Eye Development and Structure

There are three primary germ layers: ectoderm, mesoderm, and endoderm.
Ectoderm: a single cell layer thick, proliferates to form a new outer layer called periderm, and an underlying proliferating basal layer.
Mesoderm: gives rise to a loosely woven tissue called the mesenchyme.
Endoderm: the inner layer.

Week 3-10: major development of the eye involves ectoderm, neural crest cells, and mesenchyme.
Ectoderm: forms the lens, conjunctival and corneal epithelia, eyelids, and lacrimal apparatus.
Mesenchyme: forms the remaining eye structures.

Day 22: two small grooves develop on each side of the developing forebrain.
Day 23: optic pits appear on the neural tube, which develop into optic vesicles.
Day 25: the optic vesicles extend from the forebrain toward the surface ectoderm.
The optic vesicles invaginate to form the optic cup, which gives rise to the retina, iris, and ciliary body.

The optic cup develops into a double-layered structure.
The outer layer becomes the retinal pigment epithelium (RPE).
The inner layer becomes the neural retina.
The cells of the neural retina differentiate into photoreceptors (rods and cones), Müller supporting cells, and bipolar neurons.

The lens develops from the surface ectoderm.
The lens placode invaginates to form the lens pit, which eventually forms the lens vesicle.
The lens vesicle separates from the surface ectoderm and forms a ball of epithelial cells.

The retina develops into 10 distinct layers.
The outer layer remains as a single layer and becomes the retinal pigment epithelium.
The inner layer undergoes a complicated differentiation into the other nine layers.

The rim of the optic cup extends forward between the mesenchyme and the anterior lens surface.
The anterior chamber forms and deepens.
The mesenchyme surrounding the optic cup condenses into two layers: the choroid and the sclera.
The corneal endothelium, stroma, and epithelium develop from the mesenchyme and surface ectoderm.

The anterior rim of the optic cup gives rise to the non-pigmented epithelium of the iris and ciliary body.
The outer layer forms the pigmented epithelium of the iris and ciliary body.
The stroma of the iris and ciliary body develops from neural crest cells.

Aphakia: failure of the surface ectoderm to invaginate and form the lens.
Cataract: failure of the primary lens fibers to elongate and abnormal disposition of the lens fibers.
Peter's Anomaly: failure of the lens stalk to disintegrate and release the ball of epithelial cells.
Microphthalmos: failure of the neural ectoderm to fuse with the surface ectoderm.
Congenital cystic eye: failure of the optic vesicle to invaginate.### Embryonic Development of the Eye
The iris is derived from the mesenchyme and the ciliary muscle is derived from the neural crest.
The color of the iris is determined by the amount of melanin distributed in the stroma of the iris (posterior epithelium).
Microcoria is a condition where the dilator pupillae muscle fails to form, and Aniridia is a condition where the rim of the optic cup fails to develop.

At the 6 mm (31/2 week) stage, a network of capillaries encircles the optic cup and develops into the choroid.
By the third month, the intermediate and large venous channels of the choroid are developed and drain into the vortex veins to exit from the eye.

The vitreous body forms in the center of the optic cup posterior to the lens.
Vitreous is comprised of a gel-like substance called vitreous humor derived from cells of neural crest origin.
Secondary vitreous is secreted by the mesenchyme and compresses the primary vitreous and remains of the hyaloid vasculature into a thin column called Cloquet's canal.
Collagen secreted by mesenchyme forms the sclera.

Extraocular muscles develop from three preotic somites.
Each preotic somite is supplied by its own cranial nerve (III, IV, and VI).
The somite supplied by the III cranial nerve forms 4 of the 6 extraocular muscles, while the remaining two each give rise to one muscle.

The eyelids begin to form at the 6th week from the neural crest cells and surface ectoderm.
The meibomian gland and accessory glands of the lids form from lid epithelial cells that migrate into the lid stroma.
The lacrimal gland is formed by an epithelial invasion of the prospective orbital space.

At 3 weeks, a bulge in the diencephalon wall forms.
At 4 weeks, the optic vesicle forms, and at 4.5 weeks, the optic cup forms.
At 5 weeks, the lens vesicle, RPE, corneal epithelium, and primary vitreous form.
At 6 weeks, the neural retina begins to differentiate, and the lens vesicle cavity is filled with primary fiber cells.
At 7 weeks, the corneal endothelium forms, and at 7.5 weeks, the ganglion axons reach the LGN.

Neuroectoderm derivatives: pigmented epithelia of retina, ciliary body, and iris, sensory retina, optic nerve, and iris sphincter and dilator muscles.
Surface ectoderm derivatives: lens, corneal epithelium, conjunctiva, and caruncle.
Head mesenchyme derivatives: blood vessels, corneal stroma, descemet's membrane, endothelium, ciliary muscle, sclera, and extraocular muscles.

Coloboma: a condition where the choroid fissure does not completely close, resulting in a defect in the inferior part of the iris.
Congenital glaucoma: caused by abnormal development of the iridocorneal angle structures, resulting in high intraocular pressure.
Congenital cataracts: caused by improper growth of the lens fibers, often due to rubella infection or recessive mutant genes.
Congenital detached retina: occurs when the intraretinal space persists.
Partially persistent iridopupillary membrane: a condition where the iridopupillary membrane does not dissolve completely.
Persistent hyaloid artery: a condition where the distal part of the hyaloid artery does not disappear, resulting in impairment of vision and possible hemorrhages.
Microphthalmia: a condition where the eye is unusually small, often associated with other ocular defects.
Peter's anomaly: a condition where the lens vesicle does not pinch off from the surface ectoderm, resulting in a white mass where the undersurface of the cornea is connected to the anterior aspect of the lens by a stalk.