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