Physiology of lens (2), ophthalmology.ppt

lens physiology and its clinical importance.

1. Physiology of Lens
Dr Nigi Shrestha
1
st
year Resident
Department of Ophthalmology
2023/09/17

2. Layout
•Introduction
•Biochemical composition of lens
•Glucose metabolism
•Protein metabolism
•Water and electrolyte transport
•Lens transparency
•Age related changes

3. Introduction
•Capsule is permeable to water, ions and small molecules and
protein upto 70kDa
•Presence of channels, pumps and transporters in epithelial
cells and fibers enable transepithelial movement to and from
the extracellurar space

4. Water 65% of net weight
Protein
Crystalline
Alpha crystallins
Beta crystallins
Gamma crystallins
Albuminoid
34% of net weight
31.7%
53.4%
1.5%
Lipid 2.5 % of net weight
Carbohydrate
Glucose
Fructose
Glycogen
Sorbitol
Glycogen
20-120 %

5. WATER
•65% of the lens wet weight(80% is bound and remaning free)
•Relatively dehydrated
•Low amount of water to maintain the refractive index
•Lens dehydration maintain by active sodium pump
•Cortex more hydrated than nucleus
•No significant alteration in hydration with age

6. Proteins
•Highest protein content than any other organ in body
•34%(~1/3) of total weight is protein
2 attributes
1.Crystallins must be very stable structures because the
proteins of the lens are probably the longest-lived proteins in
the body
2.Crystallins must remain soluble under conditions of high
protein concentration without forming large aggregates, which
would be light-scattering centers within the lens

7. Soluble proteins(crystallins)
Alpha crystalline
•Highest molecular weight (600-800 KDa) and greatest positive
charge at alkaline PH
•Binds to partially denatured proteins and prevent them from
aggregation
•FUNCTION: inhibit complete denaturation and insolubilization of
other proteins

8. Beta crystalline
•Complex group of oligomers composed of polypeptide
•Molecular weight range from 23-32 Kda
Gamma crystalline
•Composed of monomers only
•Smallest with molecular weight of 20 Kda or less

9. Insoluble proteins
•High in old age
•Mainly albuminoid
•They can be urea soluble or insoluble
•Urea soluble- cytoskeletal protein and urea insoluble-major
intrinsic proteins
•Very large and scatter light reducing acquity
Glycoproteins
•Covalently bond
•Associated with lens cell membrane
•Contribute to the intercellular ground substance
Other proteins-nucleoprotein, phosphoprotein, lipoprotein,
fluorescent protein

10. Aminocids
•Proteogenic and non proteogenic amino acids
•Higher concentration than of aqueous and vitreous humor
•Actively transported to lens mainly from aqueous humor
•Ratio of concentration of aqueous to lens is acidic> neutral>basic
Proteogenic NonproteogenicAbsent
Alanin
Leucin
Glutamic
acid
Aspartic
acid
Glycine
Valine
Tyrosine
Serine
Isoleuci
ne
Lysine
Histidine
Methioni
ne
Proline
Threonin
e
Arginine
Phenylala
nine
Taurine
Alpha-amino butyric
acid
Ornithine
1-methyl-histidine
3-methyl-histidine
Homo-carnosine
Tryptophan
Cysteine
Hydroxy
proline
•The concentration in lens is not affected by aging fasting / feeding a
protein free diet

11. Carbohydrate
•Carbohydrate metabolism is highly active and complete
process
•Free carbohydrate: glucose, fructose, glycogen
•Sugar derivatives: sorbitol, inositol, ascorbic acid, gluconic
acid, glucosemine
Glucose
•Level: 20-120 mg%
•Source: aqueous humor
•1/10
th
of aqueous humor

12. Fructose
•Source: glucose
Glycogen
•Concentration varies with age
•Localized at nucleus
•Replaces the gamma crystalins likely to increase the refractive index
Sorbitol
Inositol
•Likely help in metabolism of phospholipid

13. LENS LIPID
•Lipid constitute 2.5 % of wet lens mass
•Cholesterol and other phospholipids
•Cholesterol (50-60%)
•Phospholipid-sphingomyelin, cephalin, isolecithin, glycerides
•Lioproteins
•Abundant in epithelial cells in children and in cortex in adult
•Cholesterol increases with age in nucleus and glyceride
decreases with age
•Cataract- abundant lecithine and cholesterol crystals
•Cataract- decrease in lipoprotein and increase in free lipids
•Functions
principal constituent of cell membrane
associated with cell division

14. Electrolytes Amount
Na 14-25 mEq/kg of lense matter
K 114-130 mEq/kg of lense matter
Ca 0.14 mg/mg dry weight
Anion (Cl, HCO3, phosphate , sulphate )Phosphate is the predominant anion
ELECTROLYTES

15. Organic phosphate
•ATP-used in phosphorylation of glucose
•Pyrimidine nucleotides-forms NAD, NADP- carbohydrate
metabolism
•Decrease of these organic phosphate with age contribute to
development of cataract
Ascorbic acid
•Higher concentration than aquous humor
•Contribute to oxidation reduction system of lens

16. Glutathion(γ - glutamyl cysteinyl glycine)
•3.5-5.5 mm/g wet weight of lens
•Contributes to redox reaction
•Protection against oxidative agents
•The concentration decreases with age as wet weight of lens
increase with age
•Highest concentration in lens epithelium
•Detoxififes via mercapturic pathway
•Protection of thiole group in protein( transporting membrane
protein in lens)

17. TRANSPARENCY OF LENS
Transparency is due to the absence of chromophores and the
presence of a uniform structure that scatters light minimally.
Although the epithelial cells contain large organelles that
scatter light, the combined refractive index of this layer and
the capsule is no different from the refractive index of the
aqueous, so light scatter is very small.

18. Transparency factors
•Avascularity
•Single layer of epithelial cells
•Semipermeable character of the lens capsule
• Highly packed nature of cells
•Characteristic arrangement of lens proteins
•Pump mechanism of the lens fibers
•High concentration of reduced glutathione in the lens maintain
the lens protein in a reduced state , maintaining the integrity of
cell membrane pump

19. Maintenance of Lens water and Cation Balance
•Critical to lens transparency
•Intracellular Na 20 mM and K 120
mM
•Extracellular Na 150 mM and K
5mM
•Ca is maintained at 30 mM
intracellular while extracellular it is
2 mM
•Potential difference is maintained
at -70mv intracellularly

20. Transport mechanisms of lens
Energy dependent cation pump
•Present at anterior epithelium
of lens
•Important role in cation balance
of lens
•Active extrusion of sodium
coupled with uptake of
potassium
•Mediated by membrane bound
Na-K-ATPase

21. PUMP and LEAK
HYPOTHESIS
The process of active transport
stimulating passive diffusion
net movement of Na from
posterior to anterior and K
from anterior to posterior
Transport of aminoacid is linked with
transport of cations
Aminoacids are incorporated into protein
They are either metabolized for energy or
diffused back by “leaky “ mechanism
Glucose is transported by simple diffusion
and facilitated diffusion
Occurs from both anterioe and posterior
surface

22. Lens as an osmometer
•Lens has around 145 mEq/l of cation and around 90 mEq/l of
anion
•The anion deficit is covered by acidic group of lens protein
and glycoproteins
•Increase of Na inside lens in exposure to surface active
detergent or antibiotics results in lens swelling

23. Calcium
Homeostasis maintained by Ca
2+
-
ATPase
Loss of Ca metabolism can be
damaging to lens metabolism.
Increased Ca levels leads
•Depressed glucose metabolism
•Formation of high mol.wt protein
aggregates
•Activation of destructive
proteases.

Decreased levels of calcium may
lead to cataract formation.

24. Lens metabolism
•As the lens is avascular, its metabolic needs are met by the aqueous
humor.
•Therefore lens metabolism depends on constant composition of the
aqueous.
•The avascularity also results in hypoxic environment of lens so 70% of
lens ATP is derived from anaerobic glycolysis

25. Glucose metabolism
Metabolism of glucose provides most of the energy requirements of the
lens. Glucose enters from the aqueous by both diffusion and assisted
transport

26. Snowflake cataract in
diabetes
Oil drop cataract in
galactosemia

27. Protein Metabolism
•Protein synthesis is believed to
occur in epithelial cells and
cortical fibers
•Synthesize from free amino acid
which are actively transported via
aqueous
•Protein synthesis occurs
throughout life
•Lens proteins are broken down
into peptides by endopeptidases
and then into amino acids by
exopeptidases

28. Oxidative Damage and Protective Mechanism
•Free radicals are produced during cellular metabolism and by
radiation
•Free radicals lead to lens fiber damage
•Lens are equipped with protective enzymes such as
•glutathione peroxidase
•Catalase
•superoxide dismutase
•Vitamin C and E present in lens act as free radical scavengers

29. Glutathione is found at high concentrations in the lens (3.5–
5.5 mmol/g wet weight), especially in the epithelial layer.
Roles in the lens
•Maintaining protein thiols in the reduced state to maintain
lens transparency by preventing the formation of high
molecular weight crystallin aggregates.
•Protection of thiol groups involved in cation transport and
permeability
•Protection against oxidative damage
•Removal of xenobiotics

30. AGE RELATED CHANGES
The lens grows throughout life but at a slower rate with
increasing
Morphological Changes:
• in both the weight ,thickness and diameter of the lens
•Transmission of light of lower wavelength decreases with age
and light absorbance increasing with age
•Increase in light scattering with age
•Frourescence increase with age
•No change in refractive index with age
•epithelial cells: becomes flatter & density decreases
•lens fibers
• total loss or partial degradation of a number of plasma
membrane & cytoskeletal protein
•Cholesterol : phospholipid ratio increases

31. •colorless or pale yellow to darker yellow in adulthood, and
brown or black in old age
CHANGES IN PLASMA MEMBRANE AND CYTOSKELETON
• loss of hexagonal cross-section
• loss of membrane proteins, lipids and cytoskeletal proteins
• increased lens sodium and calcium with subsequent
hydration
•Large membrane polypeptide decrease with age

32. METABOLIC CHANGES
• decreased proliferative capacity of lens epithelium
• decreased enzymatic activity (superoxide dismutase and glucose-
6-phosphate DH)
CHANGES IN CRYSTALLINS
• Increased in high molecular weight aggregates
• loss of gamma-crystallins
•partial degradation of crystallin polypeptides,
•increased crystallin insolubility
•photo-oxidation of tryptophan, and the production of
photosensitizers
•loss of sulfhydryl groups
• increased disulfide bonds in gamma-crystallins
•nonenzymatic glycation

33. Physiological Changes
•Decrease in membrane potential- from –50mV (at age of 20 yrs)
to –20mV (at the age of 80 yrs)
•sodium concentration increases
•Na
+
:K
+
permeability ratio increases by six fold
•free calcium level increase
•Absorption of both UV and visible light by the lens increases with
age

34. Accommodation Changes
The level of amplitude of accommodation at different age
•Adolescent- 12-16 D
•Adult at 40yrs- 4-8 D
•After 50yrs- 2D
•capsular elasticity decreases with age
•Stiffness of the lens substance increases
•Although the cortex increases in thickness throughout life,
very little change occurs in the thickness of the nucleus.
•The radius of curvature of the anterior capsule decreases with
age.
•The distance between the anterior surface of the lens and the
cornea decreases
•The internal apical region of the ciliary body moves forward
and inward with age decreasing the tension of zonules in
unaccommodated lens.

35. REFERENCES
American Academy of ophthalmology(lens and cataract)
Adlers Physiology (9
th
edition)
Ophthalmology Yanoff and Duker
Anatomy and Physiology of eye , AK Khurana and Indu Khurana
Clinical Ophthalmology -Jack J. Kanski (5
th
edition)

36. THANK YOU