Chapter 20: Eye

Day 22

When does the developing eye first appear as a pair of shallow grooves on the sides of the forebrain?

Optic vesicles

What outpocketings of the forebrain form when the neural tube closes?

Optic vesicle

What structure comes into contact with the surface ectoderm and induces changes needed for lens formation?

Optic cup

What double-walled structure forms when the optic vesicle begins to invaginate?

Intraretinal space

What lumen initially separates the inner and outer layers of the optic cup?

Choroid fissure

What forms from invagination of the inferior surface of the optic cup and allows the hyaloid artery to reach the inner chamber of the eye?

Hyaloid artery

What artery is allowed to reach the inner chamber of the eye through the choroid fissure?

Seventh week

During what week do the lips of the choroid fissure fuse?

Future pupil

What does the mouth of the optic cup become after the lips of the choroid fissure fuse?

Lens placode

What structure forms when surface ectoderm cells in contact with the optic vesicle begin to elongate?

Lens vesicle

What structure develops when the lens placode invaginates?

Fifth week

During what week does the lens vesicle lose contact with the surface ectoderm?

Pigmented layer of the retina

What is the outer layer of the optic cup characterized by small pigment granules?

Pars optica retinae

What is the posterior four-fifths of the inner layer of the optic cup called?

Rods and cones

What photoreceptive cells develop from cells bordering the intraretinal space?

Rods

Which photoreceptors are more numerous and more sensitive but cannot detect color?

120 million

Approximately how many rods are present?

Cones

Which photoreceptors detect color?

6-7 million

Approximately how many cones are present?

Mantle layer

Which retinal layer gives rise to neurons and supporting cells?

Outer nuclear layer, inner nuclear layer, and ganglion cell layer

What three layers are formed from the mantle layer?

Fibrous layer

What layer contains the axons of nerve cells from the deeper retinal layers?

Optic nerve

What does the optic stalk develop into?

Pars ceca retinae

What is the anterior fifth of the inner layer of the optic cup called?

One cell layer thick

How thick does the pars ceca retinae remain?

Pars iridica retinae

Which part of the pars ceca retinae forms the inner layer of the iris?

Pars ciliaris retinae

Which part of the pars ceca retinae participates in forming the ciliary body?

Sphincter and dilator pupillae muscles

What muscles develop in the tissue between the optic cup and the overlying surface epithelium?

Underlying ectoderm of the optic cup

From what tissue do the sphincter and dilator pupillae muscles develop?

Iris

What adult structure consists of a pigment-containing external layer, an unpigmented internal layer of the optic cup, and vascularized connective tissue containing the pupillary muscles?

Ciliary muscle

What muscle forms from the mesenchyme covering the pars ciliaris retinae externally?

Suspensory ligament (zonula)

What network of elastic fibers connects the ciliary body to the lens?

Ciliary muscle

What muscle changes the tension in the suspensory ligament and controls the curvature of the lens?

Primary lens fibers

What fibers form when cells of the posterior wall of the lens vesicle elongate anteriorly?

End of the seventh week

When do the primary lens fibers reach the anterior wall of the lens vesicle?

Secondary lens fibers

What fibers are continuously added to the central core after the primary lens fibers form?

Continuous addition of secondary lens fibers

How does the lens continue to grow after the primary lens fibers reach the anterior wall?

End of the fifth week

When is the eye primordium completely surrounded by loose mesenchyme?

Loose mesenchyme

What tissue completely surrounds the eye primordium at the end of the fifth week?

Inner mesenchymal layer

Which mesenchymal layer is comparable with the pia mater of the brain?

Outer mesenchymal layer

Which mesenchymal layer is comparable with the dura mater of the brain?

Choroid

What does the inner mesenchymal layer develop into?

Sclera

What does the outer mesenchymal layer develop into?

Dura mater

What structure is the sclera continuous with around the optic nerve?

Anterior chamber

What chamber forms through vacuolization of the mesenchyme overlying the anterior aspect of the eye?

Iridopupillary membrane

What inner mesenchymal layer forms in front of the lens and iris?

Substantia propria of the cornea

What outer mesenchymal layer forms continuously with the sclera?

Flattened mesenchymal cells

What cells line the anterior chamber?

Surface ectoderm

What is the origin of the epithelial layer of the cornea?

Substantia propria or stroma, continuous with the sclera

What is the middle layer of the cornea that is continuous with the sclera?

Three layers

How many layers form the cornea?

1) Epithelial layer derived from the surface ectoderm

2) substantia propria or stroma continuous with the sclera

3) epithelial layer bordering the anterior chamber

What are the three layers that form the cornea?

Iridopupillary membrane

What membrane in front of the lens disappears completely?

Posterior chamber

What is the space between the iris anteriorly and the lens and ciliary body posteriorly?

Pupil

Through what structure do the anterior and posterior chambers communicate with each other?

Aqueous humor

What fluid fills the anterior and posterior chambers?

Ciliary process of the ciliary body

What produces the aqueous humor?

Posterior chamber → anterior chamber

From which chamber to which chamber does aqueous humor circulate?

Avascular cornea and lens

What structures receive nutrients from the circulating aqueous humor?

Scleral venous sinus (canal of Schlemm)

Through what structure does aqueous humor pass from the anterior chamber to be resorbed into the bloodstream?

Iridocorneal angle

Where is the scleral venous sinus (canal of Schlemm) located?

Glaucoma

What condition can result from blockage of fluid flow at the canal of Schlemm?

Choroid fissure

Through what structure does mesenchyme invade the inside of the optic cup?

Hyaloid vessels

What vessels are formed by the invading mesenchyme inside the optic cup?

Lens and inner surface of the retina

What structures are supplied by the hyaloid vessels during intrauterine life?

Delicate network of fibers between the lens and retina

What structure is formed by mesenchyme between the lens and retina?

Transparent gelatinous substance

What fills the interstitial spaces of the delicate fiber network between the lens and retina?

Vitreous body

What is formed when the interstitial spaces of the fiber network between the lens and retina fill with a transparent gelatinous substance?

Fetal life

During what period are the hyaloid vessels obliterated and disappear?

Hyaloid canal

What structure is left behind after the hyaloid vessels are obliterated and disappear?

Optic stalk

What structure connects the optic cup to the brain?

Choroid fissure

What groove is present on the ventral surface of the optic stalk?

Hyaloid vessels

What structures are located within the choroid fissure of the optic stalk?

Nerve fibers of the retina

What fibers returning to the brain lie among the cells of the inner wall of the optic stalk?

Seventh week

During what week does the choroid fissure close?

Narrow tunnel

What forms inside the optic stalk after the choroid fissure closes?

Optic nerve

What does the optic stalk transform into as the number of nerve fibers increases and the inner and outer walls fuse?

Neuroglia

What network is provided by cells of the inner layer to support the optic nerve fibers?

Central artery of the retina

What does the portion of the hyaloid artery within the center of the optic nerve later become?

Pia arachnoid

What structure surrounding the optic nerve is a continuation of the choroid?

Dura layer

What layer surrounding the optic nerve is a continuation of the sclera?

Pia arachnoid and dura layer

What layers surround the optic nerve, with the pia arachnoid being a continuation of the choroid and the dura layer being a continuation of the sclera?

PAX6

What is the key regulatory gene for eye development?

PAX (paired box) family of transcription factors

What family does PAX6 belong to?

Paired domain and paired-type homeodomain

What two DNA-binding motifs does PAX6 contain?

Anterior neural ridge of the neural plate

Where is PAX6 initially expressed before neurulation begins?

Single eye field

What is initially present before the eye field separates into two optic primordia?

SONIC HEDGEHOG (SHH)

What signal causes the single eye field to separate into two optic primordia?

Prechordal plate

Where is SHH expressed to signal separation of the eye field?

PAX2

Which gene is expressed in the optic stalks?

PAX6

Which gene is expressed in the optic cup and overlying surface ectoderm that forms the lens?

Fibroblast growth factors (FGFs) from the surface ectoderm

What signals promote differentiation of the neural (inner) layer of the retina?

Transforming growth factor β (TGF-β) secreted by surrounding mesenchyme

What signal directs formation of the pigmented (outer) layer of the retina?

MITF

Which transcription factor directs differentiation of the pigmented retinal layer?

CHX10

Which transcription factor directs differentiation of the neural retinal layer?

Lens ectoderm

What is essential for proper formation of the optic cup?

No cup invagination occurs

What happens to optic cup formation if there is no lens placode?

PAX6

What gene does lens differentiation depend on?

Surface ectoderm

Where does PAX6 act to regulate lens development?

SOX2

What transcription factor is upregulated by PAX6 and maintained in the prospective lens ectoderm?