Ординатура / Офтальмология / Английские материалы / Basic Sciences in Ophthalmology_Velayutham_2009
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Glycogen Formation and Degradation
It is limited and minimal in ocular tissues (corneal epithelial, muller cells in retina).
Fig. 12.13: Debranching enzymes
Pentose shunt or HMP (Hexose Monophosphate) shunt (Fig. 12.14)
It serves 3 principal functions:
1.Generation of pentoses – for synthesis of nucleic acids
2.Generation of NADPH necessary for – synthesis of fatty acids, cholesterol etc, cell detoxification – by removal of destructive form of oxygen or reactive oxygen species like H2O2.
3.Recovery of certain metabolic intermediates such as fructose, glucose etc.
Two other pathways of significance in ocular tissues are
1.Gluconeogenesis in retina where there is formation of glucose from non carbohydrate sources like lacatate, due to the high demand for glucose. Mostly reversal of glycolysis with 3 exceptions.
2.Polyol pathway which is dormant normally. It is stimulated when there is excess glucose as in Diabetes mellitus, galactosemia, etc.
HMP Shunt
Aldose reductase has a very high Km for glucose – 700 times that of hexokinase ie very low affinity. Polyol dehydrogenase also has a very high Km for sorbitol. So, there is chance for accumulation of sorbitol in the cell, before it is further metabolized to fructose.
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Fig. 12.14
Glycation of proteins: occurs when there is an excessive level of glucose as in diabetes mellitus or non-enzymatic glycation as a part of aging.
PROBLEMS OF CARBOHYDRATE TRANSPORT AND METABOLISM
Diabetes and Galactosemia
Diabetes is a metabolic disorder of cellular carbohydrate uptake that affects not only the carbohydrate metabolism, but also lipid and protein metabolism. Primary effect occurs to blood vessels of brain, eyes, kidney and external limbs. In the eyes , retina, cornea and lens may be affected pathologically resulting in visual debilitation and blindness.
Diabetes occurs in 2 forms. Type I or Juvenile onset diabetes and Type II or maturity onset diabetes ~ 10% of diabetes are Type I and it is more severe. In both forms, there is inability of glucose to enter certain classes of cells in the body that are dependent upon insulin activated, transport protein systems.
In Type I, there is lack of insulin due to autoimmune destruction of β cells of pancreas and in Type II, there is problem with the insulin receptor protein. It may be less in number or less responsive receptors in normal numbers, e.g.
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obesity is one of the causes of Type II diabetes. The enlarged adipocytes secrete a protein known as tumor necrosis factor α− that inhibits insulin receptor autophosphorylation necessary for insulin response. So, the cells that depend on insulin for transport of glucose become starved for nourishment while other cells, not dependent on insulin become exposed to higher than normal cytoplasmic levels of glucose ie toxic levels.
Insulin dependent cells of our interest are cells of blood vessel walls.
Cells not dependent on insulin—lens fibre cell
To compensate for the lack of glucose, the insulin dependent cells alter their metabolism in pathological ways ( abnormal ways) to produce energy precursor in the form of acetyl CoA from the breakdown of lipids and protein. This leads to formation of ketone bodies in excess resulting in ketosis and consequent acidosis.
In the cells that are not dependent on insulin the glucose is taken up by cells in excess and is bound to proteins both extra and intracellularly. This is known as glycation and it is a permanent change. This reaction continues and produces more complex forms that result in the denaturing of protein. Glucose in higher concentration can cause DNA damage.
Galactosemia
Galactose is incorporated into glycolytic pathway via glucose 1 phosphate by the activity of 3 enzymes.
1.Galactokinase.
2.Galatose 1 phosphate uridyl transferase.
3.UDP galactose epimerase.
Glucose 1 phosphate is converted to Glucose 6 phosphate by isomerase as
in glycogen breakdown.
A deficiency in any of these three enzymes numbered in the pathway results in accumulation of either galactose or galactose 1 phosphate in tissues.
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The resulting condition is known as galactosemia. The general systemic symptoms and signs are, liver failure and mental deterioration from depletion of inorganic phosphate by the uridyl transferase deficiency, accumulating Galactose 1 phosphate. Epimerase deficiency is symptom free. Galactokinase deficiency results in accumulation of Galactose.
The ocular effects of galactosemia are as follows
Zonular or nuclear cataract development in 30% of galactosemic patients due to rapid accumulation of galactitol in the polyol pathway. The accumulated galactose is converted to galactitol due to the stimulation of aldose reductase. This galactitol is an extremely poor substrate for polyol dehydrogenase. So, the accumulation of osmotically active galactitol causes swelling of the lens fibre cell, leading to bursting of the cell resulting in cataract formation.
13 Biochemistry of
Individual Ocular
Tissue
Biochemistry of ocular tissue can be discussed under the following sections:
1.Ocular surface - lid, tear film, conjunctiva, cornea, sclera.
2.Uveal tract.
3.Aqueous humor.
4.Lens.
5.Vitreous.
6.Retina.
7.Photo transduction.
THE LIDS
Eyelids contain specialized mucus secreting glands that secrete a different type of mucus gamma MUC4 and oil secreting glands that contribute to the makeup of the tear film. Lashes and their hair follicles are important, in protecting the eye from foreign particles.
Abnormalities: E.g. Trachoma.
Defect in lid apposition due to scarring and deformation of lids leading to corneal exposure; ulceration and blindness.
CONJUNCTIVA
Specialized cells like immune cells (T and B cells, mast cells, dendrite cells) and mucus secreting cells are present in conjunctiva. The epithelium is more than a simple covering layer for the conjunctiva. The conjunctival epithelium is an intermediate type between keratinized squamous epithelium and mucosal epithelium with intraepithelial lymphocytes and dendrite cells organized as a part of mucosa associated lymphoid tissue.
Functions of Epithelium
1.Acts as a barrier to external organisms.
2.It is the source of tear mucins derived from the intraepithelial goblet cells which are under neuro endocrine control.
Clinical application
Abnormality in eptithelial and goblet cells result from Vit Adeficiency – leading to severe dry syndromes.
Conjunctival stroma: is highly vascular structure and contains aqueous veins.
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It has a superficial lymphoid layer and a deep vascular layer. Through this stroma, waste materials from anterior chamber of the eye are transported to periauricular draining lymphnodes and venous drainage systems in the neck.
SCLERA
This is the outermost coat of the eye. It is a tough and non-compressible layer of connective tissue with minimal amount of fibroblasts. Few are found to be myofibroblasts, i.e. contractile fibroblasts conferring refractive properties to sclera.
Connective tissue is made up of Type I and III collagen fibres. The fibres have variable diameter and are irregularly distributed. Hence, they cannot transmit light and sclera is non-transparent.
The Glycosaminoglycans present in sclera are dermatan sulfate and chondroitin sulfate. They are bound to core protein to form the proteoglycans of small non aggregating type and are localized to the collagen fibres. Sclera has some large, aggregating proteoglycans also viz. versican, neurocan and brevican combined with hyaluronic acid. As the amount of proteoglycans in sclera is less, it is less hydrated (70%). Uveoscleral outflow of aqueous is a bulkflow at constant rate and a portion of it drains directly transclerally. Fluid flowing across the sclera is absorbed by the matrix proteoglycans with low water binding capacity to maintain normal 70% hydration.
BIOMEDICAL IMPORTANCE OR CLINICAL APPLICATION
In Uveal effusion syndrome and Nanophthalmia – the sclera contains high levels of abnormal proteoglycans especially dermatan sulfate proteoglycans. These bind and trap large volumes of water leading to thickening of sclera. This may cause obstruction of choroidal venous drainage further increasing the swelling and water retention.
In myopia or near sightedness, the ocular globe is lengthened along its anteroposterior axis. This condition is associated with remodelling or reformation of the extracellular matrix proteins in the posterior sclera. The enzyme gelatinase A digests the scleral proteins (collagen) partially so that new scleral proteins can be formed to establish a new scleral length resulting in myopia. The inhibitor of this enzyme is less (TIMP - Tissue Inhibitor of Metallo Proteinase) and so, activity of the enzyme gelatinase A proceeds uninhibited resulting in digestion of scleral proteins. Gelatinase Ais a type of Matrix Metallo Proteinase (MMP).
PRECORNEAL TEARS
Precorneal tears bathes the ocular surface compised of conjunctival and corneal non-keratinised epithelium. Precorneal tears form a film between the inside of
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the lids and the cornea; when the eyes are closed and remain so for 15-45 seconds after the eyes are opened and then rupture stimulating the next blink. This is called tear break up time.
Functions of Precorneal tears
1.It is the lubricating fluid for the lid cornea interface.
2.It is the protective covering for cornea and forms a smooth optical surface at the air – eye interface.
3.Antibacterial medium to protect the eye.
4.Perfusion fluid washing away debris from corneal surface.
5.Temporary depository for instilling topical drugs therapeutically.
6.Supplies oxygen to corneal epithelium.
Precorneal tears, as a film are a complex mixture of 3 µm thick, composed of 3 layers viz.
1.The anterior or superficial lipid layer or surface oily layer.
2.Central aqueous layer, and
3.Posterior mucous layer.
Lipid layer is derived from meibomian gland secretion.
Composition of Human Meibomian gland lipids
Lipid component |
% composition |
|
|
Cholesteryl esters |
29.5 |
Waxes |
35 |
Triacylglycerol |
4 |
Cholesterol |
1.8 |
Fatty acids |
2.2 |
Diesters |
8.4 |
Unidentified |
rest of it |
|
|
Waxes are esters of long chain (14–36 carbons) fatty acids and long chain alcohol (16–20 carbons) derived from fatty acids.
Cholesteryl esters are esters of unsaturated and hydroxy fatty acids. Diesters are double esters, i.e. hydroxyl fatty acids esterified to (a) other
other fatty acids, or (b) one fatty acid and an alcohol, or (c) a fatty acid and cholesterol.
Fatty acids are long chain fatty acids (14–36 carbon) saturated and unsaturated (with 0-3 double bonds) inclusive of odd chain and hydroxy fatty acids.
Functions of lipid layer
1.Allows free flow of tears from their ducts to the eyelid edges.
2.Forms a film over aqueous layer.
3.Prevents speedy evaporation of tears.
4.Prevents spill over of tears at the lid margin by adhering to the eyelid skin.
5.Forms a water tight seal when the lids are closed.
6.Prevents migration of skin lipid onto the ocular surface.
7.Provides a clear optical medium.
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Abnormalities of lipid layer may result from meibomian gland dysfunction where excessive production of keratin occurs in the ductal epithelium (Keratin is a protein characteristic of skin hard coverings such as animal horns) leading on to reduction in the amount of steryl esters, increase in cholesterol and appearance of ceramides, a further complication of this process is that the epithelial cells may detach from the gland and block the flow of new lipids to the tear film. Bacterial infection in the area of blocked lipids can also occur. Staphylococcus aureus and other bacteria produce the enzyme cholesterol esterase and fatty wax esterase capable of hydrolyzing the meibomian lipids.
Central aqueous layer
The tear film contains 98 % H2O, pH 7.5, volume 6–9 µl, osmolality 310–334 m osm.
Lacrimal gland secretion provides the aqueous component of tears. The dilute aqueous solution from the exocrine acinar glands contain proteins, small molecular weight components and electrolytes as shown in table on comparison to blood.
Composition of aqueous layer in comperison to blood
Component |
Blood concentration |
Aqueous concentration |
|
|
|
Ascorbate |
1.3 mg% |
0.4 mg% |
Bicarbonate |
27 mmol /L |
23 mmol/L |
Glucose |
90 mg% |
6 mg% |
Chloride |
96–106 mmol/L |
118–135 µmol/L |
Potassium |
4.3 mmol/L |
30 mmol/L |
Sodium |
150 mmol/L |
138 mmol/L |
Protein |
7 gm% |
0.7 mg% |
Magnesium |
0.75–1.25 mmol/L |
0.7–0.9 µmol/L |
|
|
|
Functions of aqueous layer
The principal proteins present in aqueous are:
1.Lysozyme – antibacterial.
2.Tear specific pre-albumin – induces lipid spreading by interacting with lipid layer, removes harmful lipophilic molecules.
3.G protein – signal protein.
4.Lactoferrin.
5.Immunoglobulin A, etc.
Na+, K+, Cl¯: Regulate the osmotic flow of fluids from cornea to the tear film and maintain the osmolality at 310–334 mosm.
HCO¯ regulate the pH of tear film maintains at 7.5, other electrolytes are
3
enzyme co-factors.
Regulation of aqueous layer
Psycho-neuroendocrine control:
1.Neural control mediated by autonomic nervous system (instillation of drugs that modulate this system, e.g. Pilocarpine, atropine affect tear secretion).
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Direct effect on acinar cells through intracellular second messengers. Parasympathetic system increases the tear flow via its effects on the myoepithelial cells surrounding acinar cells.
2. Hormonal control
Testosterone stimulates secretion of certain tear components such as Ig A.
Reduction in tear flow in women after menopause.
3.Psychological factors also contribute for controlling tear flow.
Posterior Mucus Layer:
Mucins are secreted by conjunctival goblet cells e.g., MUC5AC which is the principal tear mucin and it is mucus glycoprotein. Mucins have larger amounts of carbohydrates with short chain sugars. The carbohydrates are attached in numerous short chains along the length of polypeptide chain. This layer is composed of glycocalyx of the epithelial cell surface e.g., MUC1, MUC4, sialomucin complex and an additional layer of tear specific nucleoproteins.
Functions: mucus imparts viscosity to the tear film, maintains the stability of tear film. Mucins support the stability of tear film by increasing tear film viscosity and by trapping, lipids within their structure, so that they can be, reused after blinking.
Clinical applications:
Abnormalities in mucus layer: Reduction or absence of mucin in tear film occur due to the following :
1.Vit A deficiency.
2.Ocular pemiphigoid (conjunctival ulceration).
3.Stevens-Johnson syndrome (an attack of the mucous membranes and skin).
4.Alkali burns.
All these conditions lead to destruction of goblet cells with consequent loss of mucin production. This results in rapid breakup of tear film even when there is adequate volume of aqueous layer of tears (Fig. 13.1).
Fig 13.1: The tear film
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CORNEA
Cornea is designed for light transmission as well as light refraction. Cornea is made of cellular and extracellular matrix of collagen and glycosaminoglycans. They are organized in such a way to reduce the disparity in their refractive indices and this enables the function of cornea.
Cornea consists of epithelium with basal lamina, Bowman’s membrane, stroma, Descemet’s membrane and endothelium. All the layers of cornea contain collagen.
Corneal Epithelium
It is six cell thick stratified layer on a basal lamina. It is the first refracting interface for transmitted light. Short wave length light are absorbed by the epithelium and majority of the light of the visible spectrum is transmitted through this epithelium. Epithelial cells express keratin, the intermediate filament that gives mechanical strength to the junctional structure between cell and basement membrane such as hemidesmosomes.
They also contain receptors for basement membrane components such as fibronectin, laminin and collagen. Hemidesmosomes are bound to corneal stroma through a band of anchoring fibrils. These fibrils are made of type VII collagen. In addition, type XVI collagen also support firm adhesion between the basal cells.
Function
1.Effective barrier to fluid transport through ion channels.
2.The arrangement of collagen fibers with proteoglycans of glycosaminoglycans such as keratin sulphate, dermatan sulphate and chondroitin sulphate provides elasticity and deformability to the cornea while maintaining high levels of transmission.
3.Synthesis of cell surface glycoproteins and the intracellular matrix intermediate filament keratin.
Clinical application
Vitamin A or retinol is required for the control of epithelial keratin expression, cell surface glycoprotein synthesis and for normal corneal wound healing. Retinol also promotes the synthesis of α-1 proteinase inhibitor which inhibits a wide range of proteolytic enzyme. So, vitamin A deficiency leads to a form of keratinisation of corneal epithelium, impaired epithelial function, loss of luster, bitot’s spots, punctuate erosions. Different types of collagen are present in the cornea. These are the matrix proteins in the cornea as shown in the Figure 13.2.
Corneal Stroma
Collagen fibres form sheets termed lamellae and have uniform diameter of 30 nm. 70% are type I collagen, 15% type V and 15% type VI collagen. Type I