Lecture 20-21: The Eye Flashcards Preview

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Flashcards in Lecture 20-21: The Eye Deck (27)
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Describe the anatomy of the 3 layers of the eye:

* Fibrous tunic:
- Sclera (Tunica fibrosa)
- Cornea
* Vascular tunic
- Choroid
- Ciliary body
- Iris
* Inner tunic
- Retina

- Refer to Figure 9-4
- See Slides 4-10


Describe the sclera

2 Layers:
* Episcleral layer:
- Loosely arranged collagen and elastic fibers:
- Highly vascular
- Attaches the lining of the eyelid (conjunctiva) to the sclera
* Sclera proper:
- Made up of interlacing collagen fibers
- Attachment site for tendons of extrinsic eye muscles
- Collagen bundles are always parallel to outer surface of sclera
* Lamina cribrosa:
- Perforated disc of sclera
- Pass through for optic nerve fibers
* Anteriorly the sclera is continuous with the cornea.
* The junction between the two is the limbus; it is highly vascularized.
* Posteriorly the sclera is continuous with the dural covering of the optic nerve.


Describe the cornea layers

* Corneal epithelium:
- Stratified, nonkeratinized squamous epithelium: 5-6 layers
- High mitotic activity in the basal layer: Turnover ≈ 1 week
* Bowman’s membrane:
- Acellular layer separates epithelium from stroma.
- Randomly arranged collagen fibers
* Corneal stroma:
- Thin layers of ordered arrays collagen fibers
* Descemet’s membrane:
- Acellular layer of collagen bundles
- Separates stroma from endothelium
* Corneal endothelium:
- Single layer of large squamous cells


I'm not starting the chegg flashcards yet because of where I am, but you should prepare those at some point today for lectures 20-21.

Do it.


Describe the choroid

* Extends anterior to ora serrata of the retina
* Highly vascularized with loose connective tissue:
- Forms choriocapillary network: Contains small blood vessels that supply cells of the retina
- Provides nutrients to outer layers of the retina
- Consists of medium and large arteries and veins as well as a widebore capillary network
- Contains melanocytes
* Bruch’s (glassy) membrane
- Acellular fused basal laminae of choriocapillaris and pigmented retinal epithelium
See Slide 18


Describe the ciliary body

* Wedge-shaped expansion of choroid peripheral to the lens
* Covered by a double layer of cells
- Innermost layer is continuous with the pigmented layer of the retina.
- Surface layer is from the sensory layer of the retina.
- Produces aqueous humor.
* Suspensory ligaments (of Zinn) extend to just behind lens equator and anchor the lens in place.
* Ciliary muscles
- Smooth muscle mass that changes the shape of the lens
- Innervated by parasympathetic neurons
* Ciliary muscles and accommodation:
- One group of muscles is responsible for flattening the lens
- A second group of muscles is responsible for allowing the lens to become thicker.
* Nonpigmentedsurface layer of cells produces aqueous humor.
- Basal infoldings
- Secrete aqueous humor into posterior chamber
- Aqueous humor:
-- Percolates through trabecular meshwork
-- Flows into canal of Schlemm

* See Slides 20-26


Describe the iris

* The iris is located anterior to the lens and separates the anterior and posterior chambers.
* Angle:
- Formed at lateral borders of the anterior chamber
- Loose connective tissue
- Part of passageway for aqueous humor
* Surrounds pupil
* Double layer of epithelium
- Inner layer = pigmented epithelial cells.
- Outer layer of radially oriented myofilaments:
-- Dilator pupillae muscle.
-- Sympathetic innervation
-- Dilates pupil
- Concentric layer of myofilaments:
-- Constricts the pupil.
-- Parasympathetic innervation
* Melanocytes:
-The more melanocytes, the darker the iris.


Describe the retina

* Two subdivisions divided by the ora serrata:
- Photosensitive region lies posterior to the ora serrata.
- Optic disc:
-- Region on posterior aspect of eye where optic nerve exits
-- Has no photosensitive retina and constitutes the blind spot
- Fovea centralis:
-- About 2.5 mm lateral to optic disc
-- Contains yellow pigment = macula lutea
-- Contains only cone-type photoreceptors
-- Area of most acute vision
* Two subdivisions divided by the ora serrata:
- Nonphotosensitive region lies anterior to the ora serrata.
-- Consists of two cell layers covering iris, ciliary body and ciliary processes
- Photosensitive region lies posterior to the ora serrata.
-- Includes rods and cones


Describe the 3 chambers of the eye, and glaucoma

* Anterior chamber:
- Posterior to cornea and anterior to iris
* Posterior chamber:
- Posterior to iris and anterior to lens
* Vitreal chamber:
- Posterior to lens
- Contains vitreous body
- Gelatinous mass made up of 99% water, hyaluronic acid, and vitrein
- Hyaloid canal is the site of a fetal artery.
* Glaucoma:
- Increased intraocular pressure due to compromised drainage of aqueous humor from anterior chamber


Describe the first 3 layers of the retina

* Pigmented epithelium:
- Adjacent to choroid
- Derived from outer (thinner) layer of the optic cup
* Inner and outer segments of rods and cones:
- Outer segment consists of rhodopsin-containing lamellae
- Inner segment consists of areas of rods and cones possessing mitochondria, RER, Golgi, and glycogen (metabolically active)
* External limiting membrane:
- Area at junction of inner and outer segments of rods and cones and outer nuclear layer.
- Includes adherens junctions between rods and cones and Mueller cells.


Describe Layers 4-7 of the retina

* Outer nuclear layer:
- Cell bodies of rods and cones (1st order neurons)
* Outer plexiform layer:
- Area of synapses between axons of rods and cones and dendrites of bipolar neurons and horizontal cells
* Inner nuclear layer:
- Cell bodies of bipolar cells (2nd order neurons), horizontal cells, amacrine cells, and Mueller cells
* Inner plexiform layer:
- Area of synapses between axons of bipolar cells and dendrites of ganglion cells.


Describe Layers 8-10 of the retina

* Ganglion cell layer:
- Cell bodies of ganglion cells (3rd order neurons)
* Optic nerve layer:
- Axons of ganglion cells
* Internal limiting membrane:
- Terminations of Mueller cell processes and their basement membrane


Describe pigmented epithelial, bipolar, and horizontal cells

* Pigmented epithelial cells
- Adjacent to choroid
- Synthesize melanin
- Apical processes surround and protect outer segments of rods and cones (photosensitive areas)
- Phagocytose and degrade lamellae from rods and cones
- Connected via tight junctions to form blood-retina barrier
- Esterify vitamin A used in formation of photosensitive pigments
* Bipolar cells:
- Conducting neurons that synapse with rods and cones
* Horizontal cells:
- Interneurons that interconnect rods and cones with each other and with bipolar cells


Describe a machine, muller, and ganglion cells

* Amacrine cells:
- Interneurons that connect ganglion cells and bipolar neurons
* Muller cells:
- Neuroglial cells that extend throughout retina
- Form external limiting membrane via zonulae adherens between Muller cells and rods and cones
- Internal limiting membrane is formed by the basement membrane of these cells
* Ganglion cells:
- Conducting neurons whose axons form the fibers of the optic nerve

* See Slides 12-14


Describe the histology of the eye

* Macula lutea:
- Yellow region surrounding fovea
- Highest visual acuity
* Fovea centralis
- Depression of visual axis
- Highest density of cone cells
- Lacks rod cells and capillaries
* Optic disc:
- Lacks photoreceptors
- Point where ganglion cells turn into optic nerve
- = “blind spot” of retina

* See Slides 16-18


What are the two types of photoreceptors?

* Rods: Use rhodopsin as photopigment
* Cones: Use three different kinds of iodopsins as photopigment


What is the general architecture of a photoreceptor?

* Spherule (rod) or pedicle (cone)
* Cell body
- Nucleus and short axon
* Inner segment
- Mitochondria and other organelles
* Outer segment
- Modified cilium
- Connected by a stalk to inner segment
- Site of stacks of disks containing photoreceptors


Describe photoreceptors disks

* In rod cells:
- Disks detach from cell membrane and become free
- Constantly renewed in rod cells
- Older disks are phagocytosed by surrounding pigmented epithelial cells.
* In cone cells:
- Disks remain attached to cell membrane


Compare rods and cones in photoreceptors

* Rods:
- Use rhodopsin as photopigment
- Sensitive to low-light intensity
- Responsible for black-and-white vision
- Lamellae of outer segment are not continuous with plasmalemma
- Axons of up to 100 rods synapse with single bipolar cell
* Cones:
- Use three different kinds of iodopsins as photopigment
- Sensitive to high-intensity light
- Greater visual acuity than rods
- Lamellae of outer segment are continuous with plasmalemma
- Each cone cell synapses with a single bipolar cell

- See Slides 23-25


Describe rhodopsin in photopigments

- Transmembrane glycoprotein
- Consists of opsin and 11-cis-retinal


Describe what happens when a photon strikes 11-cis retinal (vitamin A derivative)

1. Cis-retinal transforms into 11-trans-retinal
2. Rhodopsin becomes activated
3. Activated rhodopsin interacts with transducin (G-protein)
4. Activates cGMP phosphodiesterase
5. Closing of sodium channels
6. Hyperpolarization of the rod cell
7. Reduction in calcium ion influx
8. Decline in calcium concentration activates recoverin
9. Stimulates guanylate cyclase
10. Returns cGMP concentration to normal

- See Slides 27-28


Describe Photopigments

* Bleaching:
- Disassembly of rhodopsin into opsin and retinal after light stimulation
* Regeneration of photopigments:
- 11-trans-retinal is converted back into 11-cis-retinal.
- 11-cis-retinal is transported back to photoreceptor
- Recombines with opsin
- Rhodopsin is regenerated


Describe the histology of the lens

* Capsule:
- Insertion point for suspensory ligament
- Glycoproteins and type IV collagen
- Secreted by subcapsular epithelial cells
* Anterior epithelium:
- Simple cuboidal epithelium
- Give rise to cells that become “lens fibers”
* Lens nucleus:
- Composed of lens cells that mature into fiber-like structures:
- Devoid of nucleus and organelles
- Filled with crystalline proteins

- See Slide 31


Describe the three components of the eyelid:

* Anterior Surface
* Palpebral fascia
* Palpebral conjunctiva


Describe the anterior surface

Anterior surface:
- Covered with skin
- Glands of Zeiss: Sebaceous glands associated with eyelids
- Glands of Moll: Sweat glands; Ducts open into eyelash follicles


Describe the palpebral fascia

* Fibrous core of eyelids
* Meibomian glands (tarsal):
- Sebaceous glands not associated with hair follicles
- Open in front of free edge of eyelid
- Secretion keeps normal tear film in eye


Describe the palpebral conjunctiva

* Stratified columnar or squamous epithelium with goblet cells
* Lines inner surface of eyelids
* Continuous with bulbar conjunctiva (covers eyeball)

- See Slide 33-34