Eye embryology

Developement of eye

Early Embryology & Genetics

• Initiation: begins at 22 days of gestation as an outpocketing/extension of the diencephalon (prosencephalon). 

• Key Gene: PAX6 . 

• Gene Defects: Mutations in PAX6 or related pathways (such as RAX, PAX2) lead to severe congenital malformations like anophthalmia or microphthalmia. 

• Choroidal Fissure: A ventral groove/gap in the optic stalk through which embryonic vasculature passes; it normally closes by 6–7 weeks of gestation. 

Precess

• Early -

  • As the neural tube closes, from procencephalon the lateral outpouchings(diencephalon) expand outward to form the optic vesicles on both sides. 

  • optic vesicle expands further against the surface ectoderm, to form the narrow optic stalk

  • physical contact in surface ectoderm and vesicles induces the surface ectoderm to thicken, forming the early lens placode. 

Lens formation

• Lens placode invaginates → lens pit.

• The pit then pinches off from surface ectoderm → lens vesicle. [Lens develope from surface ectoderm]

• The lens vesicle is initially a hollow spherical structure with:

  • Anterior wall

  • Posterior wall

  • Cavity of lens vesicle

• Cells of the posterior wall of the lens vesicle become elongated by converting cuboidal cells in columnar cells

• These elongated cells grow toward the anterior wall.

• They become primary lens fibers.

• cells from the equatorial region continue to form secondary lens fibers throughout life.

• No posterior wall

• The anterior wall remains as a single layer of cuboidal cells → becomes the lens epithelium.

• Eyeball close from → superotemporal to inferior nasal direction

Structure of lens

• Lens has:

  • Anterior capsule

  • Anterior lens epithelium

  • Cortex

  • Nucleus

  • Posterior capsule

• Lens epithelium is present only anteriorly.

  • There is NO posterior lens epithelium.

• At the periphery, the lens fibers meet at the equator.

• Center of anterior part - anterior pole

• Center of posterior part - posterior pole

• The central older part is the nucleus.

• The peripheral newer fibers form the cortex.

types of nuclei of lens

1. Embryonic nucleus

   • Formed earliest.(0-3 monthsof gestation)

   • Corresponds to the earliest embryonic lens fibers. (Oldest)

2. Fetal nucleus

   • Formed during fetal life.(3-8 months of gestation)

   • Surrounds the embryonic nucleus.

3. Infantile nucleus

   • Fibers formed after birth during early childhood. (8 months to puberty)

4. Adult nucleus

   • Continues to form after childhood.(Puberty to adults)

   • Newer fibers are added around the older nucleus.(Youngest)

Lens sutures

• Sutures are microscopic junctions/gaps between lens fibers.

• They are formed because the ends of lens fibers meet in characteristic patterns.

From front surface

• Has an upright Y-shaped suture.

From back surface

• Has an inverted Y-shaped suture.

Strongest basement membrane of body

Lens capsule

During developent which vessels supply lens?

• Hyaloid artery - 3 branches

  • Anterior pupillary vessels

  • Capsulopupillary vessels

  • Posterior pupillary vessels

• After developement , lns become avascular

Developed lens nutrition

• The lens is avascular.

• Therefore, it does not have its own blood vessels.

• It receives nutrients by diffusion.

Main sources:

Anterior lens
→ mainly from aqueous humor

Posterior lens
→ mainly from vitreous humor

[Option choice → AH+VH > AH > VH]

Which connective tissue present all around eye balll,lens in orbit?

Mesenchyme. It will further differentiate in neural crest and mesoderm.

Neural Crest vs Mesoderm

Neural crest	Mesoderm
Ciliary muscle	Primary vitreous
Trabecular meshwork	Temporal sclera
Orbital bones	Extraocular muscles
Stroma of ciliary body , supercilliary lamina	Endothelium lining ocular blood vessels
Sclera (except temporal part)
Choroid
Corneal stroma
Corneal endothelium

Condensed part of viterous called. As

Hyaloid membrane

Remains of hyaloid artery affter regression

Cloquet canal

PHPV (Persistent hyperplastic primary viterous)

• During normal eye development, the primary vitreous and hyaloid vascular system normally regress.

• If this Entire artery regression/resorption does not occur → PHPV develops.

• Your notes mention:

  • Non-resorption of hyaloid artery/system

  • Association with unilateral microphthalmos.

• It can cause:

  • Vitreous haemorrhage

  • Poor visual prognosis.(because retinal detachment and cataract)

Persistent partial hyaloid vessels

• Muscae volitantes

  • A small fragment of the persistent hyaloid vessel can remain.

  • It may appear as floaters in the visual field.

  • Hence:
    Persistent hyaloid vessel fragment → Muscae volitantes (floaters).

• Mittendorf’s dot

  • The anterior part of the hyaloid vessel persists.

  • It remains attached to the posterior lens capsule.

  • Seen as a small white dot on the posterior lens capsule.

• Bergmeister’s papilla

  • The posterior part of the hyaloid vessel persists.

  • It remains attached near the optic disc on retina.

  • Appears as a small tuft/papilla over the optic disc.

Retina formation

• Optic stalk convert in optic nerve

• Optic cup is made up of neuroectoderm,so. Retina also neuroectodermal

• The developing optic cup has two layers:

  • Outer layer

    • Becomes the retinal pigment epithelium (RPE).

  • Inner layer -

    • Becomes the neuro-sensory retina.

    • Made up of 9 layers

Formation of Ciliary body

• Innermost(Inner limiting membrane) and outermost layer(Retinal pigmented epithelium) of retina further invaginate to form ciliary body

• Layers of ciliary body (5) →

  • Inner limiting membrane

  • Non-pigmented epithelium (middle 8 layer of retina form 1 layer)

  • Retinal pigmented epithelium

  • Ciliary body stroma → derived from neural crest/mesenchyme.

  • Supracilliary lamina → derived from neural crest/mesenchyme.

• Ciliary muscles →

  • use for accomodation

  • Secreted from stroma of ciliary body (Neural crest origin)

Formation of iris

• Only non pigmented and pigmented layers moves forward to form iris but Nonpigmented convert in pigmented and pigmented convert in non pigmented

• Hence, inner layer of iris is pigmented and outer is nonpigmented

• Above that mesenchyme will form stroma and anterior limiting layer

• All layers of iris →

  • Pigmented layer

  • Non pigmented layer

  • Stroma

  • Anterior limiting layer

• Iris muscles →

  • Secreted by anterior non pigmented epithelium

  • 2 types →

    • Spinchter pupillae

    • Dilator pupillae

Formation of viterous humor

Primary vitreous

• Present till hyaloid artery remains.

• mesodermal if hyaloid artery present.

Secondary vitreous

• Forms the major adult vitreous(after hyaloid artery regress).

• neuroectoderm origin as secreted by optic cup

Tertiary vitreous

• Forms the zonular fibres.

• This hyaloid zonules are condensed viterous which surrounded around lens forms suspensory ligaments of lens

• 3 parts →

  • Anterior zonular limb

  • Equatorial zonular limb

  • Posterior zonular limb

Canals and spaces in eye

1. Between pre-equatorial and post-equatorial zonule

→ Hannover’s canal

2. Between hyaloid zonules and posterior/equatorial limb

→ Petit’s canal

3. Area where lens is situated, formed by condensed vitreous

→ Patellar fossa

4. Space between anterior hyaloid membrane and patellar fossa

→ Berger’s space

Developement of cornea

• In front of lens ,2 waves of neural crest cells form to make corneal layer

• 1st wave

  • Neural crest cells migrate into the developing cornea.

  • They form the corneal endothelium and Dua’’s dasement membrane

• 2nd wave

  • Another population of neural crest cells migrates into the cornea.

  • They form the corneal stroma and Bowmans membrane

• Outer that Corneal epithelium present.

• All layers in order →

orneal structure	Origin
Epithelium(Outer)	Surface ectoderm
Bowman’s membrane	Anterior stroma / neural crest
Stroma	Neural crest
Descemet’s membrane	Corneal endothelium / neural crest
Endothelium (Inner)	Neural crest

Formatiuon of eyelids,conjunctival sac, lacriomal gland

• Eyelid formation -

  • Upper and lower eyelid folds develop around the eye from surface ectoderm.

  • They grow toward each other.

  • Eventually, the eyelids fuse temporarily

• Lacrimal gland formation -

  • The lacrimal gland develops from an epithelial bud of surface ectoderm at juction of fold.

  • It develops from the superolateral conjunctival epithelium.

  • The epithelial bud grows into the surrounding mesenchyme.

  • It branches repeatedly → forms the lacrimal gland and its ducts.

• Conjuctival sac formation →

  • The eyelids initially fuse.

  • The space between the eyelids and eyeball develops into the conjunctival sac.

  • When the eyelids separate, the conjunctival sac becomes continuous with the external environment.

Ankyloblephron

Ankyloblepharon = abnormal adhesion/fusion of the eyelid margins.

Normally:

• Eyelids fuse temporarily during development.

• They later separate.

If separation fails or is incomplete:
→ Ankyloblepharon

Symblephron

• Adhesion between the palpebral conjunctiva and bulbar conjunctiva.

Coloboma

Coloboma = defect caused by failure of closure of the optic fissure

It can involve:

• Iris

• Lens

• Eyelid

• Optic nerve

• Choroid, etc.

Important examples

Iris coloboma

• Typical inferonasal/keyhole-shaped defect.

• Due to failure of closure involving the optic fissure.

Lens coloboma

• Deficiency of lens tissue, commonly associated with abnormal zonular development.

Choroid coloboma

• Can produce a visual field defect.

Eyelid coloboma

• Defect in the eyelid tissue.

Myelinated nerve fibers

• Myelination starts around the lateral geniculate body during 7 months intrauterine life.

• It then progresses toward the eye.

• After birth, it normally reaches the lamina cribrosa.

• Normally, myelination does not cross the lamina cribrosa.

If myelination crosses the lamina cribrosa

The myelinated fibres become visible on the fundus.

They appear as:

• White

• Feathery

• Flame-like patches around the optic disc.

Clinical significance

They can produce:

• Pseudopapilledema

• Pseudopapillitis

• Enlargement of the blind spot

Summary