6.2 Development of the Eye

Question 1

The eyes appear during the 22nd day of development (4th week) as a pair of _______________ on the sides of the forebrain:
• Grooves evaginate from the forebrain to form the _______________ with the closure of the neural tube → grows laterally and come into contact with the surface ectoderm
• Contact with surface ectoderm causes changes in the ectoderm → necessary for formation of the ____________
• Optic vesicle begins to invaginate to form a _______________

Answer 1

shallow optic grooves;

optic vesicles;

lens (lens placodes) ;

double-walled optic cup

Question 2

The outer and inner layers of the optic cup are initially separated by the ________________-, which disappears during development → layers appose and fuse:
• Optic cup is connected to the forebrain by the _____________, which invaginates to form the choroid (optic) fissure → allows _____________ (branch of ophthalmic artery) to enter the optic vesicle → supplies the eye
• 7th week: lips of choroid fissure close and mouth of the optic cup becomes the round opening (future pupil)

• Distal (supplying lens): _________________
• Proximal (supplying retina): _________________

Answer 2

intraretinal space;

optic stalk;

hyaloid artery;

Degenerates during foetal life (as lens matures) ;

Persists → central artery of retina

Question 3

The optic vesicle initially lies in contact with the surface ectoderm, causing the ectoderm to elongate and thicken to form the lens placode:
• Lens placodes invaginate and develop into the ___________ → lose contact with the surface ectoderm by the 5th week → lies in the mouth of the optic cup

Answer 3

lens vesicle

Question 4

The growth of the lens is a continuous process involving both cells of the anterior and posterior walls of the lens vesicle

• Posterior: Elongate anteriorly → form ___________________ which gradually fill the lumen of the vesicle (reaching the anterior wall by the 7th week)
• Anterior: Remain ___________ → forms a simple epithelium covering the face of the lens → forms secondary lens fibres which make up most of the mature lens (continuously added to the central core)

Answer 4

long fibres (primary lens fibres);

cuboidal

Question 5

The retina develops from the two walls of the optic cup from the 6th to 8th month:
- Outer: __________ layer (characterised by small pigment granules)

-Inner: Neural layer → develops differently into:
• ______________ (posterior 4/5)
• _____________ (anterior 1/5) → divides into the pars iridica retinae (inner layer of iris) and pars ciliaris retinae (ciliary body)

Answer 5

Pigmented;

Pars optica retinae ;

Pars ceca retinae

Question 6

What is the origin of the iris (sphincter pupillae & dilator pupillae)?

Answer 6

Neuroectoderm

Question 7

What is the origin of the ciliary muscles?

Answer 7

Mesoderm

Question 8

The _____________ (part of the pars ceca retinae) is characterised by marked folding:
• Externally covered by a layer of mesenchyme which gives rise to the ciliary muscles
• Internally connected to lens by suspensory ligament/zonule (network of elastic fibres)

Answer 8

pars ciliaris retinae

Question 9

The mesenchyme overlying the developing lens splits to enclose the anterior chamber:
• Thin mesenchyme deep to the anterior chamber is the ______________, which breaks down completely early in foetal development to form the pupil
• Posterior chamber develops between the ____________________
• Pupil communicates the posterior chamber with the anterior chamber

Answer 9

pupillary membrane;

pupillary membrane and the lens

Question 10

The vitreous body develops from the mesenchyme in the space between the lens and the ______________:
• Mesenchyme invades the interior of the optic cup via the ____________ and forms delicate fibres between the lens and retina
• Interstitial spaces of this network layer fill with a transparent gelatinous substance (vitreous body/humour)
• Hyaloid vessels in this region are obliterated → hyaloid canal remains

Answer 10

inner wall of the optic cup;

choroid fissure

Question 11

what cells are present ganglion cell layer?

Answer 11

Ganglion cells*

Question 12

what cells are present inner nuclear cell layer?

Answer 12

Amacrine, horizontal, bipolar cells

Question 13

what cells are present outer nuclear cell layer?

Answer 13

Nuclei of rod & cone photoreceptors*

Question 14

___________ is the master regulatory gene for eye development (encodes a transcription factor):
• Expressed in a band in the anterior neural ridge of the neural plate before neurulation (development of nervous system) begins
• Single eye field later separates into two optic primordia (signalled by the _____________)
o SHH expression upregulates PAX2 (in the _______________) and downregulates PAX6 (expressed in the ______________ instead)
• PAX6 is important for differentiation of the lens (from the overlying surface ectoderm) after induction by the optic vesicle

Answer 14

PAX6;

sonic hedgehog (SHH) protein;

centre of the eye field;

optic cup and lens