Stroma structure

Plant cells contain a unique family of organelles, the plastids, of which the chloroplast is the prominent example. Chloroplasts have a double membrane envelope, an inner volume called the stroma, and an internal membrane system rich in thylakoid membranes, which enclose a third compartment, the thylakoid lumen. Chloroplasts are significantly larger than mitochondria. Other plastids are found in specialized structures such as fruits, flower petals, and roots and have specialized roles. 

The cornea is the first structure of the eye to be in contact with incident light. It is composed of five distinct layers lying parallel to its surface the outer epithelium, which is continuous with the epithelial layers of the conjunctiva the epithelial basal lamina the keratocyte-containing stroma, which is a collagen structure arranged so that it is transparent Descemet s membrane and, finally, the endothelium adjacent to the aqueous humour. 

One of the key parameters for correlating molecular structure and chemical properties with bioavailability has been transcorneal flux or, alternatively, the corneal permeability coefficient. The epithelium has been modeled as a lipid barrier (possibly with a limited number of aqueous pores that, for this physical model, serve as the equivalent of the extracellular space in a more physiological description) and the stroma as an aqueous barrier (Fig. 11). The endothelium is very thin and porous compared with the epithelium [189] and often has been ignored in the analysis, although mathematically it can be included as part of the lipid barrier. Diffusion through bilayer membranes of various structures has been modeled for some time [202] and adapted to ophthalmic applications more recently [203,204]. For a series of molecules of similar size, it was shown that the permeability increases with octa-nol/water distribution (or partition) coefficient until a plateau is reached. Modeling of this type of data has led to the earlier statement that drugs need to be both... 

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],... 

In another approach, Parnigotto and coworkers reconstructed corneal structures in vitro by using corneal stroma containing keratocytes to which corneal epithelial cells from bovine primary cultures were overlaid [73], However, this particular corneal model did not contain an endothelial layer. This model was histochemically characterized and the toxicity of different surfactants was tested using MTT methods. This stroma-epithelium model has been reported to show a cornea-like morphology, where a multilayered epithelial barrier composed of basal cells (of a cuboidal shape) and superficial cells (of a flattened shape) is noted. Furthermore, the formation of a basement membrane equivalent and expression of the 64-kDa keratin were reported, indicating the presence of differentiated epithelial cells. The toxicity data for various surfactants obtained with this model correlate well with those seen by the Draize test [73], However, this corneal equivalent was not further validated or used as a model for permeation studies. 

Cintron, C. The molecular structure of the comeal stroma in health and disease. In Chandler, J.W., Sugar, J., Edelhauser, H.F. (eds.) External Diseases Cornea, Conjonctiva, Sclere, Eyehds, Lacrimal System, vol. 8. Mosby, London (1994)... 

These vessels end in arcade-like structures at the limbus. The corneal stroma is made of three different main layers that differ in density of collagen and type of packing. The Bowman s membrane of the anterior stroma is part of the basal membrane of the corneal epithelium and accounts for 5% of the thickness of the central 500-600 pm cornea. The corneal stroma consists of highly ordered, horizontally organized and noninterconnected coUagen I and X fibriUae that are kept in a hydrated state with a water content of 72-78% and an osmolarity of 420 mOsmol/kg [1]. 

Langefeld, S., Reim, M., Redbrake, C., Schrage, N.F. The corneal stroma, an inhomogenous structure. Graefes Arch Clin Exp Ophthalmol 235(8), 480 85 (1997)... 

The edema may be benign, thus enabling a complete examination of all the structures of the anterior chamber iris, pupil, and lens. The edema may be medium when it makes the cornea translucent and then only a faded iris can be observed. It may also be important, and then it creates a porcelain Uke cornea, meaning a white cornea, in which none of the structures of the anterior chamber are visible. This stromal edema must not be taken for an epithelial necrosis due to coagulation, which is sometimes present in some specific types of bums. The epithelium with a necrosis can and must be taken off with a cotton wool stick or a scarificator. This epithelium is easy to separate from the eye, in the form of a whitish squame and then lets the transparent stroma to appear (Fig. 7.10). 

The soluble electron carriers released from the reaction centers into the cytoplasm of bacteria or into the stroma of chloroplasts are reduced single-electron carriers. Bacterial ferredoxin with two Fe4S4 clusters is formed by bacteria if enough iron is present. In its absence flavodoxin (Chapter 15), which may carry either one or two electrons, is used. In chloroplasts the carrier is the soluble chloroplast ferredoxin (Fig. 16-16,C), which contains one Fe2S2 center. Reduced ferredoxin transfers electrons to NADP+ (Eq. 15-28) via ferredoxin NADP oxidoreductase, a flavoprotein of known three-dimensional structure.367 369... 

The sclera is the outer white tough part of the eye, which is an important structural element, with the site of insertion of extraocular muscles. It covers 80% of the exterior surface and is white and nontransparent. It borders the transparent cornea at the pars planar. The sclera is divided into three layers episclera, stroma, and lamina fusca. Only a limited number of blood vessels, originating from arteriolar branches of the anterior ciliary vessels, are found and superficial vessels are mainly confined to the loose outer episclera. Scleral permeability approximates that of the corneal stroma and has been shown to be permeable to solutes up to 70 kDa in molecular weight [14]. 

A single layer of flat hexagonal cells of the corneal endothelium covers the posterior corneal surface and hydrates the cornea. The corneal endothelium can allow diffusion of molecules of dimensions up to 20 nm.65 The stroma has a highly organized hydrophilic tissue structure that comprises 90 percent of cornea.63 It has an open structure that can allow molecules up to 500,000 Da in size to pass through. However, the stroma may be a diffusion barrier to lipophilic drugs.65,84 Drug-melanin interactions such as the one with timolol can form a barrier and reduce bioavailability.85... 

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