The Eye
Inner nuclear layer
(bipolar layer) Region where cell bodies of bipolar, horizontal, and amacrine cells reside
Iris layers 1 and 2
1. Anterior limiting layer is a discontinuous layer of stromal cells, consisting of stellate shaped fibroblasts and melanocytes 2. Stroma: vascularized loose CT containing melanocytes and fibroblasts
Name the 4 refractive elements of the eye in the order that light passes through.
1. Cornea 2. Aqueous Humor - fluid space 3. Lens 4. Vitreous Humor - gelatinous to help keep eye shape (important for focal length)
Five layers of cornea.
1. Epithelium 2. Bowmans membrane 3. Lamina Propria 4. Descemets membrane 5. Corneal Endothelium
The Tunics (Coatings) of the Eye
1. Tunica fibrosa: fibrous outer coat ∴ Cornea => Sclera 2. Unveal or Vascular middle coat ∴ Iris=> Ciliary body => Choroid layer 3. Retinal coat inner layer ∴ Pigment epithelium and Neural retina ∴ Remains connected to CNS via the optic nerve
2nd and 3rd Corneal layers
2. Bowman's membrane is a thin acellular layer of randomly arranged collagen. Serves as bacterial invasion barrier 3. The Lamina Propia is the thickest layer consisting of regular arranged lamellae collagen.
Iris layers 3 and 4
3. Muscular layer: Two band of smooth muscle: ∴ Spincter pupillare (distince circular band at margin of pupil) ∴ Dialator pupillae (thin shee of radially oriented SM near posterior border of Iris) 4. Posterior Epithelium: double layer of cuboidal pigmented cells
4th and 5th corneal layers
4. Descemet's membrane is a thick basement membrane of the final endothelial layer 5. Corneal endothelium is simple squamous inner lining of the cornea. ∴ actively transport water out of the stroma to maintain corneal clarity. ∴ linked by desmosomes and occluding junction. ∴ responsible for secreting Descement's membrane which they are bound by hemi-desmosomes.
Anterior lens surface
Acellular elastic capsule collagen associated with the basement membrane of simple cuboidal epithelium which lies just deep to the capsule. Lens is avascular, composed entirely of modified epithelial cells (lens fibers) Epithelial cells form a GERMINAL ZONE at equatorial rim of biconvex surfaces and divide slowly throughout life.
How is the size of the pupil aperture Increased?
Bt contraction of the myoepithelial pupillary dilator muscle (looks like wheel spoke) which is under SNS control.
How is the size of the pupil aperture Reduced?
By contraction of *sphincter papillae*, around margin of the pupil under PNS control
During accommodation for Distance Vision
Ciliary body is relaxed and the lens and Zonules of Zinn are stretched by the natural tension of the choroidal attachement.
Optical disk (or papilla)
Circular region near the central portion of the retina where the axons of retinal ganglion cells collect to form optic nerve Here RGC axons pierce the parenchyma to leave eyeball Causes BLIND SPOT as there are no photo receptors in this region.
Ganglion cell layer
Contain cell bodies of the retinal ganglion cells - output neurons
Outer plexiform layer
Contain: synaptic processes of the photoreceptors. Devoid of cell bodies
Describe the inner segment of the photoreceptor cells
Contains most of the intracellular oganelles (no nucleus), K channels
External limiting membrane
Contains muller cells (glial support cells) Not really a membrane, composed of zonula adherens junctions between cylindrical cytoplasmic processes of Muller cells and photoreceptors.
When eyes are at rest
Default is distance vision: The elastic choroid pulls the ciliary body backward, where the eyeball has a large diamete, pulling outward and back on Zonules of Zin For near vision, ciliary muscle contract and stretch choroid to pull zonular attachments forward to a region of small diameter.
Choroid sub layer notes (don't need to memorize names)
Has loose connective tissue Numerous malanocytes that serve to absorb scattered light so it doesn't bounce back if not hitting retina. Glassy membrane serves as another layer. contains choriocapillaris
Retina as first visual processing center
If each axon carried the ouput of each photoreceptor into the brain, optic nerve would need to be more than one inch in diameter.
Ora serrata
Juncture where the overlying neural retina cover on the back of the eyeball ends. No need when light can't reach it at any angle
Muller cells
Large glia cells that extend throughout all 10 layers of the Retina. Their apical cytoplasmic processes wrap around the photoreceptor serving to insulate them from one another. Their nuclei are found in the inner nuclear layer.
Vitreous Chamber
Large space behind lens with VH Adherent to peripheral retina and ciliary epithelium
The surface ectoderm develops into the external eye structures
Lens eyelids conjuntiva outer epithelial layer of the cornea
Lens and its suspension
Lens is Translucent and sitting suspended behind the iris by the Zonules of Zinn, which are suspensory ligaments allowing for focusing ability
Describe basic pathway of light
Light enters the anterior aspect of the eyeball Passes through the refractive elements of the eye At the posterior aspect of the eye, light falls upon a specialized photo-sensitive neuroepithelium = RETINA
Counterintuitive aspect of retinal organization
Light must pass through almost all of the layers BEFORE it reaches the photo receptors and the photoreceptors are oriented so that they point away from the incoming light.
Canal of Schlem
Lined with simple squamous epithelia from the canal, aqueous veins carry the fluid to the conjunctiva to be released into venous blood.
Eye development: Lens development
Optic cup grows outward to contact surface epithelium of embryo This point of contact buds off a small mass of tissue which form the lens vesicle from which the lens develops
During accommodation for Near Vision
PNS causes ciliary muscles to contract in a manner that releases tension on zonules of zin, allowing the lens to increase its focusing power by increasing its curvature.
Pathway of visual information
Photoreceptors --> bipolar cells --> ganglion cells Horizontal and amacrine cells mediate lateral interaction across the surface of the retina (intraneural net reduces needed size of our eyes) A single ganglion cell connected to many photoreceptors. Encode whether contour, edge, color, brightness, and position in space
Photoreceptors polarization
Photoreceptors are HYPERPOLARIZED by light and chronically release neurotransmitter. Light brighter than the environment causes them to reduce the rate of transmitter release, where as darker cause an increase
Outer nuclear layer
Region where nuclei of photoreceptor cells reside
Inner Plexiform layer
Relatively clear layer where the processes of bipolar, amacrine, and ganglion cells interact synaptically to process visual information
10 retinal layers
Retinal Pigment epithelium Photoreceptor layer External Limiting membrane Outer nuclear layer Outer plexiform layer Inner nuclear layer Inner plexiform layer Ganglion cell layer Retinal Axon layer Internal limiting membrane
Retinal cells and staining
Retinal cells types are not clearly distinguishable in standard H&E prep except that in some instances their nuclei reside in specific layers.
Photoreceptor cells: Inner fiber
Slender process that extends and forms synaptic contact with the underlying bipolar cells. Spherule- rods, Pedicle- cones
What is the 1st Corneal Layer
The Epithelium is the outer most layer of stratified squamous non-keratinizing epithelium connected with intercellular bridges. *Has PAIN sensitive nerve fibers (blinking)* The superficial epithelial cells are flat squamous cells connected by desmosomes and have microvilli on apical surface to maintain tear film The basal cells of the corneal epithelium are capable of rapid division and replace damaged corneal tissue. TO time is 7-10 days.
What embryonic layer forms the supporting and vascular coat of the eye?
The mesoderm - closes over the posterior aspect of the optic cup and forms the supporting and vascular coat of the eye
Blockage of aqueous drainage can result in....
increased intraocular pressure (glaucoma) resulting in decreased blood flow and ischemia of the retina (blindness)
Photoreceptor cells: Cell body
the cell body proper of the photoreceptors is filled almost entirely with the nucleus
Important functions of ciliary body: Aqueous Humor
• Aqueous humor is filtered out of BV into the posterior chamber by the capillaries of the ciliary process (chief source of nutrients for the avascular lens and cornea) • The aqueous humor is transported out of the interior of the ciliary body by the pigment epithelial cells of the ciliary body whose basement membrane provide a "blood-aqueous barrier" • The aqueous humor flows through the posterior chamber into the anterior chamber • Drained from the anterior chamber by passing through network of fibrous channels in the lateral aspect of the ciliary body termed the Microtrebecular Meshwork • From the sieve-like meshwork the aqueous humor empties into the annular Canal of Schlemm located near the attachment of the ciliary body to the sclera
Lens Fibers and crytallins
• Lens Fibers: Lens cells lose their nucleus and intracellular organelles and elongate into transparent structures that persis through life and not replaced • The cytoplasm of these lens fibers is packed with transparent proteins called crystalline • The lateral borders of these fibers are connected by knob and socket like depressions (tight and gap junctions)
Ciliary body
• Thickening of the Choroid which forms a ring around the eye on the inner aspect of the sclera just behind the iris •Loose CT and SM covered by a double cuboidal epithelium consisting of a superficial non-pigmented layer and a deep pigmented layer •Two apical surfaces of these two cell layers face one another
Photopigment
• Vitamin A derivative • Composed of retinene and opsin • Rods have rhodopsin (dim light) and cones have photopsin (bright light) • Cones are sensitive to RED, GREEN, or BLUE
Photoreceptor distribution
• cones predominate in central retina • rods predominate in peripheral retina • cones are most dense in fovea centralis since it is devoid of blood vessels and the retinal layer is thinner-->light can gain easier access to cones in this area
Lens focusing power with aging
• with age, lens fibers harden and lens losses ability to change shape causing PRESBYOPIA • Lens also becomes semi-opaque: cataracts
Ciliary process
•The inner edge of the ciliary body terminates in a series of slender ridges-CILIARY PROCESSES •These are attached to the suspensory ligaments of the lens via a series of slender fibrillar processes=Zonules of Zinn **when ciliary contract you change tension of zonules of zinn (relax) NOT PULL •The outer edge of the ciliary body is anchored to the choroid layer from which it emerges •ciliary muscles attach to choroid anteriorly
Cornea
∴ Anterior 1/6 of tunica fibrous ∴ first optical element ∴ continuous with sclera at the LIMBUS, but its greater curvature allows for *2/3 of eye focusing power* ∴ Relatively avascular, recieving nutritent from aqueous humor. ∴ Low hydration level (for clarity)
3 chambers of the eye
∴ Anterior: Cornea to Iris ∴ Posterior: lens, iris, and ciliary body ∴ Vitreous chamber: bound by lens and posterior wall, filled with gelatinous vitreous body Anterior and Posterior layers contain aqueous humor which is produced by the epithelium of the ciliary body
Photoreceptor cells: outer segment
∴ Composed of dense vertical stacks of membrane bound disks from repeated infolding of the apical surface of the cell membrane ∴ This is the light-sensitive region of receptors. ∴ Constantly being turned over with tends sloughing off to be phagocytosed by pigment epithelial cells ∴ Disks contain a vitamin A derivative photopigment
Iris: Layers
∴ Disk shaped diaphragm that sits between the anterior and posterior chambers of the Eye ∴ Finger like projections that control how much light comes in through pupil (gap in iris) ∴ Contains 4 Layers: 1. Anterior limiting layer 2. Stroma 3. Muscular 4. Posterior epithelium
Eye color
∴ Epithelial cells are heavily pigmented to prevent light from entering the eye, except through the pupil and reduce scatter. ∴ Melanin in the STROMA determines eye color ∴ Reduced melanin = blue ∴ Increased melanin = brown
Diabetic retinopathy
∴ Leading cause of blindness in USA ∴ Uncontrolled plasma glucose in tunics of the eye causes vascular edema and altered retinal and choroidal vasculature ∴ Scar tissue-induced retinal detachment Edema and vascular leakage proliferative revascularization
The walls of the eye are composed of what 3 layers?
∴ Outer (supporting) cornea--sclera ∴ Middle (VASCULAR) Iris - ciliary - choroid ∴ Inner: Retinal divided into Retinal pigment epithelium (nutrients) and neural retina (photoreactive)
Cells of the neural retina
∴ Photo receptors: rods and cones ∴ Bipolar cells (transmit impulse from photorecptors to ganglion) ∴ Horizontal cells (interconnect photorecepters laterally) ∴ Amacrine cells (distribute impulses from bipolar cells laterally to retinal ganglion cells) ∴ Ganglion cells: axons exit retina and form optic nerve
Photoreceptor layer
∴ Populated by cells specialized for the transduction of light energy into neural impulses. ∴ Rods and cones ∴ Have 6 parts: Outer segment, cilium, innersegment, outer fiber, cell body, and inner fiber
Neural activity in the retina
∴ Principally graded hyperpolarization and depolarization in retinal neurons (counter intuitive) ∴ Only ganglion cells reliably sustain nerve impulses ∴ Synaptic circuits of retina encode visual info for transmission to brain taking the form of varying frequencies of APs in ganglion cell axons.
Blood/retina barrier, light scattering, and basement membrane
∴ Provided by epithelial basement membrane with tight junctions ∴ Cell contains MELANIN granules that prevent light from being scattered (along with choroid pigment) ∴ No firm attachement between RPE and underlying photoreceptor layer, allowing for separation with trauma (detached retina).
Function of ciliary body: lens
∴ Regulated shape of lens via ciliary muscles to accommodate the eye for close and distant vision ∴ Muscles controlled by PNS ∴ counter intuitive working: Contraction of ciliary muscles decreases tension on the attached zonules of Zinn and a rounding of the lens for close vision.
Blood supply to retina
∴ Retinal artery and vein enter eyeball at optic disk ∴ Vessels bifurcate at the disk to supply upper and lower portion of the inner 2/3 of retina, not supplied by choroid. ∴ retinal capillaries from these vessels ramify in the ganglion and fiber layers
Retinal pigment epithelium
∴ Single layer of cuboidal-columnar cells, but has no free surface under retina (apical side face retina for nutrients) ∴ Base of cells attached to Bruch's membrane of choroid ∴ Apical surface have microvilli and cylindrical cytoplasmic sheaths that enclose ends of retinal photoreceptors and nourish out segments of rods and cones and phagocytose pieces of outer segments
Choriocapillaris
∴ Single layer of wide fenestrated capillaries that nourish the surrounding tissues including the outer 1/3 of the adjacent retinal coat ∴ With advancing age, innerocular pressure can squash the choriocapillaries, which keeps nutrients from getting to photoreceptors (then THEY DIE!!!) - Photoreceptors not replaced
Fovea
∴ Small depression in the retina where visual acuity is best. ∴ Ganglion cells particularly dense here ∴ minimum diffusion of light ∴ This is what takes on that postmortem yellow spot (Maculae Lutea)
Eye Develpoment: Optic cup formation
∴ The anterolateral outpocketing of the primitive neural tube in the 3rd week of gestation called the optic vesicle at the level of the future diencephalon. ∴ The optic vesicle is attached to the neural tube via optic stock. ∴ The optic vesicle invaginates causing the anterior and posterior walls to come together to form the optic cup.
Sclera
∴ Tough white fibrous tissue covering the *posterior 5/6* of the tunica fibrous ∴ Forms the capsule for the back part of the eye ball Has 3 layers
Uveal coat: choroid layer
∴ Vascular layer of the eye ∴ just internal to sclera, extend posteriorly from the ciliary body across the entire posterior aspect of the eye ∴ 4 layers of connective tissue ∴ Fenestrated capillaries allow tissue fluid to circulate freely in the CT
Eye development: The posterior wall becomes
∴ pigment epithelium of the retina ∴ ciliary body ∴ iris
