Anatomy and physiology of vitreous 6 marks answer writing covering everything
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Anatomy and Physiology of the Vitreous
1. Introduction / Overview
The vitreous body (corpus vitreum) is a transparent, avascular, gel-like structure that fills the vitreous chamber - the largest compartment of the eye (posterior segment), occupying approximately 4/5 of the total volume of the eyeball (~4 mL). It lies between the lens anteriorly and the retina posteriorly.
2. Embryology
The vitreous develops in three stages:
| Stage | Name | Origin |
|---|---|---|
| 1st (Primary vitreous) | Vascular vitreous | Mesenchyme + hyaloid vessels enter via choroid fissure; fibrillar network forms between lens and retina |
| 2nd (Secondary vitreous) | Avascular vitreous | Secreted by the retina/ciliary body; makes up the bulk of adult vitreous |
| 3rd (Tertiary vitreous) | Zonular fibers | Derived from the non-pigmented ciliary epithelium; strictly these become the zonules of Zinn |
- The hyaloid artery supplies the primary vitreous during fetal life; it regresses during gestation, leaving behind the hyaloid canal (Canal of Cloquet) - a potential space running from the optic disc to the posterior pole of the lens capsule.
- If regression of the hyaloid system is incomplete: persistent hyperplastic primary vitreous (PHPV).
(Langman's Medical Embryology)
3. Gross Anatomy / Structure
Shape and Dimensions
- Volume: ~4 mL
- Roughly spherical posteriorly, with an anterior patellar fossa (a concavity that accommodates the posterior surface of the lens)
- Separated from the lens by the hyaloidocapsular ligament (ligament of Wieger/Egger's ring) - a 9 mm annular attachment
Zones (from outside in)
- Vitreous cortex (peripheral gel) - densest collagen concentration; in contact with the ILM of retina
- Vitreous base - 3-4 mm wide zone straddling the ora serrata (2 mm anterior, 2 mm posterior); site of strongest vitreoretinal adhesion; persists even after PVD
- Central vitreous - less dense, more liquid (fluid vitreous or vitreous humor)
- Hyaloid canal (Cloquet's canal) - runs centrally from the optic disc to the posterior lens capsule
Boundaries
- Anteriorly: Posterior lens capsule, zonular fibers, pars plana
- Posteriorly: Retinal ILM
- Anterolateral: Ciliary body
4. Composition
Vitreous is 99% water. The remaining 1% consists of:
| Component | Detail |
|---|---|
| Collagen fibrils | Mainly type II (also IX, V/XI); form a structural scaffold; densest in cortex and base |
| Hyaluronic acid (HA) | A glycosaminoglycan (GAG); occupies spaces between fibrils; responsible for gel properties and viscoelasticity |
| Hyalocytes | Sparse cells in the peripheral cortex; synthesize collagen and HA; have well-developed rER and Golgi |
| Fibrocytes/macrophages | Occasional cells at periphery |
| Inorganic ions | Similar to plasma (Na+, K+, Cl-, etc.) but lower protein content |
The interaction between collagen fibrils (providing structural framework) and hyaluronic acid (providing viscosity and hydration) maintains the gel state. With age, HA dissociates from collagen, leading to synchysis (liquefaction) and syneresis (shrinkage/condensation).
(Junqueira's Basic Histology; Histology: A Text and Atlas)
5. Sites of Vitreoretinal Adhesion
Adhesion is mediated by fibronectin, laminin, and collagen at the ILM.
Strong adhesions:
- Vitreous base (strongest - persists after PVD)
- Optic disc margins (fairly strong)
Moderate adhesions:
- Perifoveal region (fairly weak)
- Along major retinal vessels (weak)
- Posterior lens capsule (Egger's ring)
(Kanski's Clinical Ophthalmology, 10th ed.)
6. Functions (Physiology)
| Function | Mechanism |
|---|---|
| Optical | Transmits and refracts light; maintains transparency (avascular, acellular) |
| Structural support | Maintains globe shape and intraocular pressure from within |
| Lens support | Patellar fossa stabilizes lens in position |
| Retinal support | Holds the sensory retina against RPE via gentle pressure |
| Metabolic/Diffusion barrier | Acts as a diffusion barrier for nutrients and metabolites between retina and lens; low metabolic activity maintains optical clarity |
| Shock absorption | Gel consistency dampens mechanical impacts |
| Antiangiogenic | Normally inhibits neovascularization within the vitreous cavity |
7. Age-Related Changes (Clinically Important)
- Synchysis - Progressive liquefaction of gel due to dissociation of collagen-HA network; fluid-filled lacunae form
- Syneresis - Collapse and condensation of residual gel
- Posterior Vitreous Detachment (PVD) - Separation of the posterior vitreous cortex (posterior hyaloid membrane) from the ILM posterior to the vitreous base
- Incidence: <10% under age 50; ~65% over age 70
- Can present with floaters (Weiss ring at disc), photopsia
- Complicated PVD may cause retinal tears (at sites of abnormal adhesion) leading to rhegmatogenous retinal detachment
8. Clinical Correlates
| Condition | Vitreous Involvement |
|---|---|
| Asteroid hyalosis | Calcium-lipid deposits suspended in vitreous; benign |
| Synchysis scintillans | Cholesterol crystals in liquefied vitreous; settles on movement |
| Vitreous hemorrhage | Blood in vitreous cavity; obscures fundal view |
| PHPV | Failure of primary vitreous regression; white pupil at birth |
| Endophthalmitis | Infection of vitreous cavity |
Summary for exam: The vitreous is a 99% water, avascular gel filling ~4 mL of the posterior segment. Its scaffold of type II collagen and hyaluronic acid maintains transparency and viscoelasticity. Key anatomical landmarks include the vitreous base (strongest adhesion), Cloquet's canal, and Egger's ring. Functions include optical transmission, structural support, retinal apposition, and metabolic barrier. Age-related synchysis and syneresis lead to PVD, the commonest vitreoretinal event.
Sources: Kanski's Clinical Ophthalmology 10th ed. | Junqueira's Basic Histology 17th ed. | Histology: A Text and Atlas | Langman's Medical Embryology
Anatomy of vitreous humor 6 page answer
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This is a fully sourced, 6-page-equivalent comprehensive answer covering every aspect of vitreous anatomy. Here is what each section covers:
What this answer covers (11 sections):
- Introduction - overview, key facts table
- Embryology - 3 stages of vitreous development, hyaloid system, Cloquet's canal, clinical anomalies (PFV, Mittendorf dot, Bergmeister's papilla)
- Gross Anatomy - shape, subdivisions (cortex, vitreous base, central vitreous, Cloquet's canal), boundaries, Egger's ring with measurements
- Microscopic Anatomy / Histology - water, collagen (types II, V, IX, XI), hyaluronic acid biochemistry, other proteoglycans (opticin, versican), fibronectin/laminin; hyalocytes (location, origin, ultrastructure, function), fibroblasts, macrophages
- Sites of Vitreoretinal Adhesion - physiological (vitreous base strongest, optic disc, perifoveal, vessels) and pathological (lattice, tufts, etc.)
- Blood-Retinal Barrier - inner and outer components, convective flow
- Age-Related Changes - synchysis, syneresis, PVD (mechanism, epidemiology, Weiss ring, complications)
- Physiology / Functions - 9 functions in table format
- Clinical Anatomy / Surgical - pars plana entry, intravitreal injections, vitreous substitutes table
- Clinical Conditions - 10 key conditions with anatomical basis
- Summary Table - exam-ready revision
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