s layer and the epitheUiun are called bullae, which appear like small bUsters on the front surfece of the cornea. With time and the normal growth of epithelial cells, these bullae are pushed anteriorly in the cornea and erupt at its surfece. 

If the patient has limited visual potential because of other factors, pain relief may be provided by surgical intervention, such as a conjimctival flap procedure, anterior stromal pimcture with a 20-gauge needle, or PTK. Electrocautery of Bowman s layer and partial trephination of the cornea have also been reported as successful methods of pain control. 

Examination shows a dusting of gray-white deposits in Bowman s layer or a slight hazing of the cornea early in the course of the disease. It typically starts at 3 and 9 o clock and progresses slowly toward the center, usually taking several months to years to coalesce and form a complete band across the interpalpebral cornea (Figure 26-13). The deposit is separated from the limbus by a clear zone and develops the characteristic Swiss cheese appearance because of the multiple clear areas within the plaque. 

Reports show some variation in the characteristics of band keratopathy. There may be two morphologic types, with the first type presenting with an intact and smooth epithelium, little discomfort, and deposition of the calcium at the level of Bowman s layer. The second type presents with unstable epithelium in a painful eye. The deposits in the second type tend to extend into the stroma. Band keratopathy occurs much fester in patients... 

RCEs are reoccurring episodes of spontaneous breakdown or sloughing of the epithelial layer of the cornea. RCEs are caused by poor adhesion complexes between the epithelial basement membrane and Bowman s layer. 

Patients with chronic RCE and widespread ABMD benefit from therapeutic modalities that treat larger areas of the cornea. PTK has been shown to be an effective treatment for these patients, resulting in decreased symptoms and increased visual acuity. PTK is useful for corneal erosions that affect the visual axis, and it can be combined with photorefractive keratectomy. One drawback of PTK is the expensive equipment required to perform the procedure. PTK removes superficial tissue of Bowman s layer to allow the formation of a new basement... 

The differences in physiology and sensitivity to tested chemicals between rabbit and human eyes. Indeed, the eyes of rabbits appear to present several anatomical and physiological differences when compared to the human eye such as lower tear production, lower blink reflex, thinner cornea, larger corneal surface area, lack of Bowman s layer, larger conjunctival sac, different tear constituents and the presence of nictitating... 

Macular corneal dystrophy types I and II have also been characterized histo-chemically. In normal corneas, high levels of sulfated keratan sulfate were detected in the stroma. Bowman s layer, and Descemet s membrane with low levels in the keratocytes, epithelium, and endothelium. Furthermore, in normal corneas, negligible levels of labeling for A-acetyllactosamine (unsulfated keratan sulfate) were detected. In macular corneal dystrophy type I corneas, sulfated keratan sulfate... 

Superficial cells Wing cells Basal cells Bowman s layer... 

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