development of eye and ear

7/30/2019 Development of Eye and Ear

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Development of eye and ear

B.Sc. 2012


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Embryonic development

Week 3: Beginning development of the brain, heart, blood cells, circulatorysystem, spinal cord, and digestive system.

Week 4: Beginning development of bones, facial structures, and limbs(presence of arm and leg buds); continuing development of the heart (whichbegins to beat), brain, and nervous tissue.

Week 5: Beginning development of eyes, nose, kidneys, lungs; continuingdevelopment of the heart (formation of valves), brain, nervous tissue, and

digestive tract.

Week 6: Beginning development of hands, feet, and digits; continuingdevelopment of brain, heart, and circulation system.

Week 7: Beginning development of hair follicles, nipples, eyelids, and sexorgans (testes or ovaries); first formation of urine in the kidneys and firstevidence of brain waves.

Week 8: Facial features more distinct, internal organs well developed, thebrain can signal for muscles to move, heart development ends, external sexorgans begin to form.

By the end of the embryonic stage, all essential external and internalstructures have been formed. The embryo is now referred to as a fetus


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Development of eye

The eyes of all vertebrates develop in a patternwhich produces an "inverted" retina, in which theinitial detection of light rays takes place at theoutermost portion.

The eye is derived from the neural tube(neuroectoderm), from which arise the retinaproper and its associated pigment cell layer;the mesoderm of the head region, whichproduce the corneoscleral and uveal tunics;and the surface ectoderm, from which comesthe lens.


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Eye development

Earliest stage of eye development is the formation ofthe paired optic vesicles on either side of theforebrain.

These growing diverticula expand laterally into themesoderm of the head and develop a stalk-likeconnection to the main portion of the rudimentarycentral nervous system.

In humans, this process begins at about 22 days ofdevelopment; as the vesicles continue to grow, theirconnection to the brain becomes progressively

narrower and more stalk-like. The forming stalks will eventually become the

rudiments of the optic nerves.


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Eye development

As this is occurring, the surface ectoderm thickensto form a lens placode, a region visible on thesurface of the embryo. This transformation istriggered by the proximity of the optic vesicle, in atypical example of induction.

Once the formation of the lens placode has begun,the expanding optic vesicle begins to invaginate toform a cup-shaped structure, and also to fold alongits centerline, enclosing a small amount ofangiogenic mesenchyme as it does so.

This mesenchyme forms the hyaloid artery and vein,which supply the forming lens; and later, in the fullyformed stage will become the central artery and veinof the retina.


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Eye development

As the cup invaginates and folds, it is forming twodistinct layers.

The inner layer of the optic cup will eventually formthe retinal tunic, including its light-sensitiveelements.

The outer layer of the optic cup will form the pigmentepithelium layer, which lies outside the sensitiveportion.

The uveal and corneoscleral tunics eventually willdifferentiate from the surrounding mesoderm.

Meanwhile, the surface ectoderm of the lens placodehas thickened and is beginning to differentiate twodistinct areas.


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As the optic cup from the lateral wall of the forebrain comes close to the

overlying non-neural ectoderm, the terminal induction of the lens placode takes

place. The lens placode invaginates to form the lens vesicle. Fiber cells form

and the crystalline gene family is activated.


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http://eyemakeart.files.wordpress.com/2009/06/eye-stages_in_the_evolution_of_the_eye.png


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Eye development SUMMARY OF MAIN EVENTS

The optic vesicles extend from the Diencephalon

Optic vesicles come into close proximity to epithelialectoderm

Optic vesicle thickens & folds as optic cup

Lens placode forms from epithelial ectoderm Lens placode infolds as future lens

TIME LINE 22 Days--Optic Groove Appears

24 Days--Optic Vesicle

26 Days --Optic Cup & Lens Placode

28 Days--Further folding OC & LP

33 Days--Sensory & Pigmented Retina

33 & 36 Days--Lens distinct


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Eye development

Formation of lens

The first is the lens vesicle, and invagination of the surface placode that will separateand form the lens proper.

As the lens rudiment detaches and drops into position, a space forms external to itthat will become the anterior chamber of the mature eye.

The surface ectoderm and the mesoderm beneath it differentiate into the cornea andthe eyelids.

The process of forming the eye by inversion of the optic vesicle and formationof the optic cup is common to all of the vertebrates. It produces an eye that hasan inverted retina. The light-sensitive elements are located at the outer regions,and the neural connection to the brain must consequently come from the innerregion, perforating through the rest of the layers in its course. The enclosure ofthe hyaloid artery and vein in the stalk provides for blood supply to the retina.

The formation of the sclera, the cornea and the uveal tunic also haveimplications for the nature of the fully-formed eye. Since mesoderm is the onlyembryonic tissue with angiogenic potential, i.e., the capacity to form bloodvessels, its participation in the formation of the uveal tunic and the sclera isnecessary.

The cornea, although it fuses with the sclera, is derived solely from ectoderm,and it is therefore avascular in its final form. This has clinical significancebecause it isolates the cornea from the immune system, creating a "privilegedsite" suitable for transplantation..


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Induction in eye development

Chordamesoderm Induces Neural Tube

Optic Cup Induces Lens

Lens with or without the Optic CupInduces Cornea

Other combinations induce othercomponents

Eyes form at proper Place & Time Eye components appear in proper position

& orientation


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Lens Differentiation

Lens needs to clarify to allow light to pass

Lens needs to be proper shape to focus light

Cells at posterior side elongate

Cells need to multiply to fill in this space

Cells fill with clear crystallin proteins so light can passthrough as well as be focused by the lens

Germinative Region: Cells are actively dividing

Region of Cell Elongation: Cells are beginning to change

shape & Differentiate Cells differentiate into Fibre Cells

Fibre Cells Densely Packed in Centre as Lens Nucleus


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Pattern Generation

migration of retinal ganglion axonsStudied in non-mammalian vertebrates

does apply to mammals

neurons synapse in optic tectum of midbrain

position of ganglion cells in inner margin of

retina determined by cadherins on cell

membranes

growth of axon is directed both by presence

oflaminin and N-CAM on migratory surface

netrin-1 made by cells at optic disc also aidmigration (cells form optic nerve at disc)

axons growing on glial cells toward optic

tectum oriented by N-CAM, cadherins and

integrins

when reaching optic chiasma decision to go

straight or cross brain governed by ephrin

proteins and L1 and laminin adhesionmolecules

each axon goes to a specific site in tectum


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Pattern Generation

Axons distinguish between different

regions of optic tectum BY gradient of

transcription factors which specify cells

along a dorsal ventral axis

dorsal retinal cells have high levels of

TF Tbx5 while ventral cells have high

levels ofPax2 TF induced byparacrine factors BMP4 and retinoic

acid from nearby tissues

in tectum gradient ofephrin proteins

exists highest in posterior tectum

ephrin protein repulse axons from

temporal retina but not nasal retina

activity dependent synapse formation

also important here


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Neural crest formation

Muscle & dermis

Tissue-specific transcription factors act in a hierarchical and

combinatorial network to specify different cell fates.

Pax3 in Neural crest and

Dermatomyotome

expression of MITF

depends on PAX3


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Embryonic induction and PAX6

expressionlink between experimental embryology and genetics

Signal from optic cup induces placode to develop

Pax6 initiates and orchestrates program of eye development in

optic cup and lens placode

Human heterozygous-aniridia

homozygotes-complete absence of eye

Eyeless gene in Drosophila

DNA binding domainschanges in gene expression


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Ear development

Externally the pharyngeal archesare initially the most obviousexternal feature.

Sensory placodes are present onthe surface (but not obvious).

Otic placode lies in themesenchyme in the neck region

The middle ear ossicles (bones)are derived from 1st and 2nd archmesenchyme.

The space in which these bones

sit is derived from the 1stpharyngeal pouch


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Origin of Three Ear Parts

inner ear otic placode then otocyst

middle ear 1st pharyngeal pouch

1st and 2nd arch mesenchyme

outer ear 1st pharyngeal cleft

6 surface hillocks


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Origin of inner Ear

Otocyst

pair of surface sensory placodes (otic placodes) in the head region

placodes fold inwards forming a depression

pinches off entirely from the surface

fluid-filled sac or vesicle (otic vesicle, otocyst)

vesicle sinks into the head mesenchyme

Inner Ear Development

vesicle then extends and folds

membranous labrynth cochlea

utricle and saccule

endolymphatic duct

semicircular canals

Then innervated by CN VIII

embedded in developing temporal bone


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Middle Ear development

ossicles (bones)

arch 1 - malleus, incus

arch 2 - stapes

muscles (mesoderm)

arch 1 - tensor tympani

arch 2 - stapedius

tympanic cavity

first pharyngeal pouchextends as tubotympanic

recess


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Outer Ear development

surface hillocks (auricular hillocks)

three on pharyngeal arch 1

three on pharyngeal arch 2

external auditory meatus (canal)

1st pharyngeal cleft (groove)


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Development Timing

Week 3 - otic placode, otic vesicle

Week 5 - cochlear part of otic vesicle elongates

(humans 2.5 turns)

Week 9 - Mesenchyme surroundingmembranous labrynth (otic capsule) chondrifies

Week 12-16 - Capsule adjacent to membranous

labrynth undegoes vacuolization to form a cavity(perilymphatic space) around membranous

labrynth and fills with perilymph


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http://embryology.med.unsw.edu.au/Notes/images/senses/hearingcartoon.jpg

development of eye and ear

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