Lens and zonular apparatus Flashcards
What is the lens?
highly organised system of specialised cells (so-called lens fibres) which constitutes an important component of the optical system of the eye
What is the overall function of the lens?
fulfils the function of altering the refractive index of light entering the eye to focus on the retina
What is the refractive power of the lens in dioptres? How does this compare with the cornea?
15 dioptres (less than cornea)
What property does the lens possess that the cornea doesn’t, despite its weaker refractive power?
can change shape under the influence of the ciliary muscle and thus alter its refractive power
What happens to the range of dioptric power of the lens with age?
diminishes
What is the transparency of the lens a result of?
the shape, arrangement, internal structure and biochemistry of the lens cells or lens fibres
What is decreased transparency of the lens konwn as?
cataract
What is the anatomical location of the lens?
lies behind the iris and in front of the vitreous body
encircled by the ciliary processes
held in place by the zonular fibres laterally, anterior vitreous face posteriorly (patellar fossa), and the iris anteriorly
What are three things that hold the lens in place?
- zonular fibres laterally
- anterior vitreous face posteriorly (patellar fossa)
- iris anteriorly
What process in fetal development leaves the lens transparent?
it is normally transparent and avascular following regression of the pupillary membrane and tunica vasculosa lentis late in fetal development
What provides nourishment to the lens?
the aqoueous and vitreous humours
What is the shape of the lens?
biconvex, ellipsoid structure with differing radius of curvature on the anterior and posterior surfaces
What is the anterior vs posterior curvature of the lens?
- anterior: approx 10mm (range 8-14mm)
- posterior: 6mm (range 4.5-7.5mm)
What are the centre points of the anterior and posterior surfaces of the lens known as?
anterior and posterior poles - connected by an imaginary axis
What distance does the anterior pole lie from the posterior corneal surface?
3mm
What separates the anterior and posterior surfaces of the lens?
the equator
What is the appearance of the equator of the lens and why?
ridges (indented) appearance caused by the zonular fibres
What is the diameter of the lens in the adult eye?
10mm
What is the axial length of the lens in the adult?
4mm
What changes happen to the lens through life?
it continues to grow (0.023mm per year) and alters shape throughout life; becomes rounder with age, especially after age 20 years
What are the 3 parts of the lens?
- capsule
- anterior or lens epithelium
- lens fibres
What is the lens capsule?
a thickened, smooth, basement membrane produced by the lens epithleium and lens fibres
completely envelops the lens and has regions of variable thickness
What are the 2 things which produce the lens capsule?
- lens epithelium
- lens fibres
What are the thickest parts of the lens capsule and what is its thickness here?
pre- and post-equatorially (17-28µm)
What are the thinnest parts of the lens capsule and what is its thickness here?
posterior pole (2-3µm) more so than anterior pole (9-14µm)
What is the appearance of the lens capsule under ultrastructural examination?
fibrillar or lamellar appearance
What are 2 things that the interfibrillary matrix of the lens capsule consists of?
- basement membrane glycoproteins (type IV collagen)
- sulphated GAGs responsible for period acid-Schiff-positive staining properties in histological sections
What type of collagen is present in the lens capsule interibrillary matrix?
collagen type IV
What property do the sulphated GAGs in the lens capsule responsible possess?
period acid-Schiff-positive staining properties in histological sections
What shape does the lens capsule cause the lens to assume when not under tension of the zonules, due to its elastic properties?
together with the cortex causes lens to assume a more rounded shape
Label the following on the image:
- lens capsule
- embryonic nucleus
- fetal nucleus
- adult nucleus
- adult cortex
- equator
- central zone
- germinative zone
- transitional zone
- lens bow
- lens epithelium
good job!
What is cataract?
loss of the normal lens transparency, besides the normal age-related yellowing
What may cause cataract?
may be caused by cumulative damage by ultraviolet light or perturbations in lens fibre biochemistry
opacification may be the result of damage or disruption of the capsule, the lens fibre configuration or the lens epithelium
What type of epithelial cells form the lens epithelium?
simple cuboidal epithelium
Over which areas of the lens is the lens epithelium found?
restricted to the anterior surface of the lens
What changes does the simple cuboidal epithelium of the lens undergo in different regions of the lens?
the cells become more columnar at the equator
as they elongate, the apical portion comes ot lie deeper to other, more anteriorly positioned, lens cells
What are the elongated lens cells konwn as?
lens fibres
How do the equatorial lens epithelial cells become transformed into lens fibres?
the cell nucleus and cell body sink deeper into the lens as further cells are laid down externally
Where is mitotic activity greatest in the lens epithelium and what is this area konwn as?
pre-equatorial and equatorial lens epithelium
= germinative zone
What are the shape and dimensions of lens fibres?
each lens fibre is 4 x 7 µm wide
hexagonal prismatic band in cross-section
up to 12mm in length (up to 1000x longer than its width)
How are the lens fibres orientated in the lens?
the apical portion of the lens cell (or lens fibre) passes anteriorly, basal portion posteriorly
What changes hapen to the lens cells for it to move position?
the cell nucleus migrates anteriorly as the cell is pushed deeper in the lens, hence creating the anteriorly oriented lens bow
meridionally oriented lens fibres extend the full length of the lens, meeting at anterior and posterior sutures
deeper (older) lens fibres are anucleate
Where are the meridionally oriented lens fibres located?
extend the full length of the lens, meeting at the anterior and posterior sutures
Which lens cells (lens fibres) in the lens are anucleate?
the deeper (and hence older) lens fibres
What process allows continual growth of the lens?
addition fo superficial strips of new cells
What is the arrangement of lens fibres within the lens due to continual growth?
series of concentrically arranged laminae, similar to the layesr of an onion
How do different macroscopic regions of the lens differ in life?
the outer cortex of the lens has a softer consistency than the hard central nucleus
How are lens fibres connected to adjacent lens fibres?
they are tightly packed with little intercellular space
neighbouring cells are linked by ball and socket cytoplasmic interdigitations and numerous gap junctions
What is thought to e the purpose of the gap junctions between lens fibres?
they may aid maintenance of centrally positioned cells (via intercellular and molecular coupling or metabolic cooperation) some distance from the source of nutrition (aqueous humour)
What are 3 cellular organelles that superficially located lens fibres are rich in?
- ribosomes
- polysomes
- rough endoplasmic reticulum
What do the cell organelles oft he superifically located lens fibres (ribosomes, polysomes, RER) help them to achieve?
actively synthesise unique lens proteins, lens crystallins
What cellular structures are rich in all lens fibres?
cytoskeletal elements oriented parallel to the long axis of the cell
What can disturbances in the biochemistry of lens fibres / their arrangement lead to?
opacification known as cataract
What holds the lens in position?
a complex 3D system of radially arranged zonules (zonsules of Zinn or the suspensory ligament of the lens)
Where do the lens zonules attach to the lens capsule?
2mm anterior and 1mm posterior to the equator
present for a full 360 degrees of the lens circumference - resemble circular trampoline
Where do the lens zonules arise and what is their course to reach the lens?
arise from the pars plana region of the ciliary epithelium and pass forward closely related to the lateral surfaces of the ciliary processes, then attach to the lens capsule
What is the clinical importance of understanding the lens zonules’ distribution and the tension forces they are able to withstand?
important in modern day cataract surgery where the phacoemulsification probe exerts considerable downward force on the zonule
What forces are normally transmitted by the zonules in humans and what function does this allow in humans?
the zonules normally transmit the radial forces that flatten the lens, allowing the eye to focus on distant objects
What is thought to be the role of lens zonules which are present in non-accommodating species?
role in lens centration
What is the structure of the lens zonules?
they consist of dense, glassy bundles 5 - 30 µm in diameter
each bundle consists of a series of fine fibres (0.35 - 1 µm in diameter), themselves composed of 10-12 nm diameter microfibrils
microfibrils → fine fibres → glassy bundles → zonules
What do the 2 microscopic images show?
A = macroscopic view of ciliary processes (CP) and intervening lens zonules (arrowheads) inserting into lens capsule in a monkey eye. L=lens, PP=pars plana
B = Electron micrograph showing the close association of zonular fibres (F) to the non-pigmented epithelial cells (NPCE) of the ciliary processes. Note the material similar to zonular fibres beneath the basal lamina of the epithelium (arrowheads).
What other human structure do the zonules most closely resemble?
biochemically unique but closest to the periodontal ligament of the teeth
Which glycoprotein are zonules composed of principally?
fibrillin, a 350 kDa cysteine-rich glycoprotein
(although described as elastic fibres, do not contain significant amounts of elastin)
What mutations in Marfan syndrome causes ocular manifestations of this condition?
mutations in the fibrillin-1 gene
Which cells in the eye synthesise and maintain the zonular fibres that suspend the lens?
cells of the non-pigmented ciliary epithelium
Where do the zonules fibres emerge from proximally and where are the distal portions connected?
proximal end: emerges from basal lamina of the ciliary epithelial cells
distal portions: connect to the lens capsule near the lens equator. in humans, fibres appear to insert directly into the capsule, tips terminate below the capsular surface
What are the lens zonules synthesised and maintained by?
cells in the non-pigmented ciliary epithlium (same as the zonular fibres)
At what stage is the lens zonule synthesised and when does it develop?
synthesised during embryonic and early postnatal development
likely zonular proteins turn over slowly, if at all
What is the name of the condition of the lens that occurs in Marfan syndrome?
ectopis lentis
What is Marfan syndrome?
condition characterised by disturbances in connective tissue and skeletal elements
What is ectopis lentis?
the zonules are disrupted due to mutation in fibrillin-1 gene; the lens is dislocated or malpositioned within the eye
What is the state of the ciliary body/ zonules in the non-accommodated state?
the ciliary body is relaxed and therefore maintains tension on the zonules
What is the state of the ciliary body/ zonules in the accommodated state?
movement of the ciliary body causes slackening of the zonules, whereupon the lens assumes an increased anterior curvature, with resultant increase in refractive power (to focus on a near object), owing to elasticity of the lens capsule and the outer cortical layers
What do some authorities believe are the two types of lens zonules?
main zonules and ‘tension’ zonules
tension zonules are placed under tension during accommodation
What is presbyopia?
ageing changes in the eye - condition develops around age 40-50 years when elasticity of the lens markedly decreases and there is associated atrophy of the ciliary muscle fibres
lens fails to change shape sufficiently during accommodation
becomes evidence as decreasing ability to read i.e. use near vision
At what age does presbyopia typically develop?
age 40-50 years
What are the 2 key physical changes in the eye in presbyopia?
- decreased elasticity of the lens
- atrophy of ciliary muscle fibres
How does presbyopia become evident through symptoms?
decreasing ability to read i.e. use near vision
What structures are the posterior lens zonules associated with?
collagenous material of the anterior hyaloid membrane
What is the role of the zonules running perpendicular to the main zonule stream?
form circumferential bands near the base of the ciliary processes or in the pars plana (posterior zonular girdle) and over the apices of the ciliary processes (anterior ciliary girdle)
What is the anterior zonular girdle?
where zonules runing perpendicular to the main zonule stream form circumferential bands over the apices of the ciliary processes
What is the posterior zonular girdle?
where zonules runing perpendicular to the main zonule stream form circumferential bands near the base of the ciliary processes or in the pars plana
