Lecture 44 Flashcards
Eye anatomy
2.5 cm diameter in adult, grows during childhood
Muscles keep the eye in the socket and allow the eye ball to move around
Tough to cut through eye
Vitreous body is gelatinous substance that fills cavity of the eye. Lens is in front of vitreous body and is embedded in aqueous humour (salty fluid that resembles extracellular fluid). Aqueous humour is in anterior chamber (space in front of iris)
bw iris and cornea.
Iris gives eye color
Anterior chamber is connected to posterior chamber (space behind iris). Anterior and posterior chambers are connected via pupil (opening of iris)
Conjunctiva is loose CT that covers the eye and is continuous with inside of eyelid
Ciliary muscle is around the lens, and are connected to suspensory ligaments (aka zonula fibers). Suspensory ligaments attach the lens and suspend it in front of the vitreous body
Sclera is tough area that surrounds the eye (outer most layer) (rich in blood vessels).
REMEMBER: when talking about structure of eye, we say an inner or outer layer. Inner layer is closer to centre of eye.
Eye development
- Neural plate invaginates in the embryo until it detaches from the surface (ectoderm), becoming the neural tube
- Neural tube (neuroectodermal) gives rise to spinal cord and brain
- The part of the neural tube at the head end of the embryo starts to form vesicles. It bulges out at head end to form optic vesicle.
- Optic vesicle starts to hit the ectoderm. Where it hits the ectoderm causes invagination
- The invagination gets deeper and soon detaches. Through invagination, it forms a cup, which later becomes the retinal pigment ep and the neural retina; these are 2 layers, and form a part of the eye
- the retinal pigment ep and the neural retina are connected through optic stalk to the neural tube and to brain + spinal cord. The retinal pigment ep and the neural retina are
part of the nervous system
- Ectoderm invagination also detaches and becomes the lens
- lens is ectodermal. Everything else in between is mesodermal
ALTernative explanation:
‐ Derived from neuroectoderm, surface ectoderm and mesoderm
‐ As neural tube forms, optical vesicles are formed at cranial end
‐ Connection to forebrain constricts – optic stalk
‐ Contact to surface ectoderm induces lens placode
‐ Invagination of optic vesicle & lens placode forms optic cup with retina and retinal pigment epithelium (RPE) plus lens vesicle
‐ Neuroectoderm: retina, RPE, iris, ciliary body epithelium
‐ Surface ectoderm: lens and corneal epithelium
‐ Mesoderm: corneal endothelium & stroma, sclera, choroid, blood vessels, etc.
Tunics of the Eye
• Corneoscleral coat (cornea and sclera, ~1mm thick, protects and shapes the eye)
- sclera is continuous with cornea (transparent and the most anterior eye part)
- border bw sclera and cornea is limbus
- sclera is continuous with dura mater (meninges that wrap brain).
‐ Cornea (convex, transparent)
‐ Sclera, (hard), the “white” of the eye, often blue in kids and yellow in elderly, tough, opaque white layer.
• Uvea (choroid, ciliary body and iris; these are vascular layer)
- Uvea is dark brown due to melanocytes and blood vessels
- choroid is layer under sclera that has blood vessels and melanocytes, nutures the retina
- uvea contains structures that are continuous with choroid, which are ciliary body and iris
‐ Ciliary body: smooth muscle (radial and concentric) – lens accomodation
‐ Iris: smooth muscles (radial and concentric) – aperture: pupil (black) – light adaption
• Retina (thin layer of 2 components: Retinal Pigment Epithelium (RPE) and neural retina)
- most inner layer is retina
- fovea is place of highest visual acuity
- macula is area around fovea
Choroid
Choroid: 0.1-0.25 mm thick
Inner to outer: Pigmented ep, Bruch’s membrane, Choriocapillaris, Choroidal stroma, sclera
Choroid is Bruch’s membrane, Choriocapillaris, Choroidal stroma
• Bruch’s membrane
– Inner most
– Basal lamina of opposing RPE and endothelial cells
– separates retina from choroid
– gets thicker with age
– contains basement membrane of RPE and endothelial cells of the choroid
– 2 basal laminae & collagen & elastin fibres
• Choriocapillaris
– contains small fenestrated capillaries that give nutrients to tissue and outer retina
– all nutrients need to cross bruch’s membrane too which can cause trouble as bruch’s membrane becomes too thick with
age
– connective tissue, melanocytes, blood vessels, nerves, lymphatic channels
– nutures retina
• Choroidal stroma
– Contains large vessels, nerves,
collagen, fibroblasts and melanocytes
– large blood vessels surrounded by collagen and elastic fibres, smooth muscle cells, neurons, melanocytes
Also, any light not absorbed by retina or RPE will be absorbed by choroid to prevent damage to tissues outside of eye
Ciliary body
Uveal portion
- continuation of choroid (choroid is continuous with ciliary body and iris)
- Contains ciliary muscles (radial & circular) (continuous with iris) and fenestrated capillaries to give nutrients to muscle - Accommodation!
- if radial muscle constricts (get shorter), ciliary bodies are pulled to outside
- if circular muscle (like a sphincter) constricts, the circle gets smaller (also, the fibers loosen up and relax), so ciliary bodies move to inside of eye
- the ciliary processes are connected to the fibers that suspend the lens
Neuroepithelial portion (continuation with retina) - Contributes the two layers of the ciliary epithelium
There is double layer of ep cells that cover ciliary body and iris. Ciliary processes:
- inner layer of epithelium (towards centre of eye) is non-pigmented and faces posterior chamber
- outer layer of epithelium is pigmented and faces stroma of ciliary body
ADDitional info:
‐ Stroma: radial and circular smooth muscles – accommodation, vascular region with fenestrated capillaries
‐ Epithelium (inner: non-pigmented, continuous with neural retina; outer pigmented, continuous with RPE). Secretion of aqueous humour from epithelium of ciliary processes, anchoring of zonula fibres suspending the lens
Iris
Iris is continuation of ciliary body. Contains muscles:
- Dilator papillae - sympathetic activation
- Sphincter/constrictor pupillae – parasympathetic innervations
• Delineates/outlines the anterior and posterior chambers
- anterior and posterior epithelium, (the latter is continuous with retina and RPE, but both
layers pigmented)
• Circular aperture (pupil)
– Pupil opened & closed by smooth muscles
– Pupil is central aperture. Iris colour due to melanocytes. Blue eyes - little pigment.
• Four layers
– Anterior limiting membrane
(incomplete, fenestrated layer)
– Stroma (collagen, nerves, vessels, smooth muscle, melanocytes for eye color)
– Muscle layer (myoepithelial cells)
– Posterior epithelium (two pigmented epithelial layers)
- Sphincter pupillae is under parasympathetic control – Adaption!
- Dilator pupillae is under sympathetic control
- hallucinogens activate SYN
- atropine activates SYN, making pupils widen, to shine light in eye to see retina
Bright = small pupil, vice versa.
- done by smooth muscle (radial and
circular (sphincter) muscle).
- Radial muscle constricts = bigger pupil.
- Circular muscle constricts = smaller pupil
Chambers of the Eye
Eye has a diameter of max. ~2.4 cm
The 3 Chambers are interconnected
• Anterior chamber
– Space between cornea and iris
• Posterior chamber
– Small space between iris and lens
• Vitreous cavity
– Behind the lens
Aqueous humour = watery substance in the posterior and anterior chambers
Vitreous humour = transparent gel in vitreous cavity, shock absorber
Refraction and reflection in eye
If light hits eye, it hits cornea, then anterior chamber, pupil, posterior chamber, lens, vitreous cavity, retina. On its way, the
light is reflected or refracted so smaller inverse picture of environment is put on retina
The main refraction is provided by the cornea
Most of the refraction or reflection is the cornea. Lens can adjust the refraction. Lens is suspended on zonula fibers to ciliary bodies.
- if we look far away, the lens is stretched flat by zonula fibers.
- if we constrict the zonula fibers, the fibers will loosen up and the lens will go to original shape that is round to see closer objects.
- so, lens can adjust refraction depending on how close the object is to eye.
- but cornea does most of the refraction
Pathway of Aqueous Humour
Summary:
Aqueous humour is secreted by ciliary bodies, flows from posterior chamber to anterior chamber, and is drained close to trabecular meshwork into aqueous veins
Details:
• Fluid produced by epithelium of ciliary processes
• Flows from posterior chamber through the pupil to the trabecular meshwork
• Fluid percolates though endothelium of Canal of Schlemm
• Aqueous veins drain the canal
Aqueous humour fills posterior chamber and anterior chamber
Glaucoma
Glaucoma – obstruction of aqueous humor drainage – high intraocular pressure
If someone has high blood pressure, the trabecular meshwork or drainage of aqueous humour is obstructed, which means more aqueous humor is secreted and pressure inside eye will rise.
Rise in pressure is dangerous, leading to blindness, also called glaucoma, also gives
pain and headaches
Microscopic Structures - Cornea
Cornea: 0.5-1.0 mm thick
Transparent, rich in nerve endings, no vessels, wet with tears
Cornea has no blood vessels, is avascular. Cornea can be transplanted
Outer to inner cornea layers:
- Corneal epithelium
- constantly sheds
- contains free nerve endings
- ectodermal, continuous with conjunctive epithelium, proliferates, fast turnover, rich in free nerve endings, but no blood vessels or lymphatics, anchored to Bowman’s membrane by hemidesmosomes - Bowman’s layer (collagen) aka Bowman’s membrane
- Type I collagen fibrils, not elastic,barrier for infections, does not regenerate - Corneal stroma
- contains Schwann cells and fibroblasts (stroma has collagen I and V)
- (around 60 collagen (type I and V) layers, fibroblasts, transparent due to rectangular alignment) - Descemet’s membrane
- does not regenerate
- gives cornea a round shape
- thick posterior basal lamina, formed by corneal endothelium, meshwork, responsible for curvature of cornea - Corneal endothelium
- single cell layer, metabolic exchange, squamous epithelial cells, tightly connected to prevent flow of aqueous humor
Lens
‐ biconvex, avascular, transparent
‐ capsule: thick basal lamina
‐ subcapsular epithelium: layer of cells only at the anterior surface, cells differentiate into
fibres
‐ Lens fibres: derived from epithelium, cells elongate and loose nucleus & organelles as
cells migrate into the centre of the lens
‐ suspended by zonula fibres – keeps lens flat. Release of fibres – lens fattens
(accommodation)
The lens is transparent.
The anterior lens epithelium is formed by flattened cells
When epithelial cells reach the equatorial region of the lens, they being to divide by mitosis and change shape by flattening to become lens fibers. So, at equatorial region where zonula fibers are also attached, the ep cells become lens fibers.
- Cell fibers on the outer layer are called CORTICAL LENS CELL FIBERS and have a cell nucleus
- But moving even further inward, the cells lose the nuclei and are called GHOST Cell nuclei and fibers are called NUCLEAR LENS CELL FIBERS. The cells become ghost
cells bc they lose their organelles, becoming lens fibers.
Zonular fibers extend from the ciliary epithelium to the equatorial region of the capsule and hold the lens in place
The lens capsule is a transparent basement membrane-like structure enclosing the lens substance
As we age, things are deposited in the lens, which makes the lens less transparent. So the lens of old person looks more yellow.
- if you take lens out, it can go from yellow in old person to blue
Cataract
Lens can form cataracts, which are inclusions in the lens which disrupt the light going through the lens.
If cataract is too strong, it is removed and artificial lens is inserted
Sclera
like cornea, but no rectangular orientation of collagen (therefore opaque) thicker, no Bowman’s membrane, vascularized
