DEVELOPMENT OF THE EYES AND EARS

VERTEBRATE EYE DEVELOPMENT

• Single eye field in anterior neural plate 

  • Pax6

  • Shh 

• Day 22 gestation (week 3)  –  2 optic grooves from forebrain 

• Grooves reach the surface ectoderm @ day 25- 28 (around week 4) - optic vesicles 

• Vesicles and surface ectoderm meet and ectoderm thickens forming the lens placode 

• Invagination of optic vesicles forms the optic cup (distal) and optic stalk/optic nerve (proximal)

  • RA 

  • Inner layer of cup becomes retina 

  • Outer layer forms retinal pigment epithelium (RPE)

• Invagination of surface ectoderm, pinches off, forms lens vesicle

Eyelids: weeks 5 – 28

• Week 5: initiate formation from surface ectoderm and neural crest infiltrated mesenchyme 

• Week 8:  top and lower eyelid fused shut to protect developing eye 

• Week 26 – 28:  eyes blink

Lens: weeks 6-7

• Fibers formed form posterior cells of the lens vesicle and fill the lens 

• Hyaloid artery nourishes lens development and will regress  

Sclera & Choroid: around weeks 6 - 7

• Choroid from proximal inner layer of mesenchyme of cup and is directed by RPE

• Sclera from distal outer layer of mesenchyme of cup

Cornea:  around weeks 7-8

• Lens induces the formation 

• Corneal epithelium (outer layer) from surface ectoderm epithelium 

• Stroma and endothelium (inner layer) from the migration of neural crest cells 

Iris: around weeks 9 – 15

• Anterior border of the optic cup  

• Iris color - level of melanocytes within the stroma

SIGNALING MOLECULES/TRANSCRIPTION FACTORS

• Wnt and FGF

• BMP

• Retinoic Acid 

• Pax 6 – master regulator, single eye field formation, involved in the lens, cornea and retina formation. 

• Shh – divides eye field into two eyes, located in forebrain, promotes Pax2 in stalk but represses Pax6 in cup  

• BMP4, FGF8, and Delta – influences surface ectoderm to form lens placode

Eyelids

surface ectoderm and mesoderm 

Cornea

surface ectoderm and neural crest cells 

Sclera

neural crest cells 

Iris

neural ectoderm, mesoderm, and neural crest cells

Lens

surface ectoderm 

Retina and RPE

neural ectoderm

Choroid

mesoderm and neural crest cells 

Optic nerve

neural ectoderm

RESEARCH PAPER: MOVE IT APPROACH

• Objective: does specific transcription factor drive lens development by misexpressing in non-lens tissues

• Methodology: used in vivo electroporation in chick embryos to introduce lens related genes it to optic vesicle and ectoderm

• Expirmental design: Misexpress transcription factors known to be involved in lens development (e.g., FoxE3, Sox2, Six3, Etv5) in embryonic chick eye tissues.

• Techniques Used:

  • In vivo electroporation to deliver genes into targeted tissues.

  • Gene misexpression to test for sufficiency in lens induction.

  • Microscopy and molecular markers to assess resulting tissue changes.

• Findings:

  • Misexpressed genes induced lens-like characteristics in non-lens regions.

  • Confirmed that several transcription factors are sufficient to trigger lens development outside of their normal context.

DEVELOPMENTAL DEFECT OF THE EYE 

• Cyclopia 

  • Congenital disorder 

  • Single eye located in the center of the face 

  • Mutation in Shh gene 

  • Failure of the single eye field to separate 

  • Lethal condition 

WHEN AND HOW THE INNER EAR IS FORMED

• Ear development begins early in the 4th week of gestation when the otic placode recieves FGF and Wnt signals to envaginate to form the otic vesicle. 

• The dorsal side of the vesicle will receive Wnt signals from the hindbrain to form the semicurcular canals at week 8.

• The ventral side will receive Shh signals from the notochord to form the Saccule and the Cochlea at week 8

WHAT SIGNALING MOLECULES ARE INVOLVED EAR

• Sonic Hedgehog determines the dorsal-ventral axis of the inner ear, specifying which region will form the cochlea and which will form the semicircular canals

• Notch signaling specifies sensory regions and the differentiation of hair cells which will form the sensory receptors of the inner ear

• FGF is essential for forming the shape of the inner ear

• BMP signals help to generate the sensory organs of the inner ear

Inner Ear

• The middle ear forms during week 6, when the first pharyngeal cleft grows inward until it meets the first pharyngeal pouch where it forms the tympanic membrane.

• The bones of the middle ear are formed from through endochondral ossification of nearby mesenchyme

OUTER EAR

• The outermost part of the ear or the pinna forms in week 8 from six swellings that are derived from pharyngeal arch one and two. 

• These ectodermal swellings grow and fuse around the external auditory meatus.

WHERE THE TISSUES OF EAR FORMATION DERIVE FROM

• The inner ear is derived from an ectoderm placode.

• The middle ear forms the ear canal from invaginating ectoderm, the bones of the ear from mesenchymal mesoderm, and the tympanic cavity from an endodermal pharyngeal pouch.  

• The outer ear is derived from ectodermal growths.

RESEARCH PAPER: FIND IT EAR

• Objective: identify & characterize specific cell types within otic organoids that resemble inner ear sensory cells. 

• Methodology: 

  • Utilize immunostaining techniques detect the presence of hair cell markers such as Myosin VIIa (MYO7A)

• Experimental Design: Grew inner ear organoids from human stem cells.

• Techniques Used:

  • Immunohistochemistry (IHC) was employed to detect specific protein markers:

    • MYO7A (Myosin VIIA) for hair cells

    • SOX2 for supporting cells

  • Confocal microscopy was used to visualize and confirm the spatial localization of labeled cells within the organoid structure.

• Findings:

  • Hair cells (MYO7A-positive) were present.

  • Supporting cells (SOX2-positive) were surrounding them.

  • The pattern looked like the natural layout in a real inner ear.

DEVELOPMENTAL DEFECT OF THE EAR

• Microtia

  • Congenital ear deformity 

  • External part of ear (pinna or auricle) not fully developed

  • Varying degrees

  • Conductive or mixed hearing loss 

  • Ear canal absence can occur as well

  • Cause: vascular insults or medications during pregnancy