Posterior chamber Flashcards
What is the volume of the posterior chamber?
0.06ml
What are the boundaries of the posterior chamber?
Anterior: heavily pigmented posterior iris
Peripherally: ciliary body
Posteriorly: lens and zonules
Why is the sulcus between the iris and ciliary body significant?
Useful for putting lens in cataract surgery if there are issues putting it in the capsule
What are the two sections of the ciliary body?
Anterior, Pars plicata: contains ciliary processes, 2mm, 60-80 ciliary processes
Posterior, Pars plana: avascular, good for injections, 4mm length
Describe the shape of the lens
Biconvex transparent structure
Thin outer capsule
Medium dense cortex
Firm inner hard nucleus
What is the ciliary body?
A ring of tissue in the anterior segment of the eye
Forms middle part of uveal tract
What is the uveal tract?
Vascular, pigmented, middle layer of the eye
Formed by choroid, ciliary body and iris
Brings most of the nutrients to the eye and absorbs light
What is the shape of the ciliary body?
In 3D is a ring
In cross section, triangle at iris root and apex at ora serrata
How big is the ciliary body?
6mm in length
Runs from 1.5mm posterior to corneal limbus to 7.5-8mm posterior to this temporally and 6.5-7mm nasally
What is the ciliary body continuous with?
Iris anteriorly Choroid posteriorly (scalloped to fit into tooth like edge of ora serrata)
What are the relations of the ciliary body?
External: 1.5mm posterior to corneal limbus
Internal: anteriorly: anteriorchamber, iridocorneal angle and scleral spur
Medial: posterior chamber and zonular ligaments
Zonules attach to surface of pars plicata
Equator of the lens is 0.5mm from ciliary processes
Posterior: vitreous
What are the histological layers of the ciliary body?
Ciliary epithelium
ciliary stroma
Ciliary muscles
Supraciliaris
What are the two layers of the ciliary epithelium?
Non pigmented layer
Pigmented layer
Describe the non pigmented layer of the ciliary epithelium
Continuous with pigmented posterior epithelium of iris
Anterior continuation of neural retina
Continues anteriorly to line anterior chamber
Basement membrane faces posterior chamber
Junctions between cells represent primary blood aqueous barrier in ciliary body (from zonular occuldens and adherens)
Extensive foldings-interdigitate neighbouring cells
Well developed golgi apparatus, many mitochondria and extensive endoplasmic reticulum
Describe the pigmented layer of the ciliary epithelium
Anterior continuation of retinal pigment epithelium
Continuous with anterior epithelium of the iris
Basement membrane faces ciliary stroma
Packed with melanosomes
Small golgi apparatus, many mitochondria
Basal lamina infolded suggesting ion transport
How are the non pigmented and pigmented layers of the ciliary epithelium similar?
Both layers make aqueous
2 layers may be coordinated
Both cubical cells
Describe the ciliary stroma
Loose connective tissue
Rich in blood vessels and melanocytes
Contains ciliary muscle
Core of ciliary process
What type of muscle is the ciliary muscle? What are the 3 main groups?
Smooth muscle
Longitudinal: major, most external, closest to sclera, inserts into scleral spur
Oblique/radial: runs from first to 3rd layer, radiate out from scleral spur
Circular: most internal, like a sphincter, close to peripheral edge of the lens
What is supraciliaris?
Pigemented collagen, containing fibroblasts and melanocytes
What is the blood supply to the ciliary body?
Ophthalmic artery
2 long posterior ciliary arteries pierce sclera near optic nerve, go anteriorly between sclera and choroid at peripheral edge of the iris, forms major arterial circle (with 7 anterior ciliary arteries)
Venous drainage: via vortex veins (mainly) and anterior ciliary veins
Capillaries near ciliary epithelium are fenestrated
What is the parasympathetic nerve supply to the ciliary body?
Parasympathetic: supplies ciliary muscle
Preganglionic neurons in Edinger-Westphal nulear, axons travel via occulomotor nerve to synapse in ciliary ganglion
Post ganglionic parasympathetic fibres pass forward in the short ciliary nerves (6-10)
What is the sympathetic nerve supply to the ciliary body?
Vasomotor supply to blood vessels
Preganglionic neurones synapse in superior cervical ganglion, postganglionic travel via nasociliary nerve/ long ciliary nerves
Some pass the ciliary ganglion and pass forward in short ciliary nerves (2-3)
What is the sensory supply to the ciliary body?
Function unknown
From nasociliary nerve via long and short ciliary nerves
What are the functions of the ciliary body?
Aqueous production: actively produced in ciliary epithelium
Accomodation: contraction of ciliary muscle causes ciliary rings to decrease in diameter, ciliary body moves forward, tension decreases on zonules, lens becomes more convex-> increased power-> accomodation
Production and maintenance of zonular ligaments (by non pigmented epithelium)
Which two embryological tissues form the ciliary body?
Neuroectoderm: forms 2 layers of epithelium covering the ciliary body
Mesenchyme: on edge of optic cup differentiates into connective tissue of ciliary body, SM of ciliary body and suspensory ligaments
When does the ciliary body reach its adult dimensions?
7 years old
Linear relation between ciliary body and axial length of the eye
Pars plana and sclerotomy site will vary with age
Why do the iris, ciliary body and anterior choroid get inflamed together?
Share a common blood supply
Called uveitis
When can the position of the ciliary body lead to glaucoma?
Plateau iris
What are the two key risks of trauma to the anterior chamber in relation to the ciliary body?
Angle recession: split between layers of ciliary muscle
Cylodialysis: split between sclera and ciliary body
Which embryological tissue does the lens develop from?
Ectoderm thickens into lens placode, then invaginates into lens pit
Cells in posterior end of lens pit lengthen to form primary fibre cells
How does the embryological lens differentiate into primary fibres and epithelium?
The primary fibre cells elongate and a space forms between them and the overlying lens epithelium
Why does the lens grow throughout life?
Only the epithelium of the lens divides (just above the equator)
Epithelial cells which shift below the equator differentiate into secondary fibre cells which pile up throughout your life
How are the fibres within the lens arranged?
Very regularly
What shape do the apical fibres meeting at the anterior pole of the embryological lens make?
Y shaped sutures
Start out with Y shaped sutures then become more complex as lens grow
How does lens develop polarity and growth patterns?
Relationship of lens to optic cup
Vitreous promotes fibre differentation
Aqueous promotes proliferation of epithelial phenotype
Confirmed by lens inversion experiment 1963
Describe the 1963 lens inversion experiment
They took a chicken lens and inverted it so the epithelial cells face the retina and the original epithelial cells differentiates into a new mass of fibre cells
Which hormone present in the vitreous induces fibre differentiation?
FGF
Describe the epithelial cells in the lens capsule?
Mitotic figures present
Cobblestone pattern
Below lens equator cells begin elongating and pack very regularly
Describe the fibre cells of the lens
Very aligned tram line arrangement
In transverse section: individual fibres hexagonal (2 large 4 short faces) and interlocked
From corners of fibre cells membrane protrusions interlock
NO ECM
How do the fibre cells of the lens pack in with no ECM and how does this reduce light scatter?
The membrane protrusions out of each short face link
Plus the hexagonal shape allows them to stack together, reducing intercellular space and reducing light scatter.
Which plasma membrane complexes are contained in the long face of the fibre cells of the lens?
Periplakin, periaxin, ezrin, desmoyokin
Which plasma membrane complexes are contained in the short face of the fibre cells of the lens?
N- cadherin and cedherin-11
What are the main cytoskeleton components of lens fibres?
Microtubules (tubulin)- which run along entire length of fibre Microfiliaments (actin)- accumaltes at the corners but also runs entire length of the fibre Intermediate filaments (filensin/phakinin)- run entire length of fibre
How do fibres change as they move further towards the lens nucleus?
They initially have nuclei and organelles but these are lost towards the centre as they interfere with transparency
What is the embryological origin of the lens nucleus fibres?
Primary fibres which appear during embryological development
Which two factors enable the lens to remain transparent?
Lens fibre cells must have regular packing and remain very organised
Lens proteins are 90% crystallins which do not form aggregates. Aggregation is associated with cataract
What are the three types of crystallin (lens protein)
Alpha- first crystallin to be detected. Produced by epithelial cells and fibre cells
Beta-only in fibre cells. Appears second.
Gamma- only in fibre cells. Last crystallin to be detected
How does the iris develop?
Optic cup rim does not form neuroretina or retinal pigment epithelium
Becomes epithelial layers of iris and ciliary body
Migrating mesenchymal neural crest cells become associated with the 2 epithelial layers of the rip of the optic cup. Becomes stroma of iris and ciliary body.
How does aqueous flow from the pars plicata of the ciliary body into the anterior chamber?
Travels between iris and the lens
What are the components of the iris in crossection?
Anterior border layer/ stroma: nil epithelium (direct communication between aqueous and iris stroma)
Iris sphincter/ sphincter papillae
Iris dilator/anterior epithelium
Posterior pigment epithelium
What are the two layers of the iris epithelium?
Anterior epithelium and dilator muscle
Posterior pigmented epithelium
How does the iris change the size of the pupil?
When sphincter/constrictor contracts-> pupil constricts
Contraction of dilator muscle-> pupil dilates
How big is the iris?
12mm diameter
What are the 3 functions of the iris?
Control of retinal illumination
Reduce optical aberrations
Increase depth of field and focus
What are the two different segments of the anterior surface of the iris?
Pupillary zone
Split by collarette
Ciliary zone (contains contraction furrows)
What is the pupillary rough?
At the pupillary margin, the posterior pigmented epithelium of the iris sits, forming a dark ring. If this extends too anterior-> ectropion uveae
How do radial streaks of the iris change with pupillary dilation?
Wavy when dilated
Straight when constricted
What are the radial streaks made of?
Trabeculae of connective tissue which overlay oval holes called Fuch’s crypts
Describe the arterial blood supply to the iris
2 circumferential circles
Peripherally located major arterial circle: formed by anastomosis of 2 long posterior ciliary arteries and 7 anterior ciliary arteries. Located in stroma of ciliary body.
Incomplete minor arterial circle: formed by radial arteries which come from the major arterial circle. Located at in the collarette.
Describe the venous drainage of the iris
Follows arteries
Minor venous circle at collarette, drains into radial veins in centrifugal arrangement towards vortex veins
What is significant about iris capillaries?
Form an integral part of blood aqueous barrier as have non fenestrated epithelium and tight junctions
What is the nerve supply of the iris?
Sensory and autonomic
Long ciliary nerves derived from nasociliary branch of ophthalmic division of trigeminal nerve. Contains sensory fibres and sympathetic fibres from superior cervical ganglion which innervates dilator pupillae and iris vessels.
Parasympathetic supply from Edinger-Westphal nucleus, synapses at ciliary ganglion before travelling via short ciliary nerve to sphincter pupillae
Where is the sphincter papillae of the iris?
Posterior iris stroma in the central pupillary zone
Anterior to posterior pigmented epithelium
What are the 3 stages of iris sphincter development?
- Initial anterior epithelial layer near pupillary margin
- Division of epithelial cells into spindle shaped protoplasmic elements, forming a mass in pupillary margin
- Differentiation of spindle shaped cells into myoblasts which form mature sphincter muscle
What is the arterial supply to the iris sphincter?
Via the major and minor arterial circles
What do the parasympathetic short ciliary nerves supply?
97% to ciliary body
3% anticholingeric to iris sphincter, causing contraction of the pupil
What is Adie’s tonic pupil?
Pupil unresponsive to light, slow to contract in light and slow to dilate in the dark. Caused by disorder of parasympathetic innervation of iris sphincter muscle
Benign but must be distinguished from Argyl robertson. Can be confirmed by hypersensitivity to cholinergic agents (will constrict when 0.1% pilocarpine is put in the eye)
What is the different between Argyl Robertson pupil and Adie’s tonic pupil?
Argyl Robertson: small pupil, no reaction to light but contracts with accomodation. Pupil WILL NOT constrict with cholinergic agent. Caused by neurosyphilitic lesion interrupting fibres from pretectal nucleus to parasympathetic nuclei
Adies: Sluggish pupil, no reaction to light will slowly contract with accomodation. Pupil will constrict with cholingeric agent.
What is the most significant gene for eye morphogenesis?
Pax6
Which hormones regulate the elongation of primary lens fibres in the embryonic lens?
BMP and FGF
How do the Y sutures formed by secondary lens fibre development differ based on polarity and what controls this process?
IGF and FGF
Anteriorly : upright Y suture
Posteriorly: upside down Y suture
What significant event occurs in week 9 gestation for the lens?
Distal hyoid artery forms tunical vasculosa lentis
AKA posterior vascular capsule. This anastomoses with choroidal veins to form capsulopupillary membrane
Nourishes lens until aqueous takes over. This network of vessels atrophies from 4 months onwards and has completely disappeared by birth
What are the dimensions of the lens?
Unaccommodated
10MM Equatorially
4MM AP diameter
What separates the lens from the vitreous humour posteriorly?
Hyaloid capsule called Wieger’s ligament and retrolental/Berger’s potential space
What is significant about the lens epithelium?
Cuboidal
Contains Na/K+ ATPAse pumps which are involved in homeostasis of lens with aqueous humour
Also have pre-equatorial epithelial cells which are porgenitors for new lens fibres
How does the thickness of the lens capsule vary?
Thickest near the equator and thinnest posteriorly
What are the zonules?
Bundles of microfibrils
Originate from non pigmented epithelium of pars plana and insert onto equatorial region of lens capsule
Facilitates accommodation and suspends lens in position
What is the neurovascular supply of the lens?
None
Supplied by aqueous humour
Dependent on Na/K ATPase
What is the triad of acccomodation?
Convergence
Miosis
Accomodation
What is Helmholz theory?
Unaccomodated eye: ciliary muscle relaxed, zonular fibres under tension which stretches the lens and makes it less convex, decreasing optic power
Accommodated eye- construction of ciliary muscle relaxes zonular fibres, makes lens more spherical and increases optic power
How does the lens protect the retina from UV light?
Blocks UV 300-400nm long
Cornea blocks shorter wave lengths
What are the main causes of presbyopia?
Increased thickness + reduced elasticity of lens capsule
Stiffening and reduced pliability of central lens
Increasing equatorial lens diameter moves lens closer to ciliary body (decreasing potential change in zonular tension)
Loss of smooth muscle cells in ciliary body
How do the 3 main types of age related cataracts differ?
Cortical: disrupted homeostasis and damage of lens epithelial cells causes water accumulation between lens fibres
Nuclear: reduced a-crystallin causes protein aggregation and oxidation of nuclear lens fibres
Posterior subcapsular: abnormal posterior suture formation or differentiation of lens fibres
Why does the lens stiffen with age?
During life, epithelial cells differentiate and become lens fibre cells
Mature cells have no organelles packed with proteins
Cells added onto preexisting lens
As new lens fibre cells are added, lens becomes stiffer (1000x over life)
What causes age related nuclear cataract?
Oxidation and colouration of proteins in the lens centre
These proteins display highest degree of oxidation of any biological tissue
90% of protein sulfhydrl are lost in advanced ARN cataract. Susceptible to irreversible damage by oxidation
How does methionine in the lens change with oxidation/ cataract development?
Methionine becomes methionine sulphoxide when oxides
Increased methionine sulphoxide as a catarct develops
What causes lens oxidation?
GSH/glutathione (body’s main antioxidant) made in cortex (has to pass via gap junction to nucleus)
Found in higher concentration in cortex than nucleus in normal lens
In cataract: GSH zero in nucleus, normal in cortex
Barrier to diffusion develops in middle age so GSH cant flow from for cortex to nucleus
What causes the barrier to diffusion between the cortex and nucleus of the lens?
Protein breakdown
Denatured proteins and clog pores
Instrinsically unstable proteins
a-crystallin acts to bind denatured proteins
However by age 40 a-crystallin is used up
How does the sucrose gradient protocol prove that with increased age there is increased protein binding to membranes and blocking pores in the old lens?
Concentration of soluble and membrane proteins are lower in older eye, particularly in the inner and core segments. Extra couple of bands at the bottom correspond with denatured protein that is bound to the membranes, increasing from outer to core
At what age does the lens start being full of denatured proteins blocking the entry of GSH into the lens core?
Between 40 and 50
How does heat effect the sucrose gradient?
Increasing the temperature from 4 to 50 degrees changes and eye from young to old
(because temperature denatures proteins)
What causes age related protein denaturation?
Heat
Deamindation (removal of amide functional group)
Racimisation (conversion from one enantiomer to another)
Truncation
Why don’t we see violet or UV light?
Lens fliters this UV light
Made in lens by tryptophan (amino acid)
300-400 UV light band most important as this is believed to cause photooxidation of the retina
What determines the colour of the iris?
Pigment in the melanocytes
Blue has less pigment in melanocytes compared to brown iris
