refractive media of the eye
learning objectives
● Understand the basics of refraction in the eye
7. Know the anatomy of the cornea and its layers
8. Know the factors contributing to corneal transparency and how this may be
affected by pathological processes
9. Know the basic anatomy of the lens
10.Understand the process of accommodation
11.Know the basic anatomy of the vitreous
12.Understand the difference between a posterior vitreous detachment and a
retinal detachment, and how they may be linked
refraction of the eye
● the change in direction of light as it passes from one medium to the another, of a
different optical density
● the refractive media of the eye are the cornea, lens, vitreous
● most of the refractive elements of the eye are in the anterior part of the eye
● the cornea and the lens are the main refractive elements
● the cornea gives you 43-46 dioptres of refraction
● behind it you have the vitreous- the vitreous is technically a refractive element
because light will be focussed on the vitreous, however it is not a powerful refractive
element
● the aqueous humour is also not a powerful refractive element
● depending on whether light hits a convex or concave surface, you either get light
coming towards them together or you get light refracting out
● Cornea ( CONVEX structure) and the lens is a ( CONCAVE structure)
● Refraction at the cornea = n2 is more dense than n1.eye is therefore designed in a
manner to allow light to keep converging until it hits the fovea
● if light is coming from a less dense medium to a higher density medium, light will
come together
● When light passes from a more dense medium to a less dense medium, it bends
towards the normal and converges
anatomy of the cornea
● cornea forms the entire 1/6th of the eyeball
● in most people the cornea measures 10.5-10.6 mm vertically
● and horizontally it measures about 11.5mm
● its the thinnest at its center which means it most at risk of being damaged
● as you go from the centre of the cornea to the sclera, the cornea becomes the
thickest at that point
● layers of the cornea →
○ the epithelium
, ○ Bowman’s
○ Stroma
○ descemet's membrane
○ endothelium
● Corneal epithelium → stratified, consisting of 5 layers,
● the most anterior surface of the cornea is the corneal epithelium which is the only
part that can regenerate itself
● the reason for the regeneration → there are limbal stem cells at the limbus where the
cornea meets the sclera, these stem cells allow the cornea to regenerate
1. corneal epithelium
● ( only layer that can regenerate ( due to the limbal stem cells- generate cells that
migrate over to damaged area of cornea)
● Consists of 5 layers ( stratified, squamous cells, each layer has layers upon layers)
● Most superficial layer is at the top called squamous cells
● Layers increase to around 10 at the limbus ( thicker outside)
● Under the thick layer, are wing cells which has gap junctions to allow cell to cell
communication ( couple of layers)
● The deepest layer (base) of the epithelium is the columnar cells ( only ones which
are a single layer) which are bound by these things called tight junctions, all they're
doing is kust keeping everything stuck together
● All of these cells are really tightly bound by tight junctions
● The epithelium is also hooked to the bowman's layer via the hemidesmosomes which
prevent epithelium from sliding away
● Conjunctival epithelium merges with corneal epithelium
● around 50-60 micrometers wide
● layers increase to around 10 at the limbus
● there are no melanocytes except at the limbus in POC
● superficial cells are flat
2. bowman's layer
● Completely acellular
● Immediately below the columnar cells of the epithelium (basement membrane under
the columnar cells)
● Made of collagen fibrils ( very stiff and tough structure
● Does Not merge with anything external - ends abruptly at the limbus
● Cannot regenerate
3. stroma-
● makes up majority of thickness and is the reason why the cornea is transparent (90%
of corneal thickness)
● Organised in layers ( lamellae) that are perpendicular to one another
● Perpendicular composition of the stroma is what makes the cornea transparent
● Cant regenerate, just filled with with scar tissue
4)descemet's membrane
● Lies on posterior part of the stroma
● Also known as the basement membrane of the endothelium
● If you cut it, it would curl inwards at some spots, towards the anterior chamber
● Can be regenerated by the endothelial cells
learning objectives
● Understand the basics of refraction in the eye
7. Know the anatomy of the cornea and its layers
8. Know the factors contributing to corneal transparency and how this may be
affected by pathological processes
9. Know the basic anatomy of the lens
10.Understand the process of accommodation
11.Know the basic anatomy of the vitreous
12.Understand the difference between a posterior vitreous detachment and a
retinal detachment, and how they may be linked
refraction of the eye
● the change in direction of light as it passes from one medium to the another, of a
different optical density
● the refractive media of the eye are the cornea, lens, vitreous
● most of the refractive elements of the eye are in the anterior part of the eye
● the cornea and the lens are the main refractive elements
● the cornea gives you 43-46 dioptres of refraction
● behind it you have the vitreous- the vitreous is technically a refractive element
because light will be focussed on the vitreous, however it is not a powerful refractive
element
● the aqueous humour is also not a powerful refractive element
● depending on whether light hits a convex or concave surface, you either get light
coming towards them together or you get light refracting out
● Cornea ( CONVEX structure) and the lens is a ( CONCAVE structure)
● Refraction at the cornea = n2 is more dense than n1.eye is therefore designed in a
manner to allow light to keep converging until it hits the fovea
● if light is coming from a less dense medium to a higher density medium, light will
come together
● When light passes from a more dense medium to a less dense medium, it bends
towards the normal and converges
anatomy of the cornea
● cornea forms the entire 1/6th of the eyeball
● in most people the cornea measures 10.5-10.6 mm vertically
● and horizontally it measures about 11.5mm
● its the thinnest at its center which means it most at risk of being damaged
● as you go from the centre of the cornea to the sclera, the cornea becomes the
thickest at that point
● layers of the cornea →
○ the epithelium
, ○ Bowman’s
○ Stroma
○ descemet's membrane
○ endothelium
● Corneal epithelium → stratified, consisting of 5 layers,
● the most anterior surface of the cornea is the corneal epithelium which is the only
part that can regenerate itself
● the reason for the regeneration → there are limbal stem cells at the limbus where the
cornea meets the sclera, these stem cells allow the cornea to regenerate
1. corneal epithelium
● ( only layer that can regenerate ( due to the limbal stem cells- generate cells that
migrate over to damaged area of cornea)
● Consists of 5 layers ( stratified, squamous cells, each layer has layers upon layers)
● Most superficial layer is at the top called squamous cells
● Layers increase to around 10 at the limbus ( thicker outside)
● Under the thick layer, are wing cells which has gap junctions to allow cell to cell
communication ( couple of layers)
● The deepest layer (base) of the epithelium is the columnar cells ( only ones which
are a single layer) which are bound by these things called tight junctions, all they're
doing is kust keeping everything stuck together
● All of these cells are really tightly bound by tight junctions
● The epithelium is also hooked to the bowman's layer via the hemidesmosomes which
prevent epithelium from sliding away
● Conjunctival epithelium merges with corneal epithelium
● around 50-60 micrometers wide
● layers increase to around 10 at the limbus
● there are no melanocytes except at the limbus in POC
● superficial cells are flat
2. bowman's layer
● Completely acellular
● Immediately below the columnar cells of the epithelium (basement membrane under
the columnar cells)
● Made of collagen fibrils ( very stiff and tough structure
● Does Not merge with anything external - ends abruptly at the limbus
● Cannot regenerate
3. stroma-
● makes up majority of thickness and is the reason why the cornea is transparent (90%
of corneal thickness)
● Organised in layers ( lamellae) that are perpendicular to one another
● Perpendicular composition of the stroma is what makes the cornea transparent
● Cant regenerate, just filled with with scar tissue
4)descemet's membrane
● Lies on posterior part of the stroma
● Also known as the basement membrane of the endothelium
● If you cut it, it would curl inwards at some spots, towards the anterior chamber
● Can be regenerated by the endothelial cells
