ANATOMY - Lens Flashcards
Where is the lens located?
In a saucer shaped space in the patellar fossa
What is the hyalodo-capsular ligament (Weiger’s ligament)
Connects the posterior pole of the lens to the patellar fossa
What is retrolental space? (Berger’s space)
Space which exists between hyaloid face and lens within circular ligament
What is the function of the lens? (3)
1) Transmits and refracts light - 35% refractive power
2) Lens absorbs UVA light < 350nm wavelength
3) It helps in accomodation
What is the radius of curvature of the anterior surface of the lens vs posterior surface of lens
Posterior surface is more curved and thinner , so it is 6mm vs 10mm anterior surface (thicker)
What is the equatorial diameter of the lens at birth vs adulthood
6.5mm at birth, 9-10mm in adulthood
What is the lens thickness at birth and at extreme adulthood (what is average rate of growth)
3.5mm at birth, 5.5mm extreme adult life (0.2mm each year)
How far is the anterior pole of the lens from the centre of the cornea?
3mm from the cornea
What is the refractive index of the lens? What is the refractive power of the lens?
index - 1.39
power - 16-17D
What material is contained within the lens capsule?
hyaline collagenous membrane that surrounds lens, elastic but contains no elastic fibres
What is the structure of the anterior lens epithelium?
Cubiodal nucleated epithelium
What is the epithelium in the equatorial region of the lens?
columnar cells –> they actively divide
What is the difference in shape between the central, peripheral and equatorial epithelial cells
central: cuboidal
peripheral: smaller and cynlindrical
equatorial: columnar
Which lens epithelial cells undergo mitosis? central? peripheral? equatorial?
central epithelial cells: undergo metaplasia in shield cataract –> glaucomfleken
peripheral zone epithelial cells: rarely undero mitosis
equatorial zone: actively mitose, and migrate posteriorly to form new lens fibres –> dysplasia of these cells called posterior subcapsular cataract (myotonic dystrophy, radiation, neurofibromatosis
What are the causes of posterior subcapsular cataract?
myotonic dystrophy, radiation, neurofibromatosis
What is the bow region of the lens?
The newly laid lens fibres from migratory cells of equatorial zone ELONGATE so that their nuclei are more anterior to the nuclei of the older, more superficial cells
What is the difference between Elschnig’s pearls and Soemmering’s rings?
POSTERIOR CAPSULAR OPACIFICATION: Residual epithelial cells migrate posteriorly and differentiate into a balloon like wedl cell –> Elschnig pearl
If become a doughnut shape configuration - Soemmering’s rings.
What is the structure of the nuclear zone lens fibres? Where do they develop from?
Primary lens fibres : develop from posterior epithelium before 3 months of age (oldest cells are in the centre)
Secondary lens fibres: develop from the equatorial zone (youngest cells are in the periphery)
Whats the difference between anterior and posterior Y sutures in the lens?
Anterior: upright erect Y
Posterior: offset inverted Y
What are the different zones in the lens? What ages are they?
see picture
What is the lens capsule produced by?
Basal portion of lens epithelium anteriorly, basal portion of posterior lens fibres posteriorly
Where do the zonules of zinn arise from and where do they insert?
Arise from the posterior end pars plana of ciliary body (up to 1.5mm from ora serrata) and insert into the equator of the lens
Whats the difference between phacolytic glaucoma and phacoanalytic glaucoma and phacomorphic and lens particle glaucoma?
Phacolytic: lens proteins leaking through intact capsule to anterior chamber in hypermature cataract and with engorged macrophages which block trabecular meshwork
Phacoanaphylactic: autoimmune granulomatous reaction to exposed lens proteins in eye following trauma or surgery (after fracture of capsule) due to trabeculitis or blockage of TM by inflammatory cells
Phacomorphic: secondary angle closure due to enlargment of cataract lens causing pupillary block
Lens particle: Acute obstruction of TM by fragments of lens material
What is the pathophysiology process behind cataract?
Break down of lens crystallins to albuminoids.
RF: Hypocalcaemia, diabetes, ionizing radiation and mechanical trauma
What is the basement membrane of the lens capsule composed of?
Elastic basement membrane rich in type IV collagen and sulphated GAGs
Which is the most important antioxidant molecule of the lens?
Reduced glutathione
Ascorbic acid metabolites can induce a cataract.
Where does snowflake cataract occur and which patient cohort does it happen in?
Appears in lens cortex - occurs in type 1 diabetes
What metabolic disturbances can cause cataract? (3)
- Defects in galactose metabolism
- Deficiency in galactose kinase
- In diabetes mellitus
What is the major component of the lens capsule and zonules to help with elastic properties?
Fibrillin
