Tear film structure

TEAR FILM STRUCTURE Lipid (oily) 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. 

At the corneal surface, two to four layers of polygonal, extremely flat, and terminally differentiated superficial cells can be found. The cell diameters are 40-60 /xm with about 2-6 /xm thickness. The corneal surface is populated with microvilli and microplicae, resulting in surface area enlargement. These structures are associated with the tear film. Superficial corneal epithelial cells are... 

The mucins of the eyelid margin trap particulate matter, which on squeezing of the lids move the material onto the base of the eyelashes. Eyelid closure results in compression of the film, temporarily increasing the thickness of the tear film mucins in the tears must support changes in dimensions and anchor the hydrated film to the surface. The elasticity of the mucins, provided by uncoiling regions in the structure coupled with the binding of water, maintains the hydration of the cornea. 

The sclera and the cornea are the toughest and outermost layers of the eye and resist the normal internal pressure of 13 to 19 mmHg. This intraocular pressure (IOP) gives the eye its shape and maintains its dimensions that are necessary for sharp vision. The sclera covers 5/6 of the eye s surface and the cornea the remaining 1/6. Although the principal structural element of both tissues comprises of type 1 collagen fibers, differences in size and orientation of the fibers, degree of hydration, and presence of mucopolysaccharides are responsible for differences in transparency. The avascular cornea receives nourishment from the tear film, the aqueous humor, and the limbal vessels. In contrast, the sclera is vascularized and is supplied by several blood vessels, particularly in the uppermost layers (episclera). 

The lipids of the external tear film namely, neutral oils, phospholipids, sterol esters, various waxes, and other lipids are requisite for regulation of evaporation. These lipids are produced by the Meibomiam and other glands. The ability of the glandular structures situated in the conjunctiva and the lid propia to produce the tear film may be impaired due to exposure to pollutants and irritants or as a result of age-related dysfunction. 

Precorneal Tear Film Corneal transparency and good visual function require a uniform eye surface. This is achieved by the tear film, which covers and lubricates the cornea and the external globe. It is about 7-8 pm thick and is the first structure encountered by topically applied drugs. The trilaminar structure of the tear film is shown in Figure 2. 

The eye is a unique structure, because several of its fluids and tissues—tear film, cornea, aqueous humor, lens, and vitreous humor—are almost completely transparent. These components of the ocular system have no direct blood supply in the healthy state. Each can be considered a separate chamber or compartment. A compartment is defined here as a region of tissue or fluid through which a drug can diffuse and equihbrate with relative freedom. Each compartment is generally separated by a barrier from other compartments, so that flow between adjacent compartments requires more time than does diffusion within each compartment. 

Figure 14-1 Structure and composition of tear film as previously proposed. (Modified from Holly FJ, Lemp MA.Tear physiology and dry eyes. Surv Ophthalmol 1977 22 70.)...

Figure 14-2 Structure of tear film as proposed by Prydal et al. (Adapted with permission from Prydal JI, Actal P, Woon H, et al. Invest Opthalmol Vis Sci 1992 33 2006-2101. Illustration by James J. Hays.)...

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. 

Systemic drugs and their metabolites may reach the cornea and lens via the tear film, limbal vasculature, and also the aqueous hmnor. Deposition may occur, as can direct toxicity to the structures of the cornea and lens. Although corneal opacities secondary to drug therapies are often irreversible with drug cessation or reduction, these opacities may signal more permanent deposits of drug in the lens and, possibly more importantly, the retina. 

The cornea serves as the front part of the AC of the eye. Its exterior is covered by the precorneal tear film, which lubricates, nourishes and protects the corneal surface. The iris and the pupil represent the posterior border of the AC. The AC angle is an important structure which is comprised of Schwalbe s line, Schlemm s canal and trabecular mesh-work, scleral spur, anterior border of the ciliary body and the iris (Figure 5.2). Aqueous humor that fills the AC is produced by the ciliary epithelium located in the posterior chamber. The fluid flows through the pupil and is drained by the trabecular meshwork into Schlemm s canal and subsequently into the episcleral vessels. This passage is named the conventional pathway. Aqueous humor is also drained by a uveoscleral pathway across the ciliary body into the supraciliary space. 

Uses Adhesive/sealant resin for use in blown and cast film extrusion applies, incl. meat/poultry/seafood/cheese pkg., cereal liners, medical/ pharmaceutical pkg., powd./granularfood and non-food pouches, carded display/skin packaging films edible oil, motor oil, other li prod, pouches, snack structures coextrusions with nylon or in other film structures as heat seal or tie layer Features High clarity grade Regulatory FDA 21 CFR 177.1330 compliant Properties Melt flow 1.5 dg/min f.p. 78 C m.p. 103 C Vicat soften, pt. 81 C ultimate tens. str. 33.1 MPa (MD) ultimate elong. 450% (MD) Spencer impact str. 29 J/mm Dart drop str. 7.9 g/pm Elmendorf tear str. 46.3 mN/pm 9% methacrylic acid... 

Anatomical characteristics and physiological mechanisms protect the eye against toxic external effects. These mechanisms include the specific structure of the cornea, blinking, baseline and reflex lachiymation, drainage, tear film composition and the corneal sensitivity. The combination of aU mechanistic, anatomical and physiological characteristics maintains the integrity of the eye, together... 

The MWD has an important effect on the output rate, extruder torque, and pressure development in the extruder. Polyolefins with a narrow MWD have a higher output rate, extruder torque, and pressure development during extrusion than those with broad MWD. The die length has no significant effect on film structure and on factors such as tensile, tear, and impact properties. 

In most Al-containing alloys, the shape of the particles was tear-drop like due to the tight surface oxide film. The typical shape was shown in Fig.l. The effect of rapid solidification on microstructures is shown in Fig. 5 for AI2CU (precursor for Raney Cu) with a small amount of Pd (11). In the case of slowly solidified (conventional) precursor, most of the added Pd was solidified as a secondary Pd rich phase shown by white dendritic structure in Fig.5 (a). On the other hand, no such secondary phase was observed in a rapidly solidified precursor as shown in Fig.5 (b). 

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