Development of the Eye - Overview
The major structures of the eye arise from three main embryonic tissues.
- Optic Cups (light blue), which are outgrowths of the forebrain, give rise to the pigment epithelium, sensory receptors and neurons in the retina, iris and ciliary apparatus.
- Surface Ectoderm (violet), gives rise to the refractive components of the eye, the lens and cornea
- Head Mesoderm (pale red) produces the sclera, choroid and vitreous body as well as the ciliary muscles
The eyes begin development as outgrowths of the diencephalon around day 22 when the optic grooves appear. These expand into the optic vesicles that invaginate to form optic cups.
Since they maintain close contact with the surface ectoderm, the optic cups induce the successive formation of the lens and epithelial surface of the cornea.
The optic cup is connected to the diencephalon as by way of the optic stalk, which will eventually become the optic nerve.
Blood vessels (hyaloid vessels) pass ventrally into the optic stalk through the choroid fissure to nourish the developing eye.
The surrounding head mesoderm also contributes to the overall structure of the eye, giving rise to the choroid, a heavily pigmented layer and the sclera, the outer covering of the eyeball.
Development of the Eye - Day 31 to Day 34
The four panels making up the slide show below review the development of the eye through days 31 to 34. They are value added for those who are interested in following the progression of eye development, but not crucial to understanding which primordia form which structures.
You can advance the slide by a mouse click or using the arrows at the right or left side of the panel.
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Start Slide Show
Around day 31, the optic vesicle begins to invaginate to form the optic cup. Simultaneously, surface ectoderm thickens to indicate induction of the lens placode.
The outer layer of the optic cup will form the pigment layer of the retina, while the inner layer of the optic cup will form the neural retina.
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By day 32, the inner and outer layers of the optic cup have expanded to narrow the intraretinal space.
The lens has become vesicular, sinking beneath the surface ectoderm.
The hyaloid vessels approach the developing eye to enter the optic stalk through the choroid fissure. They will become the central retinal vessels. -
Once the lens vesicle has sunk into the optic cup, around day 34, the surface ectoderm begins to thicken to form the epithelium of the cornea.
The inner and outer layers of the optic cup eventually fuse to obliterate the intraretinal space.
Clinically this is where most retinal detachments occur.
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In the newborn the eye is completely formed. The neural and pigment layers of the retina formed by differentiation of the inner and outer layers of the optic cup. The iris and ciliary body also form from the optic cup.
Mesoderm of the head region condenses to form the sclera and choroid layers of the eyeball.
Cornea
The cornea is the anterior, transparent surface of the eye that is continuous posteriorly with the sclera.
Three layers form it
- Layer of surface ectoderm
- Substantia propria – continuous with the sclera (mesoderm)
- Inner layer of epithelium
Anterior chamber is fluid filled space between the cornea and the anterior surface of the iris.
Posterior chamber is between the iris and the lens.
Both spaces are filled with aqueous humor.
Iris and Ciliary Body
The iris develops from both layers of the optic cup. The outer layer of optic cup forms the pigmented layer of the iris. The inner layer of optic cup forms the non-pigmented layer of the iris.
Mesenchyme of neural crest origin forms the anterior surface of the iris and is continuous with the sclera
forms from the underlying Neuroectoderm from the optic cup forms the smooth muscle of the iris, dilator and constrictor papillae muscles.
The ciliary muscles form from mesoderm of the head
Optic Nerve
The optic vesicle is connected to the diencephalon by way of the optic stalk.
On the ventral surface of the optic stalk hyaloid vessels pass into the choroid fissure to reach the developing retina.
They become enclosed within the optic nerve when the choroid fissure closes in week 7 of development and are thereafter called the central retinal vessels, which enter the eye in the region of the optic disc.
Optic nerve fibers, originating in the ganglion cells of the retina, travel back to the CNS in the optic stalk. The inner layer of the optic stalk gives rise to the glial cells of the optic nerve. This layer is continuous with the pia arachnoid surrounding the brain.
The outer layer of the optic stalk is continuous with the sclera of the eye and dura mater covering the optic nerve.
The optic nerve is considered to be equivalent to a nerve tract rather than to a typical peripheral nerve because of this relationship to the meninges.
Clinical Correlates
Retinal Detachment
Detachment of the retina, (red arrow) as shown in this ultrasound image of the eye, is a serious condition in which the neural retina separates from the pigment epithelium and choroid, which is the location of blood vessels that nourish the retina. This is a naturally occurring plane that during development began as the intraretinal space, between the inner and outer layers of the optic cup, but which is normally obliterated as development of the eye is completed. For orientation, the anterior chamber of the eye is indicated by a yellow arrow.
Coloboma Iridis
Failure of the choroid fissure to close produces this anomaly, also known as 'keyhole iris'. It is characterized by an inferior defect in the iris.
Iridopupillary Membrane
This is due to failure of mesenchyme to completely degenerate when forming the anterior chamber of the eye, leaving a network of fibrous strands covering the iris.