Embryology of the Ear and Pharyngeal Structures

Embryonic Development of the Eye

• The 22-day embryo develops a pair of shallow grooves on the sides of the forebrain.
• The neural tube closes, creating optic vesicles.
• Optic vesicles contact surface ectoderm, initiating lens development.
• Optic vesicles invaginate, forming a double-walled optic cup.
• The intraretinal space disappears, merging the two cup layers.
• Choroid fissure formation facilitates hyaloid artery access to the inner eye chamber.
• By week seven, the optic cup mouth becomes the pupil's opening.
• Surface ectoderm cells elongate, forming the lens placode.
• The lens placode invaginates into a lens vesicle, detaching from the surface ectoderm.
• The optic cup and lens vesicle form during the fifth week.

Retina, Iris, and Ciliary Body

• The outer optic cup layer is the pigmented retina.
• The inner layer (pars optica retinae) differentiates into light detectors (rods and cones).
• The mantle layer contains neurons and support cells.
• The deeper layer's nerve fibers converge toward the optic stalk.
• The anterior fifth of the inner layer (pars ceca retinae) remains a single cell layer thick.

Lens

• Shortly after lens vesicle formation, cells elongate, filling the lumen with fibers.
• Secondary lens fibers continue to develop throughout this time.

Choroid, Sclera, and Cornea

• The eye primordium is completely surrounded by loose mesenchyme.
• Loose mesenchyme differentiates into the choroid (inner layer) and sclera (outer layer).
• The anterior chamber forms via the mesenchyme splitting.
• The anterior chamber has an inner layer and an outer layer.
• In addition, it forms the corneal epithelium, stroma and endothelium.

Vitreous Body

• Mesenchyme invades the optic cup via the choroid fissure, developing hyaloid vessels.
• These support the lens and retina in intrauterine life.
• The hyaloid vessels are eventually obliterated.
• Interstitial spaces fill with vitreous humor forming the vitreous body

Optic Nerve

• The optic stalk forms the optic nerve.
• Nerve fibers grow progressively, filling the optic stalk.
• The central retinal artery is a continuation of the hyaloid artery.

Molecular Regulation of Eye Development

• PAX6 is a key regulatory gene, with DNA-binding motifs.
• It's initially expressed in a band in the anterior neural ridge.
• Later it separates into two optic primordia.
• SHH expression affects PAX2 and PAX6 expression in the eye field.
• The optic vesicle interaction with surrounding mesenchyme and ectoderm regulates development.
• FGF promotes neural retina development, and TGF-β influences pigmented retina development.
• MITF and CHX10 are downstream transcription factors.

Coats of the Eyeball

• The fibrous coat (cornea and sclera) protects the intraocular content.
• The cornea is the anterior, transparent part of the fibrous coat.
• The sclera is the posterior, opaque part of the fibrous coat.
• The vascular coat (uvea): Composed of iris, ciliary body, and choroid.
• The choroid is highly vascular, nourishing the retina.
• The retina (nervous coat) plays a crucial role in visual function.

Segments and Chambers of the Eyeball

• The anterior segment includes the lens, iris, and cornea, and two aqueous humor-filled chambers.
• The aqueous humor nourishes the cornea and lens.
• The posterior segment includes the vitreous humor and retina.