Aqueous Humour Dynamics : Aqueous Humour Dynamics Dr. Padmaja Phade Slide 2: Aqueous humour is clear colourless watery solution continuously circulated from posterior chamber of the eye through out the anterior chamber
Maintenance of IOP and pathophysiology of glaucoma revolves around aq. Humour dynamics ANATOMY : ANATOMY Ciliary body
Forward continuation of choroid at Ora serreta
Triangular in cut section
Inner side of triangle is divided into
Pars plicata- 2-2.5mm
Pars plana- 5mm temporally, 3mm nasaly Continue…. : Continue…. Microscopy
Supraciliary lamina- outer most condensed part of the stroma
Stroma- consist of collagen tissue and fibroblast with ciliary muscle, vasculature and nervs
Layer of pigmented epithelium
Layer of non pigmented epithelium
Internal limiting membrane Continue… : Continue… Ciliary processes
70-80 Whitish finger like projections
2 X 0.5mm
Composed of central capillary network with fenestrated thin endothelium and pericytes surrounded by stroma and two layers of epithelium and ILM
Inner nonpigmented and outer pigmented epithelium with juxta pposed apical surfaces
Inner nonpigmented epi. Characterised by mitochondria, zonula occludentes (ZO)and lateral surface interdigitations
The tight junctions contribute to the blood aqueous barrier Slide 8: Posterior chamber
0.06ml of aqueous
Divided into prezonular, zonular and retro zonular space Slide 9: Anterior chamber
2.5mm deep in centre,
Contains 0.25ml aqueous
ant-post surface of cornea,
Post- anterior surface of ciliary body and iris
Comunicates through the pupil with post. Chamber
Chamber volume decreases by 0.11μl/year of life
Chamber depth decreases by 0.01mm/year of life
Chamber depth is shallower in hypermtropic than myopic
Chamber depth is slightly decrease during accommodation partly by lens curvature and partly by forward translocation of lens. Angle of Anterior chamber : Angle of Anterior chamber Peripheral recess of ant. Chamber
Formed mainly by TM
Formed post. To ant. By
Schwalbe’s line Gonioscopic grading of angle width : Gonioscopic grading of angle width Aqueous outflow system : Aqueous outflow system Consist of two pathways
Trabecular / conventional outflow
Uveoscleral / unconventional outflow
It is sieve like structure bridging the scleral sulcus cosist of 3 parts
inner most, extend from iris root and ciliary body to the schwalbes line.
The trabeculea are chord like and 2-3 layer thick.
Arrangement creates 25-75μ
Each trabeculae has 3 concentric layers with central collagenous core enclosed by abasement membrane and trabecular cells. Continue…. : Continue…. 2. Corneoscleral meshwork
From scleral spur to lateral wall of scleral sulcus
Cosist of flat sheet of trabeculae with elliptical opening ranging from 5-50 μ become progressively smaller towards the schlemms canal
3. Juxtacanalicular meshwork
Outermost layer connects corneoscleral meshwork to schlemms canal
Consists of 2-5 layers of loosely arranged cells embedded in ECM (hyluronic acid and other GAG) lined on either side by endothelial cells
Offers main resistance to aqueous flow
Outer endothelial layer of juxta canalicular meshwork comprises inner wall of schlemms canal
Inner endothelial layer continue with corneo scleral meshwork Continue…. : Continue…. Schlemms canal
Endothelium lined oval channel present in scleral sulcus
Endothelial cells of inner wall are irregular and contain giant vacuoles
Collect or channel
25-35 Intrascleral aq. Vessels
Leaving schlemms canal at oblique angles to terminate into episcleral veins.
Valveless, wide at their origin and taper towards the anastomosis with venous channel.
Drain ultimately in cavernous sinus via ant. Ciliary and sup. Ophthalmic vein Formation of Aqueous Humour : Formation of Aqueous Humour Ciliary processes are the site of aqueous formation which is primarily derived from the plasma within capillaries of cilliary processes.
1. Diffusion-mol of gas/solution distribute themselves uniformly throughout the space in which they are contained by net flux of particles from area of higher conc. to area of lower conc. continuation : continuation Fick’s law of diffusion
Rate of movt.=k( c1-c2)
K is constant which depends on nature and permeability of memb, nature solute and solvent and temp.
C1- conc of substance on side with higher conc.
C2-conc of substance on side with lower conc.
2. Ultrafiltration- depends on hydrostatic pressure and solute conc of plasma in capillaries of cilliary processes
3. Secretion-active process against conc gradient water sol substances of large mol size and greater charge are actively secreted STEPS OF AQUEOUS FORMATION : STEPS OF AQUEOUS FORMATION Secretion basically depends on transfer of solute from stromal surface to post chamber
This establishes an osmotic gradient driving water passively into aqueous .
Composition of aqueous is similar to that of protein free plasma except for higher ascorbic acid and bicarbonate content. Continuation : Continuation Formation of stromal pool:
formed by ultra filtration of plasma in capillaries of ciliary processes
due to fenestrations in endothelium proteins are also present in the stromal pool
this ultrafiltrate accumulates behind the tight junctions of the NPE. Conti…. : Conti…. 2. Uptake of fluid from stromal pool
PE takes up solute(NaCl) by 2 major electroneutral processes
Na+ / H+ counter exchanger NHE1 antiport in parallel with the AE2 antiport anion counter exchanger causing Cl- influx and HCO3- eflux carbonic anhydrase II stimulates NaCl uptake by increasing the delivery of H+ and HCO3- to the 2 antiports. Conti….. : Conti….. Na +-K+ 2Cl- SYMPORT situated on basolateral membrane of of PE cells rate of transport of the 3 ions depends on their conc gradient especially on ratio of extracellular to intracellular Cl- net solute transfer through symport is zero at intracellular Cl- conc of 50mM Conti… : Conti… PASSIVE DIFFUSION
mostly water diffuses along osmotic gradient established by Nacl transport.
cell memb have low content of sphyngomyelin and cholesterol hence relative high water permeability
POTENTIAL SOLUTE RECYCLING
In order to minimize fluctuations in the cell vol due to mismatch in rates of uptake of solutes and water by PE at stromal surf and their release at aqueous surf there is autocrine regulation of these processes at the PE level Conti… : Conti… Excess fluid and solute uptake at stromal surf than release at the aqeous surf of PE , NPE
ATP release by PE and NPE
Activation of Cl- channels on stromal surf
Efflux of excess Cl- and water from PE, NPE CONTI……… : CONTI……… 3. FLUID TRANSFER THROUGH GAP JUNCTIONS
Gap junctions between PE and NPE formed by connexins Cx43 and Cx40
Gap junctions are also present within PE and NPE but are functionally less significant
Aqueous is thus formed by parallel couplets of PE NPE cell gap junctions. Conti… : Conti… 4. FLUID TRANSFER INTO AQUEOUS HUMOUR
final step in aqueous secretion.
Solutes and water are transported across the basolateral membrane of NPE.
Na+, K+ ATPase releases(70%) Na+ against electrochemical gradient into aqueous, remaining (30%/) transported passively or by ultra filtration.
Cl- is released along its electrochemical gradient through Cl- channels.
Water released along osmotic gradient established by solute transfer into aqueous through AQP1 and AQP4.
Bicarbonate exits through HCO3-/Cl- exchangers as well as Cl- channels.
K+ transported by secretion and diffusion
Ascorbic acid secreted against a conc gradient
Amino acids are secreted by 3 diff carrier proteins each for acidic , basic and neutral molecules. RATE OF AQUEOUS HUMOUR FORMATION : RATE OF AQUEOUS HUMOUR FORMATION 2.5micro liters/min in an undisturbed human eye
Rate of aqueous inflow is 2 ½ times higher during waking than in nocturnal hours
REGULATION OF AQUEOUS FORMATION
α2 receptor stimulation lowers aqueous secretion via adeylate cyclase inhibition.
epinephrine stimulates PGF2α production which lowers IOP.
β2 receptor stimulation leads to increased aqueous secretion via activation of adenylate cyclase. Conti….. : Conti….. Catecholamines
Stimulation of adenylate cyclase
Icreased c-AMP formation
activation of PK
specific protien phosphorylation
increased permeability of PE,NPE cells to solutes and water.
increased aqueous formation Conti…….. : Conti…….. 2. Ultrafiltration and diffusion
these passive mechanisms depend on blood pressure in cilliary capillaries , plasma oncotic pressure,and IOP.
vasopressin stimulates NaCl transport through PE ,NPE and thus aqueous formation.
vasopressin levels in turn are indirectly proportional to plasma osmolarity. MEASUREMENT OF RATE OF AQUEOUS PRODUCTION : MEASUREMENT OF RATE OF AQUEOUS PRODUCTION Class 1 methods
measure rate of appearance and dissapearance of a substance from aqueous
Radioactive labeled isotopes
Intravenous PAH technique
Class 2 methods
C= facility of aqueous outflow
Pv=episcleral venous pressure Conti……. : Conti……. Perfusion of eyes at a constant pressure
Perilimbal suction cup method BIOCHEMICAL COMPOSITION OF AQUEOUS HUMOUR : BIOCHEMICAL COMPOSITION OF AQUEOUS HUMOUR Water: constitutes about 99.9% of aqueous
Proteins: is about 5-16 mg/dl ie1/500 of plasma protein content (6-7g/dl)
A:G ratio is same as that of plasma
IgG and IgM arepresent
plasminogen and its proactivators are present
Amino acids:- conc varies with aqueous/plasma conc (0.08-3.14)
Non colloidal constituents:- similar to that of plasma
ascorbate, lactate, pyruvate is higher than that in plasma
conc of glucose and urea is higher than that of plasma
bicarbonate, ascorbate levels in post chamber is higher than in ant chamber
chloride conc in post chamber is lower than in ant chamber BLOOD OCULAR BARRIER : BLOOD OCULAR BARRIER BLOOD AQUEOUS BARRIER : formed by tight junctions (zonula occludens and zonula adherans) between cells of inner NPE of ciliary processes and non fenestrated epithelium of iris capillaries.
BLOOD RETINAL BARRIER :
INNER : tight junctions of retinal capillaries and endothelial cells
OUTER: tight junctions between adjacent RPE
Blood ocular barrier prevents proteins and large mol wt substances from entering the ocular cavities
Lipid solubility facilitates ocular penetration
Medium mol wt substances penetrate at a slower rate than their transit through capillary walls.
With breakdown of blood aqueous barrier protein and antibody conc of aqueous equilibrates with that of plasma to form SECONDARY OR PLASMOID AQUEOUS. Fibrinogen may cause clotting . CAUSES OF INTERRUPTION OF BLOOD OCULAR BARRIER : CAUSES OF INTERRUPTION OF BLOOD OCULAR BARRIER OCULAR TRAUMA
mechanical : paracentesis
Physical: X rays
Chemical : alkali
irritants Slide 34: PATHOPHYSIOLOGICAL
2. intraocular and corneal infections
3.anterior segment ischemia
3. Cholinesterase inhibitors
4. Nitrogen mustard FUNCTIONS OF AQUEOUS HUMOUR : FUNCTIONS OF AQUEOUS HUMOUR Maintenance of IOP
Metabolism of avascular stuctures of eye
Clearing function PHYSIOCHEMICAL PROPERTIES OF AQUEOUS : PHYSIOCHEMICAL PROPERTIES OF AQUEOUS VOLUME: 0.31 ml ( 0.25ml in AC ; 0.06 ml in post chamber)
Refractive index : 1.336
pH : 7.2 ( acidic)
Density: slightly denser than water ( 1.040: 1.025)
Osmotic pressure: hyperosmotic to plasma by 3 to 5 m osmo/l AQUEOUS HUMOUR DYNAMICS : AQUEOUS HUMOUR DYNAMICS Includes aqueous formation and drainage
Aqueous flows from post to ant chamber through pupil and in AC flows along conventional current set up due to temp difference in ant part and post part of AC.
From AC aqueous is drained by
Uveoscleral( unconventional) outflow Slide 38: Ciliary process
Aq. In post. Chamber
Trabecular meshwork ciliary body
Schlemms canal suprachoroidal space
Collector channel venous circn. Cil. Body, sclera and orbit
Episcleral veins Trabecular outflow
90% Uveoscleral outflow
10% TRABECULAR OUTFLOW : TRABECULAR OUTFLOW Drains 75 to 90% aqueous
Free flow occurs through TM till the juxtacanalicular tissue which offer some resistance to the outflow.
SPECIAL CHARACTERISTICS OF TM CELLS:
High levels of cytoskeletal actin and lower levels of microtubules
Presence of cellular vimentin and desmin
High levels of surface tPA
GAG degrading enzymes and acid phosphatases
Β2 adrenergic receptors and TIGR
Specialized endocytic / phagocytic properties MECHANISM OF AQUEOUS TRANSPORT THROUGH TM1. VACUOLATION THEORY:- vesicles and vacuoles in endothelium open and close intermittently to transport aqueous from TM cells to Schlemm’s canal : MECHANISM OF AQUEOUS TRANSPORT THROUGH TM1. VACUOLATION THEORY:- vesicles and vacuoles in endothelium open and close intermittently to transport aqueous from TM cells to Schlemm’s canal Conti…… : Conti…… 2. LEAKY ENDOTHELIAL CELLS
SONDERMAN’S CHANNELS : microtubules in TM cells help aqueous flow from corneoscleral trabecular meshwork into lumen of Schlemm’s canal .
CONTRACTILE MICROFILAMENTS :
PORES IN ENDOTHELIAL CELLS : (3μm ) about 20,000 UVEOSCLERAL OUTFLOW : UVEOSCLERAL OUTFLOW Drains 0.3μl/min
Drains 10 to 25% of aqueous
Independent of IOP
PG increase uveoscleral flow to lower the IOP FACILITY OF AQUEOUS OUTFLOW : FACILITY OF AQUEOUS OUTFLOW Pressure gradient of 10mm of Hg between IOP and episcleral venous plexus helps in drainage
C- value expressed as aq. Outflow in μl/min/mm of Hg
It represents quantitative aproximation of state of aq. Drainage system Measurements of C value : Measurements of C value Perfusion method
C= flow rate / Pi – Po
Independent of ocular rigidity and corneal curvature
C=0.28 μl/min/mm of Hg
Most commonly used non invasive method
3. Suction cup method Slide 45: 97.5% population has C value >0.18
Most glaucoma pt. has C value <0.17
Significance of C value
As adjuncting diagnosis of glaucoma
C value < 0.10 or less in angle closure glaucoma after an acute attack suggest that peripheral iridectomy may not be sufficient
Evaluation of drug mechanism and experimentally to study abnormality of various influences on aq. dynamics Slide 46: Thank You