Vitreous

Anatomy and physiology of Vitreous Humor

1. Vitreous

2. Vitreous humour is an inert, transparent, jelly –like, hydrophilic (dissolved in water) gel structure Vitreous cpmposed of approximately 99% water Vitreous cavity is bounded anteriorly by lens & ciliary body & posteriorly by retina

3. wt-4g Vol-4cc (2/3vol of entire globe) Approximately 99 % water , 1 % solid. (0.9 % salts , 0.08 % protein, 0.02% mucopolysaccharide )

4. Development of Vitreous

5. Formation of Primary VITREOUS Primary vitreous appears at the time of formation of the optic cup, is a fibrillated secretion of the retinal cells, and fills in the vitreous space with a feltwork of fine fibrils Primary cellular vitreous formation Vascularization of primary vitreous Its mixed ectodermal and mesenchymal origin

6. Ingrowth of hyaloid artery through the embryonic fissure. Ectoderm also contribute. Present in 1 st month of gestation. Regression of hyaloid artery by the end of 3 rd month leads to retraction of primitive vitreous. Hyaloid artery is no longer patent and loses its connection with disc- 7 th month. Remnants may remain as persistent pupillary membrane.

7. FORMATION OF SECONDARY VITREOUS: Sec avascular vitreous surrounds the primary vitreous. Secreted by neuroectoderm of optic cup. Formation starts from 2 nd month and completes by 5 th -6 th month. Marked by appearance of orderly array of fine dense fibres . 

8. FORMATION OF TERTIARY VITREOUS: Secreted by neuroectoderm in ciliary region. Secreted during 4 th month. Forms vitreous base & ciliary zonules .

9. Structure The vitreous is the largest and simplest connective tissuse present as a single piece in the human body Divided in 3 parts 1. Hyaloid layer or membrane 2. Cortical vitreous 3. Medullary vitreous

10. Hyaloid layer It is not a true membrane but outermost surface of vitreous 1. Ant hyaloid membrane (ant limiting membrane layer) 2. Post hyaloid membrane (post limiting membrane layer)

11. ANTERIOR HYALOID MEMBRANE / ANTERIOR LIMITING MEMBRANE  Lies 1.5 mm anterior to oraserrata With other structure Hyalocilliary zonule - anterior hyloid membrane to cilliary proccess Retrolental ligament – anterior hyaloid to lens Coronary ligament – anterior hyaloid to posterior one-third of cilliary processes circum ferentially Median ligament – anterior hyaloid to pars plana

12. POSTERIOR HYALOID MEMBRANE / POSTERIOR LIMITING MEMBRANE Extends back from vitreous based to optic disc Subhyaloid space: between internal limiting membrane of retina and posterior hyaloid . 

13. Cortical vitreous It lie in peripheral zone appox 100 micron in width It is more condensed fibrillar vitreous It contain Type 2 collagen fibrils interspersed with the sodium hyaluronate , mucopolysaccharide molecules which provide viscosity ,elasticity and tensile strenght to it 2 percent of the total vitreous vol It is the metabolic centre of the vitreous body(vitreous cells – hyalocytes ) Vitreos cells synthesize the hyaluronic acid

14. Medullary vitreous Similar to cortical vitreous except it is less fibrillar structure and cell free Forms major part of vitreous body. Homogenous but exhibit wavy lines. Watered silk appearance in slit lamp. 

15. Vitreous tract Sheets like condensation of vitreous tissue radiating from ciliary body to retina Givens onions skin

16. Vitreous tracts seen in adult vitreous space are-   1.TRACTUS RETROLENTIS – • Innermost tract attached anterior to posterior lens capsule & posterior in front of optic disc 2. TRACTUS CORONARIUM • Attached anterior to coronary ligament & posterior lateral to tractus retrolentis 3 .TRACTUS MEDIANUS • Attached anterior to ligament & posterior lateral to tractus coronarium 4 . TRACTUS PRERETINALIS • Attached anterior at ora serrata circum & posterior lateral to tractus medianus

17. ZONES OF VITREOUS SPACE RETROLENTAL ZONE Anterior boundary – patellar fosa INTERMEDIATE ZONE Anterior boundry – epiciliary portion of anterior hyaloid membrane Medially – tractus retrolentis Laterally – tractus preretinalis Coronary & median tract are present in this zone PRERETINAL ZONE Medially- tractus preretinalis Laterally - retina

18. BIOCHEMICAL COMPOSITION Composed of 3 major structural components 1. Water 2. Collagen-like fibres 3. Hyaluronic acid and glycosaminoglycans (GAGs) Macromolecules constituents are: - Collagen : Type ii collagen :90% Type ix collagen:10% - Hyaluronic acid : N-acetyl glucosamine and glucoronic acid - Soluble proteins : About 10% of serum conc. (low) And thus does not scatter light

19. Vitreous expansion and contraction Vitreous has Na+ ions, NaCl and protein molecules which neutralizes HA( hyaluronic acid)

20. Physicochemical properties Optical properties • Transparency : Fine arrangement of vitreous fibres and high water content • Tyndall effect : Presence of low amount of soluble proteins accounts for absence of light scattering • Light transmission : Transmits 90% of light between 300-1400 nm

21. Plasticity: 3 D arrangement of collagen fibres Viscoelasticity : Negatively charged hyaluronic acid molecules precipitate extreme volume changes with change in physicochemical environment Gel stability: collagen and hyaluronic acid interact to stabilize the system termed as frictional interaction Molecular sieve effects : Selectively inhibit the movement of water ,large molecules and cells

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