Year : 1998 | Volume
: 46 | Issue : 3 | Page : 159--162
Human lens epithelial layer in cortical cataract
N Kalariya, UM Rawal, AR Vasavada
Department of Zoology, School of Sciences, Gujarat University, Ahmedabad, India
Department of Zoology, School of Sciences, Gujarat University, Ahmedabad
Normal and cataractous human eye lenses were studied by morphology and protein analysis. A marked decrease in protein sulfhydryl (PSH) and nonprotein sulfhydryl (NSPH) was observed in nuclear and cortical cataractous epithelia. Moreover, decrease in PSH contents and an increase in insoluble proteins were found to be correlated only in cortical cataractous epithelium which is also accompanied by various morphological abnormalities. In nuclear cataractous epithelium, however, there was very little insolubilisation of proteins. The epithelial morphology in nuclear cataracts was almost similar to normal lens epithelium. Hence, it is assumed that the protein insolubilisation and various morphological abnormalities are characteristics of cortical cataractous epithelium. This leads us to believe that opacification in cortical cataract might initiate in the epithelial layer.
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Kalariya N, Rawal U M, Vasavada A R. Human lens epithelial layer in cortical cataract.Indian J Ophthalmol 1998;46:159-162
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Kalariya N, Rawal U M, Vasavada A R. Human lens epithelial layer in cortical cataract. Indian J Ophthalmol [serial online] 1998 [cited 2023 Jan 31 ];46:159-162
Available from: https://www.ijo.in/text.asp?1998/46/3/159/14956
The single-layered lens epithelium underlying the anterior capsule is metabolically the most active part of the lens. It is also responsible for growth of the lens through mitosis, fibre cell differentiation and protein synthesis. As it is known to be the primary site of active transport and permeability, it plays a crucial role in maintaining the levels of electrolytes and water in the lens that are necessary for retention of lens transparency. Any factor disturbing the transport processes or morphology or biochemistry of the lens epithelium would result in water accumulation in the lens and the subsequent imbalance of intracellular and extracellular ion concentration would lead eventually to cataract formation.
Two major types of cataracts are formed through very different mechanisms. Nuclear cataract is characterised by the accumulation of insoluble proteins which appear to be derived from soluble proteins. Cortical cataract is characterised by the altered electrolytes and water level.
Comparatively little is known about the etiology of human lens epithelium in these two types of cataracts, even though it is well known that the lens epithelium plays a very crucial role in maintenance of lens transparency. It has been proposed that changes in sulfhydryl (SH) contents and protein structure play an important role in cataractogenesis. However, no direct analysis of SH contents and proteins in human lens epithelium has been previously reported in these two types of cataracts.
The present work deals with an estimation of the SH contents in lens epithelia of normal eyes and in both types of cataracts. Their possible influence in epithelial protein insolubilisation and morphological changes were also studied. This work also represents an effort to discover the peculiar characteristics, if any, of these cataractous lens epithelia, which may justify the involvement of the epithelial layer in the cataractous process.
Materials and Methods
Normal (noncataractous) eyes (age 24-52 years) were obtained from the eye bank within 20-36 hours of death. The lenses were immediately removed from the eyeball and the anterior capsular epithelium detached from the lenses under dissecting microscope (Bausch & Lomb, USA).
Nuclear and cortical cataractous anterior lens capsules with adherent epithelial layer were obtained from patients (mean age 71 years for nuclear and 63 years for cortical cataract) undergoing extracapsular cataract extraction and immediately stored at -70°C. Once a sufficient number of samples had been collected, they were processed further. The type of cataract was determined by a detailed slitlamp biomiscroscopic examination prior to surgery.
The anterior lens capsules with adherent epithelium so removed were fixed immediately in acetic acid:methanol (1:3) fixative. The anterior capsule with epithelium was placed on a vaporized slide and observed under microscope. The capsules were spread uniformly using microforceps and a microspatula to prevent overlapping and folding. The epithelial surface of the anterior capsule was confirmed by the presence of prominences. The evenly spread capsules were air dried and stained with haematoxyline and eosin stains. These were observed under a light microscope. The characteristic epithelial changes were documented in photographs.
Studies of total SH (TSH), protein SH (PSH) and nonprotein SH (NSPH) of the lens epithelium were carried out by the modified method of Sedlack & Lindsay with slight modifications done at our laboratory.
The soluble, insoluble and total proteins of the lens epithelia were determined by the modified method of Lowry et al.10 epithelia were pooled to obtain a single sample.
The normal lenticular epithelial morphology [Figure:1] had a uniform distribution of cells with round nuclei. Both types of cataractous epithelia revealed typical changes. In nuclear cataractous epithelium, it was found that the uniform distribution of cells was almost similar to normal ones with varied nuclear sizes. Vacuolization in the cytoplasm was more in nuclear cataractous epithelium [Figure:2]. In cortical cataracts the epithelial changes we observed included irregularites in the distribution of cells with large nuclei, cytoplasmic vacuolization and very rare superimposed nuclei [Figure:3] and [Figure:4].
Changes in SH content
[Table:1] shows changes in the NPSH, PSH and TSH in the epithelial layer of normal, nuclear cataractous and cortical cataractous lenses. The decrease in NPSH was significant in both types of cataractous epithelia, while TSH and PSH were significantly low only in cortical cataractous epithelium compared to normal epithelial data [Table:2].
Moreover NPSH, PSH and TSH of cortical cataractous epithelia were significantly low compared to the nuclear cataractous epithelial data.
Changes in protein level
[Table:3] shows changes in the soluble, insoluble and total proteins in the epithelial layer of normal, nuclear and cortical cataractous lenses. A significant decrease was found in total and soluble proteins of both types of cataractous epithelia compared to normal epithelial levels. This decrease is accompanied by marked increase in insoluble protein in cortical cataractous epithelia only [Table:4].
A decrease in total and soluble proteins and an increase in insoluble proteins of cortical cataractous epithelium were found to be significant with the data of nuclear cataractous epithelia.
Dramatic loss of PSH and increased insolubilisation of proteins in the lens epithelium is considered to play a considerable role in the malfunctioning of human cortical cataractous epithelium.
In the present study nuclear cataractous epithelia showed an initial marked decrease in NPSH. Most of the NPSH in the lens will be reduced glutathione (GSH). The epithelium is the region of the lens where GSH concentration is found to be the highest. Epstein & Kinoshita showed that GSH is a protector for membrane SH in the lens and Kinoshita & Merola reported that GSH maintains lens protein thiols in reduced form. These results of nuclear cataractous epithelium show that despite a marked decrease in GSH, the epithelial proteins of nuclear cataracts could maintain their thiols in reduced state as it is clear from the result of proteins that the level of insoluble proteins in this type of cataractous epithelium is almost similar to that of the normal lens epithelia. This indicates that there can be some other factors which prevent insolubilisation of epithelial proteins by keeping protein thiols in the reduced state.
The observed decrease in total proteins of nuclear cataractous epithelium could be due to loss of soluble proteins. Such loss could be due to aging, leakage through lens capsule or decreased rate of protein synthesis.
Insolubilisation of proteins in cataract etiology has long been regarded to be the consequence of SH oxidation. Indeed, our results of cortical cataractous epithelia showed marked increase in insoluble proteins accompanied by a large decrease in PSH. Oxidation of SH groups of epithelial Na-K ATPase and epithelial cell membrane could alter the active transportation and cause structural deformation which leads to imbalance of electrolytes and water in the lens, finally resulting in loss of transparency.
Though a marked decrease in NPSH (GSH) of nuclear and cortical cataractous epithelium is observed, significant protein insolubilisation was found to be the characteristic of cortical cataractous epithelium only. So we presume that opacification is initiated in the epithelial region of lens in cortical cataracts. There are many cataract models which have shown that the initial changes in transparency are in the epithelial cell region.[12-14] This again leads us to speculate that there would be some other reducing factors functioning in nuclear cataractous epithelium that prevent initiation of opacification by maintaining protein thiols in reduced state. These factors could be exogenous GSH[15, 16] and/or other SH protecting enzymes such as the recently identified Thioltransferase (TTase) in bovine lenticular epithelium which is hypothesised to play a significant role in SH homeostasis by protecting the SH groups of the proteins from S-thiolation. Any damage to the epithelial transport system for exogenous GSH and/or loss of activity of unidentified SH-protecting factors in epithelial tissue could lead to insolubilisation of proteins. This in turn may alter the morphology and physiology in cortical cataractous epithelium.
Insolubilisation of proteins and SH oxidation are believed to be associated with structural deformation. Our histomorphological results were also in concordance with the biochemical results. Nuclear cataractous epithelia were found to be almost similar to the normal lens epithelium with least morphological changes. This finding agrees with earlier reports. The cortical cataractous epithelia have irregular distribution of cells, other changes include large nuclei, cytoplasmic vacuolisation as well as crenated and pyknotic nuclei. The pyknotic nuclei obviously reduce the mitosis and therefore the mitotically active superimposed nuclei found were very few.
Finally our study concludes that the degree of variation in epithelial morphology and epithelial opacification depends on the extent of SH oxidation and protein insolubilisation, which was found to be higher in high cortical cataractous epithelium and relatively lower in nuclear cataractous epithelium. Moreover extensive loss of a reducing environment provides the opportunity for loss of transparency in the epithelial cell layer followed by lens cortical region.
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