Embryology - Part 1 (The Retina)

When do the 3 primary germ layers form during embryonic development?

3rd week

→ ectoderm, mesoderm & endoderm form the embryonic plate

Which germ layers contribute to developing ocular structures?

only the ectoderm and mesoderm

How is the Neural plate formed?

Ectoderm thickens to form the neural plate, which gives rise to → CNS + ocular structures

How is the Neural fold formed?

• optic groove forms down the middle of the neural plate

• ridges on each side become neural folds

When are the Neural groove + Neural folds formed by?

Day 18

When does the first indication of eye development appear?

Day 22

How is the Neural tube formed?

1. Groove expands, causing these folds to grow towards each other

2. Neural crest cells (cells on the crest of each neural fold) separate from the ectoderm

3. Folds fuse to form the neural tube

   • Forebrain forms as part of the anterior neural tube

4. Neural crest cells migrate & form discrete “islands” of cells in the mesoderm

What are the tissues called after the Neural tube forms?

Neural tube tissue = Neural ectoderm

Surface layer above neural tube = Surface ectoderm

When is the neural tube formed by?

Day 22

How is the Optic Pit formed?

Forebrain evaginates → optic pit

What ocular structures does the Surface Ectoderm give rise to?

Lens

Epithelium of eyelids, cilia, meibomian, Zeis & Moll

Epithelium lining nasolacrimal system

Corneal epithelium

Conjunctival epithelium

What ocular structures does the Neural Ectoderm give rise to?

Iris sphincter & dilator muscles

RPE

Optic nerve fibers

Neural retina

Neuroglia

Epithelium of ciliary body

Epithelium of Iris

What ocular structures does the Neural Crest give rise to?

Corneal stroma (which gives rise to Bowman’s layer)

Uveal pigment cells

Sclera (most)

Trabecular structures

Uveal CT

Meninges of optic nerve

Corneal endothelium (which gives rise to Descemet’s membrane)

Vascular pericytes

Ciliary muscle

By day 28, what’s most visible?

Forebrain + its covering layers (mesenchyme and surface ectoderm)

What 2 structures begin forming after the forebrain coverings develop? When does this happen?

Day 28-32

1. Optic cup

2. Lens vesicle

   

Lens

What is the sequence of lens formation?

Note: This occurs after the forebrain coverings.

Lens placode → Lens pit → Lens vesicle (which closes to form the lens)

Optic Cup

What are the 2 layers of the optic cup, and what do they form?

Note: This occurs after the forebrain coverings.

1. Outer layer → future RPE

2. Inner layer → future neural retina

When does the optic cup first appear externally?

32 days

Optic Cup

What is the optic (choroidal) fissure? What vessels enter through the optic fissure?

→ groove along the inferior surface of the optic cup & optic stalk that allows blood vessels (hyaloid artery & vein) to enter and supply the newly formed lens 

Note: Optic fissure must close

By when does the Optic fissure normally close?

6th week

We know that the Optic fissure must close. What happens if it fails to close?

Iris coloboma develops

Optic Cup

How is the Optic nerve formed?

1) Neural retina starts forming from the inner layer of the optic cup, where RGCs develop

2) RGCs send out axons towards the brain

3) To reach the brain, they must pass the optic stalk

4) As more axons grow through the optic stalk, the lumen of the stalk disappears (involution)

5) Bundled RGC axons → optic nerve fibers

What do the hyaloid artery & vein develop into? What do these new vessels supply and drain?

CRA & CRV → supply & drain the inner retina

Lens

Recall: Lens placode → Lens pit → Lens vesicle (which closes to form the lens)

In detail, how is the Lens vesicle formed?

1) Surface ectoderm (SE) adjacent to the optic vesicle thickens → lens placode (LPL) forms

• Thickening occurs due to:

  • ectodermal cells elongate

  • ↑ regional cell div

2) SE progressively invaginates, forming → Lens pit (LP) & Lens vesicle (LV)

Lens

What happens to the posterior cells of the Lens vesicle (LV) during early lens development?

Elongate → 1er lens fibers

• fibres fill up the lens vesicle’s lumen → embryonic nucleus formed (small dots)

Lens

What is the lens bow? What’s the typical bow angle?

→ curved line formed by the cell nuclei of elongating 1er fibers

• 120–130°

Lens

What happens to the Anterior cells of the Lens vesicle (LV) during early lens development?

1) Anterior epithelium stays in place, while posterior epithelium elongates

2) Undergo mitosis near the equator to form “new epithelial cells”

Lens

How is the 2er lens fiber formed?

1) New cells at the equator (formed anterior epithelium mitosis) elongate anteriorly and posteriorly

2) Elongated cells → 2er lens fibers

3) 2er lens wrap around the embryonic nucleus in concentric layers → growing lens mass

4) Lens are formed where the fiber ends meet.

When are the secondary lens completed?

Week 7

Zones of Development of Lens Epithelium

Describe the Central zone in the lens epithelium.

→ occupies 80% of the front surface of the lens, sitting right under the lens capsule (not part of the lens fibers yet)

• Cells are arrested (non-dividing)

• Exposed to incoming light via the pupillary aperture

  • only zone not protected by the iris from DNA-damaging UV/visible light

Zones of Development of Lens Epithelium

Describe the Pre-Germinative Zone in the lens epithelium.

→ Lies concentric to the central zone

• Cells divide to produce new anterior epithelial cells as the lens surface increases in size with age

  • these are a source of new anterior epithelial cells for the Central zone

    

Zones of Development of Lens Epithelium

Describe the Germinative Zone in the lens epithelium.

→ Zone of active mitosis

• cells divide and migrate toward the lens equator

  • protected from DNA-damaging light since they’re behind the iris

Zones of Development of Lens Epithelium

Describe the Transition Zone in the lens epithelium.

→ divided anterior epithelial cells → 2er lens fibers

• 2er lens fiber are non-mitotic + have no nuclei

  

What do the ends of the lens fiber meet in? Where are they found?

Upright Y-suture (posterior to the anterior epithelium)

Inverted Y-suture (anterior to the posterior capsule)

When are Y-sutures visible?

3rd month

How does the lens shape change as fibers are added? How does this affect vision?

Spherical → ellipsoidal as new fibers are added

• Spherical lens = better for near vision

• Elliptical lens = better for distance vision

How is the Lens capsule formed?

from secretions of the lens epithelium

When is the lens capsule evident by?

5 weeks

Compare the following nuclei:

• Embryonic

• Fetal

• Infantile

• Adult

Embryonic: formed from the original cavity of the lens vesicle; filled by 1er lens fibers

Fetal: has Y-sutures + all fibers formed before birth

Infantile: has fibers formed from birth to puberty

Adult: has fibers formed from post-puberty to adulthood

Describe the development of Zonule Fibres. 

Origin: near the ora serrata

→ Extend from valleys b/w ciliary processes to attach to the lens capsule (near the lens equator)

• form a 90^o with the Marginal bundle of Drualt

• aka: 3er vitreous fibers (b/c arise within the vitreous)

When are Zonules formed by?

7th month

How does the Hyaloid artery develop?

Branch of the internal carotid artery enters the optic cup via the fetal fissure → forms hyaloid artery

How is the Posterior tunic (“blood supply to back of lens”) formed?

Hyaloid forms a highly branching vascular network that fills the vitreous cavity (behind the lens) → Posterior vascular tunic

• vessels + branches make up most of the space b/w the lens & neural ectoderm (developing retina)

How is the Anterior vascular tunic?

1) Branches of hyaloid artery near the lens equator anastomose with the annular vessel at the margin of the optic cup

2) Annular vessel sends loops forward onto the anterior surface of the lens → forms the anterior vascular tunic

3) These vascular networks carry nutrients to the developing lens until the aq humor begins to form

4) Hyaloid & annular vessels regress as the eye matures

Where do the Anterior tunic vessels (hyaloid + annular vessel) drain, and what structures are found there?

drain into a network found in the region that will become the ciliary body

When is the Anterior Vascular tunic formed?

7th week

When is the hyaloid vasculature fully formed?

End of 2nd month

→ Entire hyaloid system = posterior & anterior vascular tunic fully formed by then

When does the vascular primary vitreous reach its greatest extent, and what happens in the vitreous at this stage?

• By 2^nd month, 1er vitreous becomes fully vascularized by the hyaloid system

• Avascular 2er vitreous begins to form

  • has a thinner/finer fibrillar composition

  • creates a narrow zone b/w the peripheral hyaloid vessels & retina

What key changes occur in the vitreous during the 4th month of eye development?

1. Hyaloid vessels regress

2. Zonular fibers (3er vitreous) start to stretch from the growing ciliary region toward the lens capsule

Compare the 1er, 2er, 3er vitreous.

1er vitreous → vascular (contains hyaloid system)

2er vitreous → avascular, more fibrillar

3er vitreous → includes zonules (zonular fibers)

What vascular changes occur as the hyaloid system regresses?

1) Hyaloid vessels enter through the optic nerve + connect with 1er vitreous, forming small loops into the developing retina 

2) As the hyaloid system regresses:

• loops shrink and disappear

• CRA & CRV enter

What happens to the hyaloid system by the 7th month of development?

• No blood flow remains in the hyaloid vasculature

• Hyaloid system is fully reabsorbed

• CRA & CRV take over as permanent retinal vessels

What is the first retinal layer to differentiate?

RPE

Development of Neural Retina

Describe the development of the RPE. 

1) Cellular structures and melanosomes begin to appear in the outer layer of the optic cup

• Melanosomes: melanin-producing cells inside RPE

2) After week 6, the RPE is 1 cell thick, where they’re cuboidal to columnar 

• Base of each RPE cell = external toward the developing choroid

• Apex of each RPE cell = internal toward the inner layer of the optic cup

Development of Neural Retina

When does the pigmentation of the RPE occur? What is this known as?

Weeks 3 - 4

“Earliest pigmentation evident in the embryo”

Development of Neural Retina

When do the inner and outer neuroblastic layers form in eye development?

Week 7: Cells from the neural ectoderm migrate to form the Inner & Outer neuroblastic layer

By month 3: Formation of neuroblastic layers (inner and outer) is complete

• neural retinal cells will begin to differentiate in the central retina and proceed to the periphery

Development of Neural Retina

What is the fiber layer of Chievitz?

Transient, uneven, nucleus-free area that lies b/w inner & outer neuroblastic layers

Development of Neural Retina

At 2.5 months, what do the following cells differentiate with?

• Uppermost cells (above inner neuroblastic layer)

• Below Chievitz layer

• Outer neuroblastic layer

Uppermost cells → RGCs (form the optic nerve)

Below Chievitz layer → Amacrine & Müller cell

• b/w shifted nuclei of amacrine & Müller cell → Inner plexiform 

Outer neuroblastic layer → photoreceptor, bipolar & horizontal cells

Development of Neural Retina

What happens to retinal layers around midterm (4.5 months)?

Retinal lamination is complete

Development of Neural Retina

Describe the changes in the retinal layers at 4.5 months.

• RGC layer

• Inner Plexiform

• Inner Nuclear

• Outer Plexiform

• Outer Nuclear

• Photoreceptors

Development of Neural Retina

Describe the changes in the retinal layers at 5.5 months.

• Ganglion cells

• Inner Nuclear

• Outer Plexiform

• Outer Nuclear

• Photoreceptors

  

Development of Neural Retina

Describe the Newborn retina. 

→ has adult configuration + fully layered

• Nerve fiber layer: forms from RGC axons

• Vascularization extends to outer limits of the INL (arrowheads)

• OPL

  • thinner than adult retina → will thicken by ~4 years

  • well-established synapses

Development of Neural Retina

Describe the photoreceptor outer and inner segments in the Newborn retina.

• fully developed

• its tips contact RPE → outer segment renewal

  • Old outer segment discs are phagocytosed by RPE

  • New discs form at the base and migrate outward

Draw out the Retinal Development Flow chart. Mention the developing structures, retinal cells and the layers they form in the adult retina.

List the 3 Foveal Development Stages.

1) Displacement of inner retinal components → forms foveal depression

2) Migration of photoreceptors toward the center → ↑ cone packing

3) Maturation of photoreceptors

Foveal development

Describe Foveal development at 25 Weeks Gestation.

• CRA nourishes inner retina

• Choriocapillaries nourish the outer retina

• No foveal pit formed yet

Foveal development

Describe Foveal development at 30 Weeks/ 7th month Gestation.

• Depression forms at the center of the retina → foveal pit

  • occurs due to displacement of inner retinal neurons (ganglion + bipolar cells)

  • Ganglion cell layer and INL move towards macular periphery

• Cone packing increases at the center → maximum cone density

Foveal development

Describe Foveal development at 40 Weeks WG to 6 months PG.

• Foveal depression present BUT not fully deep at birth (deepens more at 6 mons)

  • 1 interrupted row of ganglion cells remains across foveal depression

  • 1–2 layers of bipolar cells still extend across the fovea

• OPL (Henle’s fiber layer) is wide and well-developed

• Elongated cone inner and outer segments

• Parafoveal area has large accumulation of ganglion and inner nuclear cells → indicates mature macula

What is the Foveola?

→ central part of fovea (site of sharpest visual acuity)

• last retinal area to mature

Describe the Foveola before birth.

• Large rod-free area

• As cone density increases as they migrate towards the centre → rod-free zone narrows

Describe the Foveola after birth.

1 week old: cone population is ~18 cones (diameter of rod-free zone ≈ 100 μm)

4–5 years: cone population increases to ~42 cones (adult level) (diameter shrinks)

• Perifovea and parafovea begin to show rod presence, but the foveola stays rod-free

Retinal Vessels

What is the pre-cursor of the CRV?

branch of the primitive maxillary vein within the optic stalk

Retinal Vessels

When do retinal vessels (CRV & CRA) begin developing?

Early 4th month

Retinal Vessels

How do retinal vessels (CRV & CRA) begin developing?

1. Primitive retinal vessels emerge from the hyaloid artery near the optic disc

2. These vessels enter the developing nerve fiber layer

3. Retinal vessels form arterioles, venules, and capillary beds

Note: Nasal retinal vessels develops before the temporal retinal (nasal matures first)

Retinal Vessels

When is the full vascular structures of the retina arterioles, venules & capillary beds developed by?

~3 months after birth

What signals and direct the retinal vessels? What do they signal and direct?

Guidance molecules signal and direct:

• growth and path of neurons

• growth and branching of retinal vessels

What is induction?

→ influence of one developing structure on another via signaling molecules

• occurs via a series of biochemical steps, not a single event

Describe the role of Induction in Embryological Development.

→ Development depends on differentiation and interaction b/w tissues

• Some tissues develop only when near another developing structure at the right time

  • Ex: Lens release signaling molecules that induce corneal development

• Contact or proximity b/w tissues allows biochemical signaling