ANATOMY OF LENS MADE EASY

Hello and Welcome to Insight Ophthalmology. Today we shall delve deep into the anatomy of lens. We shall discuss about the gross anatomy and the relevant clinical anatomy of the lens in this post. If you are an audio visual person you can visit our video on anatomy of lens.

FUNCTIONS OF HUMAN CRYSTALLINE LENS

  • It transmits and refracts the light.
  • Lens absorbs ultraviolet (UV) light of <350 nm wavelength. Thus, prevents damaging UV radiation from reaching the retina.
  • It contributes 35% of refractive power of the eye.
  • It helps in accommodation.

The lens is a transparent biconvex crystalline mass placed between the iris and the vitreous in a saucer shaped space called the patellar fossa.

image showing the human crystalline lens location between the iris and the vitreous. It is placed inside the patellar fossa

LOCATION OF LENS & IMPORTANT ATTACHMENTS Of LENS

  • The saucer shaped depression in the anterior vitreous is called the patellar fossa.
  • WEIGERT’S LIAGAMENT (HYALOIDO-CAPSULARE) :The posterior part of the lens is attached firmly to the patellar fossa with a circular ligament called the WEIGERT’S LIGAMENT ( Hyaloido-capsulare)
  • RETROLENTAL SPACE ( BERGERS SPACE) : The space between the hyaloid face (anterior vitreous) and the lens within the circular ligament is called the bergers space.
image showing important ligament and spaces around the lens, like weigert liagment, berger space and patellar fossa

IMPORTANT DIMENSIONS Of LENS

  • LENS THICKNESS is about 3.5 mm (birth) to 5.5 mm ( at extreme of age )
  • It increases by 0.2 mm every year.
  • EQUATORIAL DIAMETER of lens at birth is 6.5 mm.
  • By second decade of life , the equatorial diameter reaches about 9-10 mm and then becomes constant.
  • RADIUS OF CORVATURE OF ANTERIOR SURFACE is 10 mm.
  • RADIUS OF CORVATURE OF POSTERIOR SURFACE is 6 mm
  • Therefore the posterior surface is more curved than the anterior surface .
  • ANTERIOR POLE and POSTERIOR POLE :The centre of the anterior surface and the posterior surface is care called  the anterior and posterior poles respectively.
important dimensions of lens, like the lens thickness, equatorial diameter, radius of curvature of anterior and posterior surface of the lens

REFRACTIVE PROPERTIES Of LENS

  • The refractive index of the lens is 1.39.
  • The refractive Power is 16-17D.
  • The accommodative power of the lens decreases with age.

Accommodative power of lens :-

  • AT BIRTH : 14-16 D
  • At 25 years : 7-8 D
  • AT 50 Years : 1-2 D

 

PARTS OF LENS

Lens anatomy can be understood by dividing it into various parts: 

  • LENS CAPSULE
  • LENS EPITHELIUM 
  • LENS FIBRES 

LENS CAPSULE

  • A thin, transparent membrane surrounds the lens. This memberane is formed of hyaline and collagen. This forms the lens capsule.
  • The lens capsule is elastic but doesn’t have any elastic fibers
  • It is produced continuously through out life.
  • It is produced by the basal portion of the lens epithelium anteriorly and basal portion of the posterior lens fibers posteriorly .
the image depicts the variable thickness of the lens capsule

DID YOU KNOW?

  • The lens has the thickest basement membrane in the human body.
  • It stains with Per-Iodic schiff base ( PAS STAIN)
  • The capsule is thicker anteriorly than posteriorly.
  • It is Thicker at the Equator than at the poles
  • It is thinnest at the posterior pole.

CLINICAL NUGGET 

  •  TRUE EXFOLIATION of the lens: The anterior/ superficial lamella of the lens capsule splits of from the deeper lamella.
  • POSTERIOR CAPSULAR RUPTURE: Since the capsule is weakest at the posterior capsule, the most common place from where it ruptures is at the posterior pole. This is a common complication during the cataract surgeries and is known as the posterior capsular rupture or PCR.
image illustrating the posterior capsular rupture(PCR), complication of the cataract surgery.

LENS EPITHELIUM

  • Since the posterior epithelium of lens vesicle is used up to form the primary lens fibers during embryonic period (0-3 months); the adult lens only has an anterior epithelium and NO posterior lens epithelium 
  • The posterior epithelial cells become the primary lens fibers and form the embryonic nucleus at the center of the lens.
  • This nucleus has no sutures.

ANTERIOR LENS EPITHELIUM

  • The anterior lens epithelium consists of nucleated Cuboidal cells below the lens capsule.
  • It is the most METABOLICALLY ACTIVE part of the lens (contains all the organelles ).
  • Produces Adenosine Triphosphate(ATP)
  • The entire epithelium does not have the dividing capability.
  • The epithelium of the Equatorial region consists of  COLUMNAR CELLS and is capable of actively dividing.
  • The anterior epithelium can be divided into three zones:-

I.CENTRAL ZONE
II.PERIPHERAL ZONE
III.GERMINATIVE  ZONE / EQUATORIAL zone

CENTRAL ZONE : 

  • Shape of cells in this zone -Cuboidal in shape
  • Their number decreases with age
  • These cells are usually stable and do not show mitosis.
  • Cells in this zone can show mitosis in pathological conditions.

INTERMEDIATE ZONES/ PERIPHERAL ZONE

  • The cells in this zones are smaller in size and more cylindrical in shape.
  • They are located peripheral to the central cells.
  • These cells rarely undergo mitosis 



GERMINATIVE ZONE/ EQUATORIAL ZONE

  • Located just before the equator  (PREEQATORIAL) , equator and posterior to equator and are COLUMNAR in shape 
  • ACTIVELY DIVIDING CELLS
  • New cells that form from this zone migrate posteriorly to form new lens fibers
  • These cells continue to divide through out life.
image showing differnet zones of the anteriro lens epithelium; central zone, peripheral zone, germinative/equatorial zone and bow region of lens

WHAT IS THE BOW REGION OF THE LENS ?

  • The newly laid cells elongate to transform into the lens fibres . These fibres are pushed inwards and the new cells are laid by the germinative zone. As these lens fibres elongate their nucleus is shifted anteriorly thereby creating and anteriorly oriented bow. Therefor this region is known as the Bow region.
  • The innermost lens fibres lose all their organelles along with nucleus. This happens to avoid scattering of light and helps in lens transparency.

CLINICAL NUGGET 

  • The Epithelial cells in the central zone do not divide. However in pathological conditions they can undergo METAPLASIA and  transform into myofibroblasts.
  • Shield cataract seen in atopic dermatitis and Glaukaumafleckon seen in acute angle closure glaucoma are examples of central epithelial cells undergoing metaplasia.
  • The actively dividing cells of the equator are susceptible to irradiation
  • Irradiation leads to DYSPLASIA of these cells leading to  formation of POSTERIOR SUBCAPSULAR CATARACT.

WHAT IS POSTERIOR CAPSULAR OPACIFICATION?

  • Now that we understand what a posterior epithelium and a capsule are, we can understand exactly what a posterior capsular opacification (PCO) is.
  • After a cataract removal surgery with intraocular lens implantation, sometimes some residual epithelial cells can still remain in the capsular bag due to their sticky nature.
  • These residual cells, especially in the equatorial zone, can migrate posteriorly and differentiate into balloon-like wing cells known as the WEDL cell.
  • These cells resemble pearls stuck behind the IOL on retroillumination and are therefore known as the ELSCHNIG PEARLS.
  • Sometimes these opacifications take on a doughnut shape and are known as the SOMMERING’s RINGS.
image illustrating the two types of posterior capsular opacifications( PCO); elschnig pearls and sommerings rings

LENS FIBRES

  • Throughout life, lens fibers are still being produced; as a result, concentric layers of secondary lens fibers are produced as new fibers are put down outside of previous ones.
  • The structure of lens resembles that of an onion,
  • The cytoplasm of lens fibers is highly concentrated in proteins called crystallins, which make up around 40% of the fiber’s net weight.
  • The crystallin concentration varies from approximately 15% in the cortex to 70% in the nucleus. Therefore nucleus is more compact than the cortex.
  • Lens fibres are tightly packed with little intercellular space. Neighbouring cells are linked by ball-and-socket cytoplasmic interdigitations  and numerous gap junctions too.
  • Lens fibres are hexagonal in cross section.
  • These gap junctions help in ions and nutrient transport across the lens.
  • These gap junctions have a different packing arrangement and different connexins than normal gap junctions elsewhere in body.

The fibers of the lens are split into regions depending on the age of origin. From inside to outside we have: –

  • EMBRYONIC NUCLEUS (3 months of embryonic life).
  • FETAL NUCLEUS (3–8 months of fetal life)
  • INFANTILE NUCLEUS (last month of intrauterine life till puberty),
  • ADULT NUCLEUS (corresponding to the lens in early adult life) and
  • CORTEX consisting of the youngest fibers.

It can be observed that the central nucleus of the lens consists of the oldest cells. The periphery or cortex consists of the youngest cells.

image depicting the layers of lens, fetal nucleus, embryonic nucleus, infantile nucleus, adult nucleus and cortex
PRIMARY V/S SECONDARY LENS FIBRES
  • PRIMARY LENS FIBERS : develop before 3 months of age , from the posterior epithelium. It consists of the embryonic nucleus 
  • SECONDARY LENS FIBERS : develop from the equatorial zone/ germinative zone after 3 months. It consists of the lens fibres outside the embryonic nucleus 

SUTURES OF THE LENS and SUTURAL CATARACTS

  • The initial fibers formed surrounding the embryonic nucleus are arranged in a way that they terminate into two sutures. It means that the y sutures are seen in the FETAL NUCLEUS 
  • Upright ( anterior ) Y suture
  • Inverted ( posterior ) Y suture.
  • Opacification of fibres around the sutures leads to sutural cataract 
  • Sutural cataracts are seen in NANCE HORAN SYNDROME
image depicting the two types of sutures of the lens. Upright Y suture and Inverted Y suture . It also depicts a sutural cataract

ZONULES OF ZINN / SUSPENSORY LIGAMENTS OF LENS

  • Lens is held in place by the suspensory ligament or zonule of Zinn.
  • It consists of bundles of strands from the surface of the ciliary body to equatorial capsule. 
  • The zonules of zinn are divided into two, a thinner tuft near the hyaloid (vitreous) referred to as the HYALOID ZONULES and the thicker tuft near lens called as zonular fibres.
image showing the position of the lens zonules

RECENT CONCEPTS ABOUT ZONULAR COMPLEX 

Recent scanning electron microscopy suggests that majority of the zonules arise from the posterior end of the pars plana up to 1.5 mm from ora serrata.

 The suspensory zonular complex is divided into four zones: –

    1. PARS ORBICULARIS: lies on pars plana.
    2. ZONULAR PLEXUS: lies between the ciliary process in the region of pars plicata.
    3. ZONULAR FORK: the point of angulation of the zonules at the midzone of the ciliary valleys.
    4. ZONULAR LIMBS:
      • ANTERIOR ZONULAR LIMB: passes from pars plana to pre-equatorial part of lens.
      • POSTERIOR ZONULAR LIMB: passes from pars plicata to post equatorial part of lens.
      • EQUATORIAL ZONULAR LIMB: passes from pars plicata to lens equator.
    5. The anterior zonular limb is inserted 1.5mm anterior to the equator. The posterior zonular limb is inserted 1.5 mm posterior to the equator

CANAL OF HANNOVER And CANAL OF PETIT

  • CANAL OF HANNOVER : Space between pre-equatorial and post equatorial zonules
  • CANAL OF PETIT : Space between Postequatorial zonules and hyaloid zonules 
Image illustrating the parts of the zonules.

SURGICAL ANATOMY OF LENS

According to the surgical classification lens can be divided into the following parts from inside to outside 

  • Central hard nucleus
  • Epi-nuclear plate of variable thickness
  • A layer of Cortex
  • Capsule

HYDRODISSECTION V/S HYDRODELINEATION

  • HYDRODISSECTION : A step in cataract surgery in which fluid is pushed between the capsule and the cortex. This separates the cortex from the capsule, making cataract extraction easier .
  • HYDRODELINEATION: Here, the fluid is injected within the layers of the nucleus leading to separation of central nucleus from the epinucleus. This step lead to a formation of a golden ring .
image illustrating hydrodissection and hydrodelineation

GRADING OF LENS

Nuclear cataracts can graded according to their color and opacification using slit lamp examination. Cortical and posterior subcapsular cataracts are graded instead using retroillumination to evaluate the degree to which the intrapupillary space or posterior capsule are obscured.

HARDNESS OF LENS BASED ON COLOR

  • Grade 1 : grey
  • Garde 2 : amber , greenish yellow
  • Grade 3 : yellow
  • Grade 4 : brown/ brunescent cataract
  • Garde 5 : black/ cataracta nigra
image depicting rough estimation of hardness of lens based on the color of the lens

LENS OPACITY CLASSIFICATION 3 (LOCS III)

Latest classification for lens grading is LOC III. It compares the findings seen on slit lamp examination with a given set of pictures of the LOC 3 classification. There are a total of 6 pictures for nuclear opalescence and color; 6 pictures for cortical opacification and 6 pictures for posterior subcapsular opacification.

For better understanding you can visit the video here on anatomy of lens. 

CONCLUSION

I hope you found this exploration of the lens informative. For more in-depth knowledge on ophthalmology, follow our YouTube channel or explore our written articles at Insight Ophthalmology. I’m confident our articles will be valuable in your exam preparation. Thank you for visiting, and best of wishes for the future. 

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