Aqueous humour - Ron and Simon PDF

Preview

Summary

Vitreous Humour  Clear colourless fluid that fills the space between the lens and retina of the eye.  Fills 80% of the interior volume of the eye.  Helps maintain the shape of eye and supports retinal attachment.  Storage area for metabolites for retina and lens.  Viscoelastic – shock absorber,...

Description

     

 





Vitreous Humour Clear colourless fluid that fills the space between the lens and retina of the eye. Fills 80% of the interior volume of the eye. Helps maintain the shape of eye and supports retinal attachment. Storage area for metabolites for retina and lens. Viscoelastic – shock absorber, protecting retinal tissue during eye movement. Has 3 components: o Collagen fibres – mainly type II. o Proteoglycans – protein core and glysosaminoglycan (GAG) – GAG is vitreous in hyaluronic acid (HA). o Water Widely spaced matrix of collagen fibres and HA – produces a gel-like structure. 99% consists of water and the rest is collagen-HA matrix. Collagen fibrils can aggregate and if bundles are large enough produce floaters. Hyalocytes produce HA and collagen – function as phagocytes – common in vitreous cortex adjacent to retina. Vitreous Cortex – ~100-200um thick – thicker on anterior surface adjacent to ciliary body and lens. o Thinner adjacent to retina. o Collagen fibrils denser in inner vitreous. o Merges with basement membrane on surface of ciliary body and retina.

 Innermost vitreous – Canal of Cloquet o Site of embryonic hyaloid artery. o Site of first vitreous to form (primary vitreous – outside the canal is the secondary vitreous – primary and secondary separated by remnants of artery wall.  Anterior vitreous o Layer of dense collagen – called anterior hyaloid membrane (AHM). o Cup-shaped depression in AHM – occupied by posterior surface of lens – called patellar fossa.

 Vitreous Base o Strong attachment of anterior vitreous at pars plana epithelium and with the inner limiting membrane of retina at ora serrata = vitreous base – collagen very dense and forms a distinct layer here.  Prepapillary and premacular holes o Annular attachments around these regions – among strongest vitreal attachments.  With age – the vitreous liquefies and shrinks, collagen matric becomes stringy – entities float around the rest of vitreous, casting a shadow on the retina (floaters) . At birth the vitreous is all gel – liquid compartment increases throughout life. Pathologies  Posterior vitreous detachment (PVD) o Once liquid vitreous enters the sub-hyaloid space between vitreous cortex and retina, it strips the vitreous cortex off the retina with each eye movement. o Px often experiences a circular floater (Weiss ring) consisting of avulsed glial tissue from around the optic nerve head. o PVD can tear the retina – liquid vitreous can then leak behind photoreceptors leading to retinal detachment – this can tug on retinal blood vessels and produce haemorrhage. o Common in older eye (65% of people over 65). o Can occur in young if highly myopic.  Developmental abnormalities – usually due to incomplete hyaloid regression. o Persistent hyaloid artery o Mittendorf dot – fragment on posterior surface of lens. o Bergmeister’s papilla – remnant on optic nerve head. o Persistent hyperplastic primary vitreous – anterior part of hyaloid fails to regress and abnormal formation and attachment to posterior lens capsule.

  

  





Aqueous humour Transparent colourless fluid – anterior segment filled with this. Essential for the nutrition of the avascular cornea and lens and removal of metabolic waste products. Generates an intraocular pressure (IOP) – balanced by aqueous production and drainage – this maintains optimal flow rate and necessary for maintaining structural integrity and normal optical function of eye. Produced at a rate of 2-3ul/min (higher flow rates recorded during waking hours than during sleep). Composition – reflects the secretory activity of the ciliary epithelium and the metabolic processes within the eye. Electrolyte concentration of aqueous is similar to plasma and differs in the concentration of certain organic solutes – levels of ascorbate and lactate are much higher than in plasma. Low concentration of protein (...