The lens begins developing very early in embryogenesis. By day 25, the optic vesicle forms and by days 27-29, the lens plate and pit form. By day 33, the lens vesicle is complete. Primary lens fibers form by day 35. Secondary lens fibers form by 7 weeks of gestation. The lens continues developing postnatally between 2-8 months as the fetal nucleus forms. The lens is a transparent, avascular biconvex structure composed of epithelial cells, cortical fibers, and the nucleus. It maintains clarity and refracts light to provide accommodation through changes in shape. Aging causes morphological, physiological and biochemical changes in the lens that can lead to cataract formation over time.

1. Embryology, AnAtomy
And
Physiology of humAn lEns
HIRA NATH DAHAL

2. Lens
 Lens (Lent-Latin word – lentil - similar shape)
 Transparent, avascular, biconvex, elliptical,
crystalline body
 Maintain clarity
 To refract light
 To provide accommodation

3. Embryogenesis of Lens
 25th
day of gestation
optic vesicle forms and
enlarges to oppose with
ectoderm

4. Lens plate formation(27-29 days)
Lens pit
Lens plate
27th –29th day of
gestation lens placode or
lens plate is formed

5. Lens vesicle
formation 30 days
Forming lens
vesicle
Ectoderm
Lens vesicle
completed 33 days

6. Embryonic nucleus formation
(app. 40 days)
Primary lens fibers
(app. 35 days)

7.  35 – 40th
day posterior epithelial cells º columnar cells
º primary lens fiber (embryonic nucleus)
 49th
day (7th
wk.) cells along the equator multiply &
elongate to form secondary lens fibers (fetal nucleus)

8. Tunica Vasculosa Lentis
 1st
month of gestation hyaloid
artery gives branch to the post.
surface of lens(post. Vascular
artery - PVA) - (Mittendorf ’s
dot)

9.  PVA anastomoses with choroidal vein
(capsulopupillary portion)
 It anastomose with long ciliary artery
and form anterior vascular capsule (9th
wk.) - (persistance pupillary
membrane)

10. SUMMARY
 Begins very early in
embryogenesis
 Days 25,optic vesicle forms
from forebrain
 Days 27,lens plate
 Days 29, lens pit
 Days 33, lens vesicle
 Day35,primary lens fiber

11.  7 weeks-Secondary lens fibers
 Develop between 2-8 months: fetal Nucleus
 8 weeks-y shaped suture
 3rd
month -Zonular fibers are secreted by the ciliary
epithelium

12. Clinical Significance

13. Coloboma
Coloboma of iris Coloboma of choroid
Ocular associations

14. Lenticonus
Posterior
• Posterior axial bulge
• Unilateral - usually sporadic
• Bilateral - familial or in Lowe
syndrome, Alports syndrome
Anterior
• Anterior axial bulge
• Associated with Alport syndrome

15. Small lens
• Small diameter • Small diameter and spherical
• May be familial (dominant)
Microphakia Microspherophakia
• Systemic association
- Lowe syndrome
• Systemic association
- Weill-Marchesani syndrome

16. Ectopia lentis
SIMPLE( pupil may be normal)
Pupil may be displaced in opposite
direction (ectopia lentis et pupillae)

17.  Congenital aphakia
 Mittendorf’s dot
 Peters’ anomaly

18. ANATOMY

19. Introduction
 Lens is a biconvex, transparent crystalline structure.
 Adds 15-20 D of plus power to 43D created by cornea.(R.I
:1.386-1.41)
 Avascular with no lymphatics, no innervation
 Accommodative power and color varies with the age
 Continually growing throughout life

20.  Second major refracting unit of human eye
At birth, Weight:65- 90 mg
Equatorial Diameter: 6.4 mm
AP length: 3.5 mm
Adult lens, Weight-255 mg
Equatorial Diameter: 9 - 10 mm
AP length: 4.5-5mm
Radius of curvature: Ant surface-10 mm
Post. surface -6mm

21. Morphology of the Lens
 Biconvex its more convex
posteriorly
 Anterior surface – center is known
as anterior pole
 Posterior surface- center portion
is called posterior pole
 Optical axis (ap-pp)
 Equator (meeting point of as-ps)

22. Position of the Lens
 Located between the iris and
the viterous at the pupillary
area in saucer shaped
Patellar fossa and attached
with vitreous by ligamentum
hyaloideo-capsulare

23. Structure of Lens
1. Capsule
2. Epithelium
3. Cortex,nucleus

24. Structure Of The Lens:
 Capsule:
 Elastic, transparent basement membrane surrounding the lens
completely
 created by epithelial cells anteriorly & cortical fibers posteriorly
 Thickest near the equator and thinnest at posterior pole
 Thickest basement membrane in the body.

25. Structure Of The Lens: (contd..)
 Capsule: (contd..)
 composed of glycoprotein associated Type IV collagen
 contains Heparan Sulfate (<1%) ⇒ maintains capsular
clarity

26.  Functions:
 acts as a barrier in keeping back the vitreous
 as a barrier against fluorescein, bacteria, and
growth factors
 a source of antiangiogenesis factors

27. Capsule of the Lens
 Basement membrane of the
lens epithelium & thickest in
the body
 Elastic and transparent and
are arranged in lamellae – type
IV collagen
 Along the equator –
pericapsular membrane
(zonular lamellae)

28.  Epithelium:
 Single layer of cuboidal cells beneath the
anterior capsule
 have metabolic capacity-
to carry out all normal cell activities
to generate sufficient ATP to meet the energy
needs

29.  3 zones
a)central-cubical cells, stable, no mitosis
b)intermediate-cylindrical
c)germinative-columnar, forms lens fiber

30.  Lens Fibers-
 after terminal differentiation of epithelial cells
 increase in cell size/mass
 loss of organelles (nuclei, mitochondria & ribosomes

31.  Cortex
 Nucleus
a)embryonic
b)fetal
c)infantile
d)adult

32. Lens Fibers
 Highly organized concentric
shells
 Little extra cellular space
 2 major components:
crystallin 90% & cytoskeleton

33.  Zonule(suspensory Ligament)
 Series Of Fine Fibres Passing Between The Ciliary Body And The
Lens.
 Transmit The Force From Ciliary Body To The Lens In
Unaccomodated Eye.
 Force Is Relaxed During Accommodation.

34.  Fibres Consists Of Elastin Associated Glycoprotein Called
Fibrillin Also Found In Vascular And Other Connective
Tissues.
 Weakness Leads To Subluxation Of The Lens As In
Marfan’s Syndrome.

35.  Fibres Arise From The Pars Plana And Ciliary Valleys Of
Ciliary Body And Are Distributed To The Ant, Equatorial
And Post. Parts Of The Lens Margin.

36. BIOCHEMISTRY AND
PHYSIOLOGY OF LENS

37. Chemical Composition of Lens
 Water 66 % of wet wt
 Protein 33 % of wet wt
 Lipids 28 mg/g of wet
weight
 Na+ 17 mmol*
 Cl 26 mmol
 K 125 mmol
 Ca 0.3 mmol
 Glucose 0.6 mmol
 Lactic acid 14.0 mmol
 Glutathione 12.0 mmol
 Ascorbic acid 1.0 mmol
 Inositol 5.9 mmol
 pH 6.9
*mmol/kg of H2O

38. Membrane
 Very stable and rigid
 Lipids constitute 55 % of plasma membrane dry wt
 High content of saturated fatty acid
 High cholesterol:phospholipid ratio
 High concentration of sphigomyelin
 All contribute to tight packing of and low fluidity

39. Lens Lipid
 Lipid constitute 1 % of total lens mass
 Cholesterol (50-60%)
 Phospholipid-sphingomyelin
 Gylcosphingolipids
 Functions: principal constituent of cell membrane
and associated with cell division

40. Lens Protein
 33% of lens wet weight
 Majority in lens fibres
 2 Major groups:
a) Water soluble (80%)
crystallin – alpha(32%), beta(55%) and
gamma(1.5%)
b) Water insoluble
2 fractions
soluble in urea – cytoskeleton protein
insoluble in urea – MIP

41. 32% β 55%
γ 1.5%

42.  Water Balance:
 65% of wet weight
 Closely associated with lens protein ∴ not freely diffusible
 Intercellular water- determined largely distribution of
monovalent cations (Na+
, K+
)

43. Biophysics:
 lens absorbs light between 295 to 400 nm
 intrinsic fluorescence is due to-
 phenylalanine
 tyrosine
 tryptophan (major)
 extrinsic fluorescence is due to-
 chromophores- blue, green, yellow, orange, red
 aberration - chromatic & spherical

44. Refractive Index
Peripheral cortex;1.386
Central nucleus ; 1.41
Anterior capsular surface more R.I than post. Surface
Higher the protein content, more the ref. Power

45. Transparency of Lens
 Avascularity
 Highly ordered arrangement of macromolecular
components of lens cells and fibres
 Lamellar confirmation of lens proteins and minimal
intracellular space
 Small differences in refractive index between light
scattering components

46. Carbohydrate Metabolism
 Energy production largely depends on
glucose
 Glucose enters both by simple and
facilitated diffusion
 Anaerobic glycolysis (80%) – 2 ATP
 Aerobic glycolysis by TCA Cycle (3%)- 36
ATP
 Pentose Phosphate Pathway (5-10%) –
provides NADPH and ribose
 Sorbitol Pathway ( <5% )

48. Sorbitol pathway
Glucose +NADPH+H+ Sorbitol +NADP+
Fructose +NADH+H+
Polyol dehydrogenase
Aldolase Reductase
High levels of sorbitol and fructose
Stimulation of HMP shuntIncrease in osmotic pressure
Indrawing of waterSwelling of fibers, disruption of cytoskeletal structures
Lens opacification
Sorbitol+NAD+
Glucose +NADPH+NAD+
Fructose +NADP+
+NADH

49. Diabetes
Juvenile
white punctate or snowflake
posterior or anterior opacities
May mature within few days
Adult
Cortical and subcapsular opacities
May progress more quickly
than in non-diabetics

50. Galactose metabolism
Galactose +ATP Galactose-1-phosphate +ADP
UDP Glucose
UDP Galactose +glucose-1-phosphate
UDP glucose
Galactokinase
Galactose-1-phosphate uridyl transferase
UDP-galactose-4-epimerase
Galactitol
Increased osmolarity
Influx of water Osmotic damage to lens CATARACT

52. Protein Metabolism
 Protein concentration is higher than in other tissues
(33%)
 Protein synthesis occurs throughout life
 Synthesis occurs mainly in epithelium and surface
cortical fibers

53. 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, and superoxide
dismutase
 Vitamin C and E present in lens act as free radical
scavengers

54. Maintenance of Lens water
and Cation Balance
 Critical to lens transparency
 Water content is approx. 66 %
 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

55. Pump Leak Theory
 Combination of active transport and membrane
permeability
 Lens epithelium is site of active transport where Na/K ATPase
and Ca ATPase are present
 K and amino acid are actively taken by epithelium and
diffuse out through back of lens
 Na flows from back of lens and is exchanged actively with K
in epithelium

56.  Amino acids transport takes place by active transport
dependent on Na gradient
 Glucose enters lens by facilitated diffusion
 Waste products leave lens by simple diffusion

57. Pump-Leak Hypothesis:
Na+150mM
K+5mM
Na+20mM
K+120mM
Inward active K+
transport
Outward active Na+
transport
Passive K+
diffusion
Passive Na+ diffusion
ANTERIOR
Aqueous humor
POSTERIOR
Vitreous humor
Passive
Diffusional
Exchange of
H2
O and solutes
Inward active A.A
pumps
Passive leak
H2O and solutes
Epithelium

58. Calcium (30 mM): Homeostasis maintained by Ca2+
-ATPase
oLoss of Ca metabolism can be damaging to lens
metabolism.
o ed Ca levels leads to depressed glucose metabolism,
formation of
high mol.wt protein aggregates and activation of
destructive proteases.
ed levels of calcium may lead to cataract formation.

59. Accommodation
 Mechanism by which eye which changes focus from
distant to near focus
 Occurs by change in shape of lens mainly in anterior
lens surface by action of ciliary muscle
 Relaxation theory is the widely accepted theory of
accommodation
 12-16 D in adolescence, 4-8 D at 40 years, <2 D after
50 years

60. Age Related Changes
 Morphological Changes:-
 ↑ in both the mass & dimension of the lens
 epithelial cells- becomes flatter & density ↓es
 lens fibers- total loss or partial degradation of a no. of
plasma membrane & cytoskeletal proteins
 cholesterol:phospholipid ratio ↑es
 lens capsule- thickens throughout life (collagen type IV vs. I,
III, IV)

61. 6 months 8 yrs 12 yrs
25 yrs 47 yrs 60 yrs
70 yrs
82 yrs
91 yrs
70 yrs brown
NS
60 yrs with
Cortical cataract
Mixed NS
+ cortical

62. Age Related Changes
 Physiological Changes:-
 membrane potential- from –70mV (at age of 20 yrs) to –
20mV (at the age of 80 yrs)
 sodium concentration - ↑es
 Na+
:K+
permeability ratio ↑es by six fold
 free calcium level ↑es

63. Age Related Changes
 Biochemical Changes:-
 overall metabolic activity of the lens ↓es
 ↓ glycolytic activity
 ↓ level activity of antioxidants
 Changes in Crystallins:-
 molecular accumulation of high weight aggregates
 ↑ed insolubility