Ocular Embryology Notes

Ocular Embryology

Germinal Layers

• Eye formation starts at the end of the 3rd week.
• The eye develops from three germinal layers:
  • Surface Ectoderm
  • Neural Ectoderm
  • Mesoderm

Germinal Layer Derivatives

• Surface Ectoderm:
  • Epithelium of lens, cornea, conjunctiva, lids, lacrimal gland, and lacrimal apparatus
  • Lid derivatives: cilia, tarsal glands
• Neural Ectoderm:
  • Retina and pigment epithelium
  • Ciliary epithelium & posterior pigmented iris epithelium
  • Iris sphincter & dilator muscles
  • Neuroglial & neural components of the optic nerve
• Mesoderm + Neural Crest Cells (Mesenchyme):
  • Blood vessels
  • Sclera
  • Optic nerve sheath
  • Ciliary muscle
  • Corneal stroma and endothelium
  • Iris stroma & anterior epithelium
  • Extraocular muscles
  • Connective tissue of lids
  • Orbital walls, fat & ligaments of orbit
  • Vitreous

Week-by-Week Development

• Week 1: Fertilisation, Morula, Trophoblast, Blastocyst
• Week 2/3:
  • Implantation of blastocyst.
  • Formation of 3 primary germinal layers.
  • Neural plate and groove formation.
  • Neural folds elevate.
  • Optic pits (sulci) appear.
• Week 3:
  • Neural groove closes to form the neural tube.
  • Optic pits deepen into optic vesicles.

Week 4

• Optic vesicles push towards surface epithelium.
• Optic vesicles induce surface epithelium to form the lens placode & vesicle.
• Optic vesicle invaginates to form a double-walled cup; the choroidal fissure results.
• Pigment appears in the outer wall, and inner wall differentiation commences.
• Hyaloid artery enters choroidal fissure and vascularises the lens.
• Hyaloid artery carries mesenchyme, which with inner cup ectodermal cells, forms primary vitreous.

Week 5

• Mesenchyme coats the optic cup.
• Lens placode differentiates into lens pit, cup, and vesicle.

Week 6

• Optic cup wall differentiates into inner (nervous) and outer (pigmented) layers.
• Posterior lens cells elongate.
• Surface ectoderm forms corneal epithelium.
• The first wave of mesenchyme forms corneal endothelium and trabecular meshwork.

Week 7

• Posterior lens cells obliterate the lens cavity.
• Mesenchyme forms eyelids and extraocular muscles.
• The second wave of mesenchyme forms the corneal stroma & pupillary membrane.
• Corneal epithelium forms two layers.
• Optic fissure closes (coloboma).
• The retina begins to become layered.
• Incomplete closure of the optic fissure results in a coloboma affecting the iris, retina/choroid, and/or optic disc.

Week 8

• The third wave of mesenchyme forms the iris stroma and arterial circle (aniridia).
• The cornea changes shape; the limbus and anterior chamber form.
• Corneal stroma differentiates, and cells secrete collagenous fibrils.
• Anterior lens cells undergo mitosis.
• The optic stalk is fully occupied by nerve fibres.

Week 10

• Eyelids close.
• The formation of ganglion cell layer.
• Hyaloid blood vessels.
• Fusion of lids.

Week 12

• The lens forms secondary fibres.
• Orbicularis oculi forms.
• The anterior optic cup is complete.
• The iris sphincter muscle begins to form.
• The ciliary body & posterior iris epithelium begin formation.

Week 16

• Corneal stroma secretes Bowman’s layer.
• Mesenchymal parts of the ciliary body develop.
• Sclera & choroid continue to differentiate.
• The canal of Schlemm forms.
• Mesenchyme at the angle begins to atrophy to form the trabecular meshwork.

Week 24

• The iris sphincter pupillae has formed, and the iris dilator pupillae commences development.
• Descemet’s membrane is secreted by corneal endothelium.
• Tarsal glands of the eyelids and eyelashes are forming.
• Eyelids reopen.

Week 28

• The choroid has differentiated.
• The hyaloid artery is impermeable.
• Lacrimal canaliculi open on lid margins.

Week 32

• The hyaloid artery system disappears.
• All retinal layers are recognisable.
• The pupillary membrane atrophies.
• Foetal lens nuclei are complete.

Term

• Upper visual pathways begin myelination.
• The pupillary membrane is fully atrophied.
• The dilator pupillae is incomplete.
• The pars plana of the ciliary body is immature and continues until 5 years.

Post Term

• Accommodation is active, and infants focus on near objects.
• Corneal development continues.
• Hyaloid artery remnants float in the Canal of Cloquet.
• The lacrimal gland is underdeveloped.
• The macular region reorganises (packing of foveal cones) over the first 4 months.
• The ciliary body is complete by 5 years.
• The iris dilator pupillae muscle is complete by 5 years.
• The corneal stroma becomes less cellular.
• Lens continues to grow throughout life.

Cornea Development

• Weeks 4-5: Developing lens induces surface ectoderm to form corneal epithelium.
• Week 6: 1st wave of neural crest cells form corneal endothelium and trabecular meshwork.
• Week 7: 2nd wave of neural crest cells form corneal stroma and Bowman’s membrane.
• Week 8: Corneal stroma differentiates.
• Week 16: Corneal stroma secretes Bowman's layer.
• At Term: Corneal epithelium has 5-6 layers.
• Post Term: Corneal stroma becomes less cellular.

Crystalline Lens

• Week 4: Optic vesicle induces thickening of surface ectoderm to form the lens placode.
• Week 5: Optic cup induces lens placode to invaginate, forming the lens pit and cup.
• Week 7: Posterior lens cells lengthen and obliterate the cavity.
• Week 8: Secondary lens fibres form by mitosis of anterior epithelial cells.

Vitreous Body

• Primary vitreous forms from mesenchyme that enters the optic cup with the hyaloid artery.
• At ~2 months, its development peaks.
• Avascular secondary vitreous begins to develop.
• Primary vitreous regresses.

Iris

• Neuroectoderm from anterior cup wall forms the iris.
• Week 8: 3rd wave of neural crest cells migrate to form iris stroma.
• Week 12: Sphincter muscle forms from neuroectoderm.
• Week 24: Development of dilator muscles commences.
• Month 8: Pupillary membrane degenerates.
• Term: Dilator pupillae is still incomplete.
• Post Term: Dilator pupillae mucle is complete at 5 years.

Retina

• Weeks 4-5: Optic vesicle invaginates to form the 2 layered optic cup.
• The pigment layer of retina forms from the outer layer, and the neural layer from the inner layer.
• Anterior 1/5 of the cup forms the developing ciliary body and iris.
• Week 7: Neural layer divides into outer (ONBL) and inner (INBL) neuroblastic layers.
• The layer of Chievitz lies between the INBL and ONBL.

Optic Nerve

• Week 6: Retinal ganglion cells develop axons that extend into the optic stalk.
• Some cells of the inner layer degenerate and are replaced by invading ganglion cells, with the remaining cells forming glial cells.
• Cells of the outer layer form neuroglial cells that support the optic nerve.