Histology Lecture Q- Special Senses

What is the size of the eye (diameter)?

25mm

What suspends the eye?

six extrinsic muscles

What are the layers of the eye?

1. Fibrous tunic (corneoscleral)

2. Vascular tunic (uvea)

3. Retina

What is contained in the fibrous tunic?

Opaque, white sclera (posterior 5/6) and transparent cornea (anterior 1/6)

What is in the vascular tunic?

iris, choroid, ciliary body

What is the lens used for?

accommodation (focusing)

What is in the retina?

10 layered structure including rods and cones

Describe characteristics of rods?

low intensity, black and white

Describe characteristics of cones?

color: red (erythrolabi), green (chlorolabi), blue (cyanolabi)

Anterior chamber

between corneal endothelium and lens, iris, etc.

What are the chambers of the eye?

1. anterior chamber

2. posterior chamber

3. vitreous chamber

What is the posterior chamber?

space between the iris and lens and zonular fibers

What fills the anterior and posterior chamber?

aqueous humor

Where is the aqueous humor from the chambers drained?

canal of schlemm

What is the vitreous chamber?

clear, transparent gel filling the bulk of the eye that is 99% water with hyaluronic acid and collagen fibrils

What is the corneoscleral coat?

aka fibrous coat; provides tough, fibroelastic support for the eye

What is the cornea?

clear, transparent anterior 1/6 with smooth, curved anterior surface; refractive power greater than the lens

What are the two parts of the cornea?

1. cornea proper (avascular)

2. limbus

What is the size of the limbus and what is found there?

1 mm wide peripheral region with blood vessels and lymphatics

How does light affect the cornea proper?

It is protected from UV light due to the cells possessing nuclear ferritin, an iron-storage protein that prevents free radical damage

What is the corneal epithelium?

stratified squamous (moist)

Describe the layers of the corneal epithelium?

1. 5-6 cell layers (basal, low columnar cells)

2. 3-4 layers of “wing” cells

3. 1-2 layers of squamosal cells

What are two main characteristics of the corneal epithelium?

• sensitive

• regenerative

Bowman’s Membrane (anterior basement membrane) size?

8-10um

What is bowman’s membrane?

thick acellular layer of collagen (18nm diameter and randomly oriented)

What is another name for the substantia propria?

corneal stroma

What is the substantia propria?

60 thin lamellae of parallel collagen fibrils separated by slender, flattened fibroblasts

What accounts for 90% of corneal thickness?

substantia propria (corneal stroma)

What collagen type is in the substantia propria?

type V

How is collagen arranged in the corneal stroma?

in plates arranged at different angles so that the cornea remains transparent

In addition to collagen type v, what else is in the corneal stroma?

stellate, flat fibroblasts

What do the fibroblasts in the corneal stroma do?

secrete corneal proteoglycans (lumican) which is keratan sulfate and chondroitin sulfate linked to the protein decorin

What is decemet’s membrane composed of?

small regularly arranged fibrils and pectinate ligaments

What is the size of decemet’s membrane?

5-10um thick

What is the role of pectinate ligaments in decemet’s membrane?

extend under the sclera to assist in keeping the normal curvature of the cornea

What is the endothelium of the cornea?

single layer of cuboid cells on internal surface

What is the role of the endothelium of the cornea?

transports fluids from the anterior chamber of the stroma (diffusion of aqueous humor)

What is the sclera?

posterior 5/6; dense fibrous CT

What is the size of the sclera?

0.8-1.0 mm thick collagen fibers parallel to the surface

What is found in the sclera?

collagen fibers, elastic fibers, ground substance and flattened, branched fibroblasts

What is episcleral tissue?

loose Ct with skeletal muscle and periorbital fat

What is the substantia propria (sclera proper)?

dense network of collagen fibers

What is the suprachoroid lamina (lamina fusca)?

inner aspect of the sclera next to the choroid

What is found in the lamina fusca?

thinner collagen fibers, elastic fibers, fibroblasts, melanocytes, macrophages, and other CT cells

What is the lamina cribrosa?

where sclera is pierced by optic nerve fibers

What is the limbus?

well vascularized peripheral region where Bowman’s membrane ends

What happen when Decemet’s membrane splits?

a transition between the cornea and sclera

What is the canal of schlemm?

venous scleral sinus for outflow of aqueous humor

What is the uveal (vascular coat)?

choroid, ciliary body, and iris (with blood vessels and pigment cells)

What is the iris?

pigmented, muscular, anterior part of the vascular coat with the centrally-located pupil

What are the muscles in the iris?

sphincter (parasympathetic) and dilator (sympathetic) muscles that control the size of the pupil.

What are the iris sphincters made of?

pigmented and nonpigmented myoepithelial cells as well as typical smooth muscle cells

Describe the anterior and posterior surface of the iris?

Anterior: irregular with no cellular membrane

Posterior: surface is pigmented

What is the size of the ciliary body?

extends 6mm from the root of the iris to the ora serrata

What are the two ciliary stroma layers?

1. the outer smooth (ciliary) muscle fiber layer

2. inner vascular region

Where do you find the ciliary body?

Encircles the eye anterior to the ora serrata

What are features of ciliary bodies?

• shallow grooves called ciliary striae

• ~75 deeper ciliary ridges or grooves called ciliary processes

• ciliary stroma

What type of fibers are found in the ciliary body?

elastic fibers

What are the three functional groups of ciliary muscles?

• meridional/ longitudinal

• radial/ oblique

• circular/sphincteral

What attaches the lens to the ciliary processes and plays a role in accommodation?

zonular fibers

What is the choroid?

spongy, brown layer with extensive venous plexuses

What is the size of the venous plexus of the choroid?

0.1 to 0.3 mm thick

What is the external layer of the choroid?

Suprachoroid lamina with the epichoroid lymph space

What is found in the epichoroid lymph space?

loosely arranged collagen and elastic fibrils, fibroblasts, macrophages, lymphocytes, plasma cells and mast cells

Where is the bulk of the choroid tissue?

Vessel layer

What is in the vessel layer?

arteries, veins in loose CT with numerous melanocytes

What is internal to the vessel layer?

capillary beds called choriocapillaris with fenestrated endothelium

What structure achieves light absorption?

choroid

Bruch’s membrane is what?

thin, amorphous, refractile layer

Where is Bruch’s membrane located?

between the choriocapillaris and pigmented layer of the retina

What is the size of the Bruch’s membrane?

1um to 4um thick

What are the five layers of Bruch’s membrane?

1. the basal lamina of endothelial cells

2. .5um thick layer of collagen

3. a 2um thick layer of elastic fibers

4. a second collagen fiber layer

5. the retinal epithelial cell basement membranes

What are the 10 layers of the retina (nervous) coat?

1. pigmented epithelium

2. receptor layer

3. external limiting membrane

4. outer nuclear layer

5. outer plexiform layer

6. inner nuclear layer

7. inner plexiform layer

8. ganglion cell layer

9. optic nerve fiber layer

10. internal limiting membrane

What does the pigmented epithelium do?

absorbs light and provide nutrients to receptors

What is in the outer nuclear layer?

rods and cones

What is in the inner nuclear layer?

bipolar cell

What are photoreceptors?

rods and cones

What are rods?

slender cells used for dark vision (B/W)

What are cones?

tapered outer segments used for color perception

What are the three components of the photoreceptors?

1. the outer segment

2. connecting stalk

3. inner segment

Describe the outer segment of the photoreceptors?

intimately related to the microvilli of the pigmented epithelial cells

Describe the connecting stalk of the photoreceptors?

contains cilium with nine doublets of microtubules extending from a basal body

What is the inner segment of the photoreceptors divided into?

outer ellipsoid and inner myoid portion

What are examples of retinal cells?

• bipolar cells

• ganglion cells

• horizontal cells

• amacrine cells

• neuroglial cells

• Muller’s cells

What are the bipolar cells in the retina?

1. diffuse bipolars that go to several photoreceptors and midget

2. (monosynaptic) bipolars to a single cone cell

What are the ganglion cells in the retina?

1. with diffuse cells to several bipolar neurons and midget

2. (monosynaptic) cells toa single, midget bipolar cell

What are the horizontal cells in the retina?

in outer part of inner nuclear layer; associational neurons

What are amacrine cells in the retina?

in inner part of inner nuclear layer; associational neurons

What are neuroglial cells?

for support, insulation, and nutrition

What are Muller’s cells?

packing cells (very large) forming the external and internal limiting membrane

What is the fovea centralis?

high concentration of photoreceptors for accurate vision

What is the optic disc?

where optic nerve exits the retina and with no photoreceptors and it represents the blind spot

What are accessory structures of the eye?

eyelids, lacrimal glands, and extraocular muscles

Describe the development of the ear?

1. develops from surface ectoderm as otic placodes

2. invaginating to form otic pits

3. eventually pinch off from the surface as otic vesicles in the superior region of the pharynx

Three parts of the ear?

1. external ear

2. middle ear

3. inner ear

What structures make up the external ear?

auricle or pinna and the external auditory meatus

What is the middle ear?

air filled chamber in the temporal bone

What separates the middle ear and external ear?

tympanic membrane (ear drum)

What separates the middle ear from the inner ear?

two small membranes covering an oval window ( finestra vestibuli) and a round window (finestra cochlea)

What is the middle ear connected to?

nasopharynx