Lens & Iris Flashcards
approximately how much of the total refractive power of the eye is found in the lens?
approx. 20D (a third of the total refractive power of the eye)
what are the anterior and posterior curvatures of the lens?
anterior radius of curvature of the lens: 8-14mm
posterior radius of curvature of the lens: 5-8mm (steeper than anterior)
lens becomes flatter towards the periphery (aspheric)
what is the pH of the lens?
pH of the lens is 6.9 (more acidic than 7.2 aqueous humor and 7.4 blood)
retrolental space of Berger
the potential space between the posterior pole of the lens and the anterior vitreous (ring-shaped hyaloid capsular ligament)
lens capsule - properties
transparent basement membrane that surrounds the entire lens, thinnest at the posterior pole and thickest in a circular region around the anterior pole of the lens, primarily composed of type 4 collagen fibers and GAGs
what cells secrete the lens capsule membrane?
the anterior lens epithelium
lens capsule - function
serves as a barrier against large molecules (albumins, WBCs, RBCs, etc.) entering the lens
lens zonules
maintain the position of the lens within the posterior chamber, extend from the non-pigmented ciliary epithelium (where the zonules are produced) and insert into the anterior capsule of the lens
what are the lens zonules composed of?
microfibrils that contain fibrillin and extracellular matrix but NO true elastic fibers
capsulorhexis
opening the front of the lens capsule for cataract surgery
lens epithelium is composed of?
single layer of cuboidal epithelial cells adjacent to the anterior lens capsule (there is NO posterior lens epithelium)
these cells join together with maculae occludens and gap junctions and serve as the main site of lens metabolism
germinal zone of the lens
the pre-equatorial region of the lens (just anterior to the lens equator) that contains mitotic epithelial cells that become the secondary lens fibers
what is the lens cortex composed of?
composed of 65-70% water, 30-35% protein, and 1% other constituents
what types of proteins are found in the lens cortex?
80-90% of lens proteins are water soluble alpha, beta, and gamma crystallins that are tightly packed within the cytoplasm of the lens fibre cells. Alpha crystallins act as molecular chaperones stabilizing the beta and gamma crystallins, help them recover from injuries and prevent degradation of lens fibres and the loss of lens transparency
gradient index of refraction of the lens
the index is gradient because the crystallin concentration varies, the index is higher in the nucleus (n=1.41) and lower in the anterior lens (n=1.38), helps to reduce spherical aberrations
What is the primary protector against oxidative damage in the lens?
Glutathione
Molecular factors that significantly contribute to cataract formation with aging
increase in intracellular Ca2+, a decrease in glutathione, and a decrease in crystallins
What change in astigmatism happens as the lens ages/develops cataracts?
ATR
Where is the lens capsule the thickest and thinnest?
thickest at anterior mid-periphery
thinnest posteriorly
What are zonules comprised of? and where do they derive from?
comprised of microfibrils containing fibrillin and ECM but NO true elastic fibers. Derives from tertiary vitreous
Subluxation vs dislocation of lens
subluxation - lens moved but still in fossa
dislocated - lens falls out of hyaloid fossa (= ectopia lentis)
How does the lens capsule thickness change with age?
anterior lens capsule thickens (14um), posterior capsule stays the same (4um)
primary vs. secondary vs. “tension” zonules
primary - attach directly to ant lens capsule from NPCE
secondary - connect primaries to each other
“tension” - connect primary to the valleys in the pars plicata
Thickest and thinnest parts of the iris?
thickest in collarette region, thinnest at the iris root
Collarette
was the site of attachment for the fetal pupillary membrane during embryonic development
Two zones of the iris
ciliary (radial streaks), and pupillary zones, separated by collarette
Aniridia
bilateral, complete or partial absence of the iris, high association with glaucoma (75%, from PAS angle closure), foveal hypoplasia, nystagmus, microcornea, lens sublux, ON hypoplasia
Iris nodules
Brushfield spots - Downs syndrome
Lisch nodules - NF1
Busacca - granulomatous uveitis
Koeppe - non-granulomatous uveitis
Layers of the iris
anterior border layer, stroma, anterior pigment epi, posterior pigment epi
“ASAP”
What layers of the iris are the sphincter and dilator muscles?
sphincter in iris stroma, dilator in the anterior iris epi
What germ layers do the layers of the iris form from?
neural crest cells - iris stroma
neural ectoderm - sphincter and dilator muscles, and the ant/post pigmented epithelium
Lens embryology
Surface ectoderm: lens placode -> vesicle -> embryonic nucleus -> capsule
Pupillary ruff
formed by anterior and posterior iris epithelial layers curling anteriorly
attachments between anterior and posterior pigmented iris epithelium
these layers are attached apex to apex by desmosomes and microvilli. Iris cysts can develop if there is a separation
Canal of Hannover
aka circumlental space, located bw anterior and posterior lens zonules over the equator of the lens
Canal of Petit
aka retrolental space, located bw the most posterior lens zonules, the anterior hyaloid membrane, and the posterior portion of the ciliary body
apex of the ciliary body points which way?
posteriorly towards ora serrata
Regions of the ciliary body
Pars Plicata (corona ciliaris) - processes with valleys of Kuhnt between, NPCE here makes aqueous
Pars Plana (orbicularis ciliaris) - flatter more posterior, make oral bays, NPCE here makes zonules
Dentate processes and oral bays
Peripheral retina border - dental processes
Oral bays are the pars plana parts
Layers of the ciliary body
supraciliaris, ciliary muscle, ciliary stroma, pigmented ciliary epithelium and non-pigmented ciliary epithelium
Supraciliaris
potential space under the sclera, continuous with suprachoroid, fluid may accumulate and cause CB detachment
Ciliary muscle
largest intrinsic muscle of the eye, longitudinal muscle fibers of Brucke, radial fibers, and Muller’s annular muscle
MACI
within ciliary stroma, fenestrated, formed by anastomoses of ACA and LPCA, contributes to aqueous formation
PCE
continuous with anterior iris epithelium and RPE
NPCE
continuous with posterior pigmented iris epithelium and neural retina
site for aqueous production in the pars plicata region
