Cui Dongmei. Atlas of Histology: with functional and clinical correlations. 1st ed

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CHAPTER 20

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Eye

395

Eyelash

follicles

Gland of Moll

Eyelash

follicles

Eyelashes

Gland

of Zeis

Gland of Moll

Tarsal

plate

Palpebral

conjuctiva

Meibomian

(tarsal)

gland

Gland

of Zeis

Figure 20-4A. Upper eyelid (lower part), glands of the

eyelid. H&E, 68

Eyelids (palpebrae) consist of upper and lower eyelids. The

structural components of the upper eyelid are similar to

those of the lower eyelid, although the upper eyelid is more

mobile. Eyelashes and their follicles are visible at the margin

of the eyelid. The tarsal glands, also called meibomian

glands, are large sebaceous glands embedded in the tarsal

plate. The glands associated with eyelashes are (1) glands

of Moll and (2) glands of Zeis. The glands of Moll are

modiﬁ ed sweat glands near the base of the eyelash. They

have unbranched tubules, which begin in a simple spiral

rather than coiling in a glomerular shape, as do ordinary

sweat glands (see Fig. 20-3B). The glands of Zeis are small,

modiﬁ ed sebaceous glands that are sometimes called ciliary

glands. They are close to the eyelash follicles and empty

their secretions into the follicles.

Meibomian

(tarsal)

gland

Hair

follicles

Skin

Palpebral

conjuctiva

Orbicularis

oculi muscle

Orbicularis

oculi muscle

Duct of

tarsal

plate

Figure 20-4B. Upper eyelid (middle part). H&E, 68

The outer layer of the eyelid is covered by thin skin (see Fig.

3-13 and Chapter 13 “Integumentary system”), a keratinized

stratiﬁ ed squamous epithelium, over a loose elastic con-

nective tissue layer. The skin contains hair follicles, which

are much smaller than eyelash follicles (found only in the

lid margin; see Fig. 20-3A). Orbicularis oculi muscle ﬁ bers

are located beneath the skin. The inner surface of the lid is

a layer of palpebral conjunctiva, covered by stratiﬁ ed low

columnar epithelium, which is in contact with the eyeball.

The tarsal (meibomian) glands, embedded in the tarsal plate,

lie between the orbicularis muscles and palpebral conjunc-

tiva. Each gland has a single duct that opens at the lid mar-

gin. Their lipid secretion creates a surface on the tear ﬁ lm

that prevents lacrimal ﬂ uids (tears) from evaporating from

the surface of the eyeball. This secretion also lubricates the

cornea and edges of the eyelids.

Orbicularis

oculi muscle

(skeletal muscle)

Meibomian

(tarsal)

gland

Superior

tarsal

muscle

Orbicularis

oculi muscle

Orbicularis

oculi muscle

(skeletal muscle)

Tendon of the

levator

palpebrae

muscle

Figure 20-4C. Upper eyelid (upper part), muscular control

of upper eyelid movement. H&E, 68; inset 272

Three types of muscles control upper eyelid movement.

(1) The orbicularis oculi muscle is a sheet of striated muscle

that is oriented in a circle around the eye. It is innervated by

the facial nerve (CN VII) and is responsible for closing the

eyelids. (2) The levator palpebrae superioris muscle is a band

of striated muscle that originates in the orbit, passes forward,

and inserts into the upper eyelid. This muscle is innervated

by the oculomotor nerve (CN III) and functions to open the

eyelid and hold it up. The levator palpebrae muscle is not vis-

ible here, but its tendon is clearly seen. (3) The superior tarsal

muscle, also called the Müller muscle, is a smooth muscle

that inserts on the superior tarsal plate. It is innervated by

sympathetic nerve ﬁ bers from the superior cervical ganglion

and works with the levator palpebrae superior muscle to

raise the upper eyelid. Damage to the levator palpebrae supe-

rior muscle, the superior tarsal muscle, or their innervation

can cause ptosis (drooping of the eyelid).

CUI_Chap20.indd 395 6/16/2010 7:37:56 PM

396

UNIT 3

■

Organ Systems

Tunica Fibrosa (Tunica Externa)

Pavement

epithelium

Posterior

Stroma

Corneal

endothelium

Corneal

endothelium

Descemet

membrane

Pavement epithelium

(anterior corneal epithelium)

Pavement

epithelium

Bowman

membrane

Bowman

membrane

Descemet

membrane

Figure 20-5A. Cornea. H&E, 77.5; insets 173

The cornea is a transparent and avascular tissue,

which is composed of ﬁ ve layers: three cellular

layers (epithelial layers and stroma) and two non-

cellular layers (Bowman membrane and Descemet

membrane). These include (1) pavement epithe-

lium (anterior corneal epithelium); (2) the Bowman

membrane (basement membrane of the pavement

epithelium); (3) stroma, consisting of ﬁ broblasts (also

called keratocytes in the cornea) and alternating lamel-

lae of collagen ﬁ bers; (4) the Descemet membrane

(basement membrane of the posterior corneal epithe-

lium (endothelium); and (5) the corneal endothelium

(posterior corneal epithelium). The cornea has a rich

nerve supply from CN V. The superﬁ cial corneal

layer contains numerous sensory nerve ﬁ bers, and

irritation can cause severe eye pain. CN V also car-

ries the afferent limb of the corneal reﬂ ex.

Descemet membrane

(noncellular layer)

Stroma (cellular layer)

Endothelium

(cellular layer)

Pavement epithelium

(cellular layer)

Bowman membrane

(noncellular layer)

D. Cui

Figure 20-5B. A representation of the cornea.

Pavement epithelium is formed by stratiﬁ ed squamous

epithelium consisting of four to six layers and is about 50 mm in

thickness; it has a high capacity for regeneration. The transpar-

ency of the cornea is due to its avascularity and due to its state

of relative dehydration. Corneal endothelium is a single layer of

squamous cells. This layer functions in the regulation of water

in the stroma. The primary task of corneal endothelium is to

pump the excess ﬂ uid out of the stroma and, therefore, is criti-

cal for keeping the cornea clear. If this endothelium is damaged,

the result may be corneal swelling due to ﬂ uid retention within

the stroma and loss of its transparency.

CLINICAL CORRELATION

Figure 20-5C.

LASEK.

Laser refractive surgery reshapes the cornea in order to focus

images more accurately onto the retina. The most common

procedures include laser in-situ keratomileusis (LASIK), and

laser

-assisted epithelial keratomileusis (LASEK). Laser tech-

niques have been continuously modiﬁ ed to reduce complica-

tions and enhance surgical outcomes. Unlike with LASIK,

the most recent LASEK surgery saves the epithelium by using

an alcohol solution to weaken epithelial cell adhesion so the

epithelial layer can be lifted as a ﬂ ap. After the epithelial

ﬂ ap is moved out of the way, excimer laser energy is applied

through the Bowman membrane layer and into the upper

stroma to reshape the cornea. The epithelial ﬂ ap is then

returned to its original position. The advantages of LASEK

are a reduction of postoperative discomfort, a decreased risk

of infection, and an increase in the overall thickness of the

untouched area of the cornea.

Cornea flap

Laser beam

Exposed Bowman membrane

D. Cui

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397

Figure 20-6A. Pavement epithe-

lium (anterior epithelium). EM, ×6,000

The anterior corneal epithelium is a

stratiﬁ ed squamous epithelium that

covers the external surface of the

cornea. The lack of keratinization

and the uniform thickness of the

epithelium contribute to the essen-

tial transparency of the cornea. The

epithelial cells have a fairly fast

turnover rate of about one week.

The surface cells are not perfectly

smooth, but rather have small

microvilli (only a few are preserved

here) that serve to anchor the vitally

important tear ﬁ lm. Not shown in

this ﬁ eld are free nociceptive nerve

endings that penetrate the epithe-

lium and provide the afferent limb

of the corneal blink reﬂ ex. Bowman

membrane is the commonly used

term for the thick basement mem-

brane at the interface between the

anterior corneal epithelium and the

corneal stroma.

Corneal

stroma

Bowman

membrane

Collagen

fibers

Basal

epithelial cells

Nuclei of the

epithelial cells

Nuclei of the

epithelial cells

Basal

epithelial cells

Collagen

fibers

Bowman

membrane

Corneal

stroma

Anterior (Fig. 20-6A)

Posterior (Fig. 20-6B)

Anterior (Fig. 20-6A)

Posterior (Fig. 20-6B)

Figure 20-6B. Corneal endothe-

lium (posterior corneal epithelium).

EM, 4,500

The posterior corneal epithelium is

sometimes called the corneal endothe-

lium. It is a simple squamous epithe-

lium that faces the aqueous humor

of the anterior chamber. Descemet

membrane is the commonly used

term for the sharply deﬁ ned base-

ment membrane of the posterior

epithelium. Descemet membrane is

unusual in that it consists largely of

an orderly network of type VIII col-

lagen, a relatively rare collagen type.

The epithelial cells are joined by

tight junctions, and they control the

movement of water, ions, and metab-

olites between the stroma and the

aqueous humor, the source of nutri-

tion for the corneal stroma. Orderly

layers of type I collagen ﬁ brils form

the bulk of the corneal stroma. The

extremely ﬂ attened ﬁ broblasts that

produce and maintain the stroma

are called keratocytes.

Corneal stroma

Nuclei of

endothelial cells

Keratocytes

(fibroblasts)

Descemet

membrane

Collagen

fibers

Keratocytes

(fibroblasts)

Nuclei of

endothelial cells

Collagen

fibers

Descemet

membrane

Corneal stroma

CUI_Chap20.indd 397 6/16/2010 7:38:02 PM

398

UNIT 3

■

Organ Systems

Refractive Media of the Eye

Posterior

Ciliary

body

Cornea

Lens

Iris

Lens cortex

ens

cortex

Lens

nucleus

Anterior chamber

Posterior chamber

Fig. 20-7C

Fig. 20-7B

Fig. 20-7D

Lens cortex

Lens

nucleus

Figure 20-7A. Overview of the lens. H&E, 11

The lens is a biconvex, avascular, ﬂ exible, and colorless transparent

structure. Anterior to the lens is aqueous humor and posterior to it is

the vitreous body. The lens is composed of the lens capsule, subcap-

sular epithelium, and lens ﬁ bers. The peripheral region of each lens

ﬁ ber contains a nucleus and organelles, which form the bulk of the

lens called the lens cortex. In the central part of the lens, the ﬁ bers

lose their nuclei and organelles; this region is ﬁ lled with crystalline

proteins and is called the lens nucleus. The lens is held in place by

zonular ﬁ bers (position indicated by dashed lines; see Figs. 20-8A

and 20-11C). The anterior chamber is the space between the cornea

and the iris. The posterior chamber is a narrow space between the

iris and the posterior zonular ﬁ bers of the lens. These chambers are

ﬁ lled with aqueous humor and communicate through the pupil.

Figure 20-7B. The anterior region of the lens. H&E, 272; inset

680

The anterior lens is covered by a thickened lens capsule (see Figs.

20-8B and 20-10A), a transparent basement membrane that envel-

ops the entire lens. Beneath it is a single layer of ﬂ at, squamous cells

(subcapsular epithelium).

Figure 20-7C. The posterior region of the lens. H&E, 272;

inset 680

The posterior lens is covered by a thin lens capsule (see Fig. 20-8B).

No subcapsular epithelium lies beneath the capsule in this region.

The posterior lens surface is in contact with the vitreous body, a

transparent gel, which contains water, collagen, and hyaluronic acid

and ﬁ lls the interior of the globe posterior to the lens.

Figure 20-7D. The equatorial region of the lens. H&E, 272;

inset 544

The cells of the subcapsular epithelium in the equatorial region of

the lens are increased in height, and most cells are cuboidal in shape.

The sizes of the lens ﬁ bers and their nuclei are increased in this

region. The equatorial surface of the capsule is connected to the

zonular ﬁ bers, which hold the lens in place (Fig. 20-8A).

Posterior lens surface

Posterior lens

suface

Lens cortex

Posterior

Posterior lens

suface

Lens cortex

Lens capsule

Subcapsular

epithelium

(cuboidal cells)

Subcapsular

epithelium

Lens fibers

Nuclei of

peripheral

lens fibers

Lens capsule

rte

Subcapsular epithelium

(anterior lens epithelium)

lens epithelium

Lens cortex

lens epithelium

CUI_Chap20.indd 398 6/16/2010 7:38:04 PM

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Eye

399

Figure 20-8A. Lens and zonular ﬁ bers. H&E, 34

The zonular ﬁ bers are also called the zonules of Zinn. The

combination of all the zonular ﬁ bers is referred to as the suspen-

sory ligament of the eye. These ﬁ bers form a connection between

the ciliary body and the equatorial (lateral) region of the lens (see

Fig. 20-11C,D). One end of each zonular ﬁ ber is attached to a

ciliary process, and the other end of the ﬁ ber is embedded in the

capsule of the lens. The capsule is thicker on the anterior and

lateral surfaces than on the posterior surface. The zonular ﬁ bers

have an important function during accommodation, which is to

adjust the tension of the lens (thereby changing the curvature of

the lens) to enable an object to be focused on the retina. Some

ciliary muscle ﬁ bers are arranged in a circle at the base of the iris.

Contraction of these ﬁ bers decreases the diameter of the circle

and, therefore, decreases the tension on the zonular ﬁ bers so the

lens becomes more round (the curvature increases). Relaxation

of the ciliary muscle increases tension on the zonular ﬁ bers and

the lens becomes ﬂ attened (curvature decreases).

Zonular

fibers

Ciliary

muscle

Ciliary

processes

Ciliary

body

Iris

D. Cui

Subcapsular

epithelium

Anterior lens capsule

Posterior

lens capsule

Lens nucleus

Lens

cortex

Nucleus of

lens fibers

Lens

fibers

Zonular

fibers (relaxed)

Zonular

fibers (stretched)

Lens curvature

increases

Lens curvature

decreases

Posterior

Figure 20-8B. A representation of the lens and its function.

The lens is transparent and is composed of a lens capsule, sub-

capsular epithelium, and lens ﬁ bers. The function of the lens is to

focus light on the retina. When focusing on a distant object, the

ciliary muscle is relaxed, tension on the zonular ﬁ bers increases,

and the anteroposterior thickness of the lens decreases. To focus

on a near object, the ciliary muscle contracts to release the ten-

sion on the zonular ﬁ bers and the thickness of the lens increases.

Ciliary muscle contraction also pulls the choroid forward to aid

in focusing objects on the retina. (The actions of the ciliary mus-

cle are also discussed in Figs. 20-8A and 20-11B.) The ability of

the lens to accommodate decreases with age (presbyopia). The

most common disorder associated with the lens is the cataract.

Figure 20-8C.

Cataract. H&E, 51

Cataract is a condition of opacity in the lens of the eye. Crystallin

protein in the lens degenerates, becoming insoluble and opaque. In

cataractous lenses, lens ﬁ

bers are edematous and sometimes necrotic.

These changes alter the normal continuity of the lens ﬁ bers. The

capsule may become wrinkled. The inset photomicrograph shows

a cluster of altered protein in the anterior lens cortex. Cataract is

usually an age-related disorder that causes partial or total blindness

if left untreated. Risk factors include age, smoking, alcohol use,

sunlight exposure, diabetes mellitus, and systemic corticosteroid

use. Cataracts are usually bilateral and progress slowly. The visual

acuity decrease is directly related to the density of the cataract. Types

of cataracts include senile cataract, congenital cataract, traumatic

cataract, toxic cataract, and cataract associated with systemic

disease. The senile cataract is the most common form. Medical

intervention consists of removing the opaciﬁ ed lens from the eye

and implanting an artiﬁ cial intraocular lens.

D. Cui

Wrinkled capsule

Abnormal anterior

lens cortex tissue

Normal anterior

lens cortex tissue

Degenerating

protein

Disrupted

lens fibers

Necrosis

CLINICAL CORRELATION

CUI_Chap20.indd 399 6/16/2010 7:38:09 PM

400

UNIT 3

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Organ Systems

Tunica Vasculosa (Tunica Media)

Posterior

Fig. 1-1C

Fig. 20-9B

Fig. 20-9C,D

Anterior chamber

Cornea

Iris

Posterior chamber

Position of

zonular fibers

Ciliary

body

Lens

Figure 20-9A. Overview of the iris and nearby structures. H&E,

11

A low-magniﬁ cation photomicrograph of the anterior part of the eye

is shown. The cornea, iris, lens, ciliary body, and their anatomical

relationships are reviewed here. The iris arises from the anterior part

of the ciliary body and separates the anterior and posterior chambers.

The iris covers part of the anterior surface of the lens and forms the

pupil, which regulates the amount of light that enters the eye.

Figure 20-9B. Iris. H&E, 136

The anterior iridial border (anterior surface of the iris) consists of a

discontinuous layer of ﬁ broblasts and melanocytes. Beneath it is a

thick layer of loose connective tissue, called the iris stroma (stroma

iridis), which contains some ﬁ bers, cells (ﬁ broblasts and melano-

cytes), and blood vessels. The posterior surface of the iris is covered

by two layers of heavily pigmented epithelial cells, which com-

pletely block light coming into the eye (except the light through the

pupil). These are the anterior iridial epithelium (anterior pigment

epithelium) and the posterior iridial epithelium (posterior pigment

epithelium). (See also Fig. 20-8D.)

Figure 20-9C. Constrictor pupillae muscle and its function.

H&E, 272; inset 628

The pigment epithelium of the iris is partially in contact with the

lens capsule. There is a thick layer of concentrically oriented smooth

muscle in the stroma of the iris, which is called the constrictor

pupillae muscle (sphincter muscle). It is innervated by postgangli-

onic parasympathetic ﬁ bers from the ciliary ganglion and serves to

decrease the pupillary size when the eye is exposed to strong light.

Figure 20-9D. Dilator pupillae muscle and its function. H&E,

473; inset 680

The dilator pupillae muscle of the iris is composed of radially

arranged myoid processes of myoepithelial cells of the anterior pig-

mented epithelium, located more peripherally in the iris than the

constrictor pupillae muscle (also see Fig. 20-8B). It is innervated by

postganglionic sympathetic ﬁ bers from the superior cervical gan-

glion and serves to increase pupillary size when the light is dim.

Anterior iridial border

Iris stroma

(stroma iridis)

iridial

epithelium

Constrictor

pupillae

muscle

Dilator

pupillae

muscle

Lens

Posterior

iridial

epithelium

Iris stroma

Constrictor

pupillae muscle

Lens

Lens capsule

Iris stroma

Lens

Anterior iridial

border

Constrictor pupillae

muscle

Constrictor pupillae

muscle

iridial

epithelium

Posterior

iridial

epithelium

Melanocytes

Dilator

pupillae

muscle

iridial

epithelium

Posterior

iridial

epithelium

Dilator pupillae

muscle

Dilator pupillae

muscle

CUI_Chap20.indd 400 6/16/2010 7:38:11 PM

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Eye

401

Figure 20-10A. Anterior surface

of the lens. EM, 3,600

A simple epithelium, the lens epi-

thelium, covers the anterior surface

of the lens. The heights of the cells

vary, ranging from squamous near

the center of the anterior surface to

columnar at the edge (equator) of

the lens. Because the lens develops

from a ball of epithelial cells (lens

vesicle) that invaginate from the

surface ectoderm (lens placode),

the basement membrane of the epi-

thelium covers the surface of the

lens as the lens capsule. The part

of the lens capsule at the equator

of the lens serves as the insertion

for zonule ﬁ bers of the lens suspen-

sory ligament. Behind the lens epi-

thelium is an orderly array of lens

ﬁ bers. Each lens ﬁ ber is a highly

elongated, crystallin-ﬁ lled remnant

of an epithelial cell that extends the

full thickness of the lens. The lens

ﬁ bers in this view are seen in cross

section.

Lens fiber filled with

epithelium

Nuclei of the

lens epithelial cells

Lens

capsule

Lens

cortex

Lens fiber filled with

protein crystallin

Lens

capsule

Lens

epithelium

Nuclei of the

lens epithelial cells

Lens

protein crystallin

cortex

Myoid

processes

Posterior

pigment

epithelial cells

pigment

epithelial celsl

Nucleus

Melanin

granules

Junction boorder

Posterior

iridial

epithelial cells

iridial

epithelial cells

Melanin

granules

Nucleus

Nucleus of

stromal cell

Junction border

Myoid

processes

Figure 20-10B. Posterior surface

of the iris. EM, 4,600

The posterior surface of the iris

is covered by double epithelium

derived from the inner and outer

layers of the lip of the original optic

cup. The cells of the posterior irid-

ial epithelium are tall and densely

packed with melanin granules. The

cells of the anterior iridial epithe-

lium are more complicated in shape.

Part of the cytoplasm of these cells

contains melanin granules, simi-

larly to the posterior epithelial cells;

however, these cells also extend con-

tractile processes into the adjacent

iridial stroma. These myoid pro-

cesses are ﬁ lled with actin ﬁ laments,

and, because of their radial orien-

tation, the diameter of the pupil

is increased when they contract.

Therefore, the myoepithelial cells

of the anterior iridial epithelium

collectively constitute the pupillary

dilator. At the border between the

posterior and anterior iridial epithe-

lia ([PE and AE, respectively] inset),

the apices of the epithelial cells con-

tact each other.

CUI_Chap20.indd 401 6/16/2010 7:38:15 PM

402

UNIT 3

■

Organ Systems

Posterior

Fig. 1-1C

Posterior

iliary muscleC

Ciliary

muscle

Ciliary processes

Ciliary

body

Zonular fibers

Lens

Iris

Sclera

Limbus

(corneoscleral

junction)

Cornea

Anterior chamber angle

(iridocorneal angle)

chamber

Posterior

chamber

Ciliary

muscle

Lens

Ciliary

processes

Zonular fibers

Pigmented

epithelium

Unpigmented

epithelium

Ciliary

muscle

Ciliary

muscle

AnteriorPosterior

Lens

D. Cui

Ciliary

processes

Trabecular

meshwork

Canal of

Schlemm

Canal of

Schlemm

Sclera

Bulbar

conjunctiva

Ciliary

epithelium

Trabecular

meshwork

chamber angle

Retina

Ciliary ring

(pars plana)

Ora serrata

Ciliary process

(pars plicata)

Zonular fibers

Zonular fiber

Ciliary process

Ciliary

muscle

Ciliary

muscle

Figure 20-11A. Overview of the ciliary body and nearby

structures. H&E, 19

The ciliary body is located internally to the anterior margin of the

sclera. The transition between the cornea and sclera is the limbus

(corneoscleral junction). This is an important landmark for eye

surgery procedures. The surface of the anterior portion of the cili-

ary body (ciliary process) has zonular ﬁ bers attached to it and is

in contact with the aqueous humor. The surface of the posterior

portion of the ciliary body is in contact with the vitreous body.

Figure 20-11B. Ciliary processes and the ciliary muscle.

H&E, 87; inset 348

Ciliary processes have loose connective tissue cores and are covered

by two layers of epithelium: (1) a nonpigmented layer and (2) a

pigmented layer. The apical surfaces of the two epithelial layers

face each other. Their basal surfaces each rest on a basement mem-

brane, one bordering the ciliary stroma and the other bordering

the aqueous humor. The cells are ﬁ rmly connected by junctional

complexes. The ciliary muscle contains three smooth muscle ﬁ ber

groups: (1) longitudinal muscle ﬁ bers, which stretch the choroid

to alter the opening of the anterior chamber angle for drainage of

aqueous humor; (2) radial muscle ﬁ bers, which increase tension on

the zonular ﬁ bers and cause the lens to ﬂ atten, allowing the eyes to

focus for distant vision; and (3) circular muscle ﬁ bers, which relax

the tension on the zonular ﬁ bers and cause the lens to become more

convex to accommodate for near vision. Ciliary muscles are inner-

vated by parasympathetic nerve ﬁ bers of the oculomotor nerve.

Figure 20-11C. Ciliary body, transverse and posterior views.

H&E, 34; inset 62

The ciliary body consists of (1) the ciliary ring (pars plana), the

region that contains a ring of smooth muscle (ciliary muscle)

surrounded by loose connective tissue and covered by the ciliary

epithelium and (2) the ciliary processes (pars plicata), ﬁ nger-

like structures which contain many fenestrated capillaries that

produce aqueous humor. Aqueous humor ﬂ ows from the pos-

terior chamber through the pupil to the anterior chamber, then

passes into the trabecular meshwork and ﬁ nally into the canal of

Schlemm (see Fig. 20-12C).

CUI_Chap20.indd 402 6/16/2010 7:38:16 PM

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Eye

403

Figure 20-12C.

Glaucoma.

Glaucoma is a group of eye diseases that produce elevated

intraocular pressure, usually because of obstruction of the aqueous

humor outﬂ ow (see Figs. 19-11B and 19-12C). Glaucoma

results in damage to the optic nerve and is a major cause of

blindness. (1) Open-angle glaucoma is the most common type.

Fragments from normal cell degeneration become deposited

within the trabecular meshwork and endothelial lining of the

canal of Schlemm and reduce the absorption of aqueous ﬂ uid.

Intraocular pressure rises slowly over a long period of time.

Peripheral vision may be reduced before the patient is aware of

the loss. As the pressure rises, the optic disk becomes cupped.

(2) Acute angle-closure glaucoma is also common. Occlusion

of the anterior chamber angle occurs when the peripheral iris

obstructs aqueous outﬂ ow. Intraocular pressure can rise quickly

and reach very high levels. Patients may present with severe

headache and eye pain, malaise, nausea, and vomiting. Immediate

medical intervention is necessary to prevent vision loss.

J.Lynch

Zonular fibers

Vitreous body

Ciliary process

Canal of Schlemm

Aqueous vein

Trabecular

meshwork

Cornea

Posterior

chamber

Anterior chamber angle

Sclera

Ciliary

muscle

Figure 20-12A. Posterior portion of the ciliary body.

H&E, 34; inset 102

The ciliary body lies posterior to the root of the iris, anterior

to the ora serrata, and interior to the sclera (see Fig. 20-1).

The ciliary body is triangular in shape. The anterior portion is

thick and the posterior portion gradually becomes thinner and

ends at the ora serrata (Fig. 20-12B). The two cell layers of the

ciliary epithelium cover the entire surface of the ciliary body.

Ciliary

muscle

Ciliary epithelium

(pars planum)

Bulbar

conjunctiva

Ciliary epithelium

(pars planum)

Ciliary epithelium

Sclera

Ora serrata

Ciliary epithelium

Retinal

pigmented

epithelium

Bruch

membrane

Bruch

membrane

Retina

Pigmented

ciliary

epithelium

Basement

membrane

Ciliary epithelium

Bruch

membrane

Retinal

pigmented

epithelium

Sclera

Basement

membrane

Pigmented

ciliary

epithelium

Figure 20-12B. Ora serrata. H&E, 34; inset 102

The ora serrata is a denticulate border (junction) between

the ciliary body and the retina; this is an important anatomic

landmark for the ophthalmologist. The extended ciliary epi-

thelium from the ciliary body is shown on the right side of

the picture. The anterior portion of the retina is shown on

the left side of the picture. The pigmented ciliary epithelium

and its basement membrane are continuous with the retinal

pigmented epithelium and the Bruch membrane.

Interference with the normal ﬂ ow of aqueous humor leads

to increased intraocular pressure, and a serious medical

condition, glaucoma, may result. The most likely sites of

blockage of the ﬂ ow of aqueous humor are at the trabecular

meshwork and the endothelial lining of the canal of Schlemm

rather than the venous collector system.

Figure 20-12C. Outﬂ ow of aqueous humor.

Aqueous humor is produced in the posterior chamber by the

epithelium that lines the ciliary processes of the ciliary body

(see Fig. 20-11B,C). It ﬂ ows through the pupil from the pos-

terior chamber to the anterior chamber (yellow arrow), where

it is absorbed into the trabecular meshwork (Fig. 20-11B). The

aqueous humor then diffuses through connective tissue and epi-

thelial tissue into the canal of Schlemm. Aqueous veins connect

the canal of Schlemm to episcleral veins, where the aqueous

humor is absorbed into the body’s venous circulation.

CLINICAL CORRELATION

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404

UNIT 3

■

Organ Systems

Retina (Tunica Interna)

Fovea

centralis

Sclera

Choroid

Macula

lutea

Substantia propria

Episclera

Choroid

Choriocapillaris

Nerve fiber layer (9)

Ganglion cell layer (8)

Inner plexiform layer (7)

Inner nuclear layer (6)

Outer plexiform layer (5)

Outer limiting layer (3)

Photoreceptor layer (2)

Outer nuclear layer (4)

Pigment epithelium

layer (1)

Pigment

epithelial

cells

Inner limiting membrane (10)

Bruch

membrane

Choroid

Choriocapillaris

Nerve fiber layer

Ganglion cell layer

Inner plexiform layer

Inner nuclear layer

Outer plexiform layer

Outer limiting layer

Photoreceptor layer

Outer nuclear layer

Pigment epithelium

layer

Inner limiting membrane

Nuclei of cones

Nuclei of rods

Choroid

Figure 20-13A. Overview of the retina; fovea. H&E, 17

The retina is a multilayered sheet of neural tissue covering the

inner aspect of the posterior two thirds of the eyeball (Figs.

20-13B,C and 20-15A–C). There are regional variations in its

structure: the macular (central) region is generally thicker than

the peripheral region, and cone receptors predominate in the cen-

tral retina, whereas rods are more numerous in the periphery. The

fovea centralis is a small depression in the central retina caused by

the displacement of the superﬁ cial layers, thereby allowing incom-

ing light more direct access to the photoreceptors in this area. The

photoreceptors here consist entirely of miniature cones, and there

are no blood vessels in this region. These characteristics permit

maximum visual acuity to be achieved in the fovea. The macula

lutea is the region immediately surrounding the fovea centralis.

Macula lutea means “yellow spot”; this region appears yellow in

the living retina when viewed with an ophthalmoscope.

Figure 20-13B. Macular region of the retina and the retinal

layers. H&E, 184; inset 368

The macular region of the retina is relatively thick, and its

ganglion cell layer contains many layers of nuclei. However,

both the macular and peripheral regions of the retina have

the same histologic layers: (1) the pigment epithelium layer, a

layer of cuboidal cells that contain melanin granules; (2) the

photoreceptor layer, external segments of rods and cones; (3) the

outer limiting layer, a junction complex between the Müller cells

and photoreceptor cells; (4) the outer nuclear layer, containing

nuclei of the photoreceptor cells; (5) the outer plexiform layer,

containing processes of photoreceptor cells, bipolar cells, and

horizontal cells; (6) the inner nuclear layer, containing nuclei of

bipolar cells, horizontal cells, amacrine cells, and Müller cells;

(7) the inner plexiform layer, containing processes of the cells in

the adjacent layers; (8) the ganglion cell layer, containing nuclei

of the ganglion cells; (9) the nerve ﬁ ber layer, containing axons

of the ganglion cells; and (10) the inner limiting membrane,

which is the basement membrane of the Müller cells.

Figure 20-13C. Peripheral region of the retina and distribu-

tion of the rods and cones. H&E, 184; inset 694

The peripheral part of the retina is thinner than the macular

region and its ganglion cell layer becomes a single layer of nuclei.

The rod photoreceptors are more numerous in the peripheral

retina. The nuclei of rod cells are small and round and spread

throughout the depth of the outer nuclear layer. The cone photo-

receptors are present in both the center and periphery of the

retina but are most highly concentrated in the fovea and macula.

The nuclei of cone cells are large and ovoid in shape and often

located at the base of the outer nuclear layer. Rod and cone cells

are both photoreceptor neurons. Rods are specialized for motion

detection and vision in dim light. Cones are specialized for ﬁ ne

visual acuity and color vision. Note the difference in thickness

between the macular and peripheral regions of the retina and the

relatively low number of ganglion cells in the peripheral retina.

CUI_Chap20.indd 404 6/16/2010 7:38:23 PM