The eye is formed from three different germ layers, namely neuroectoderm, surface ectoderm and mesoderm with contribution from the neural crest cells. The structures originating from the different layers are:
Retina including the retinal pigment epitheliumQ
Epithelium of ciliary body
Epithelium of the iris
Sphincter and dilator pupillae musclesQ
Secondary and tertiary vitreous
Epithelium of conjunctiva
Epithelium of cornea
Skin of eyelids
Neural crest cells
Stroma, Descemet's membrane and endothelium of the cornea
Angle of anterior chamberQ
Stroma of the irisQ
Ciliary body and choroid
Walls of the orbit
Connective tissue of the orbit
- The development of the eye starts at about the third week of gestation. The neural tube which forms the forebrain gives rise to one diverticulum on either side known as the optic vesicleQ.
- The optic vesicle (neuroectoderm) meets the surface ectoderm which shows an area of thickening called the lens placode.
- The optic vesicle invaginates to form the two layered optic cupQ. Eventually, the inner layer of the cup forms the neurosensory retinaQ whereas the outer layer forms the retinal pigment epitheliumQ. It then continues backward as the optic nerve with its meninges to the brain. The anterior end of the cup later differentiates into the ciliary epithelium, iris epithelium and muscles of the irisQ.
- The invagination of the optic cup however remains incomplete inferonasally in the form of a fissure known as the embryonic fissureQ. Through this fissure, the hyaloid artery passes to provide nutrition to the developing ocular structures. Eventually, the hyaloid artery disappears and the embryonic fissure closes. The space between the lens and optic cup becomes filled by a clear jelly called the vitreous which is mainly secreted by the neuroectoderm.
- After formation of the lens vesicle, there is migration of the waves of neural crest cellsQ. These cells eventually differentiate into the cornea, angle structures and stroma of the iris and ciliary bodyQ.
- While the ectodermal events are taking place, the mesoderm surrounding the optic cup differentiates to form the sclera, extraocular muscles and orbital structuresQ.
Embryonic Remnants in the Eye
- Mittendorf's dotQ: It is the remnant of the anterior end of the hyaloid artery and remains attached to the posterior pole of the lens.
- Bergmeister papillaQ: It is the remnant of the posterior end of the hyaloid artery. It remains attached to the optic disc associated with some glial tissue.
- Persistent hyperplastic primary vitreous (PHPV): Failure of the foetal vasculature to regress is called PHPV (explained in detail in the chapter on Retina).
- Coloboma: Failure of the embryonic fissure to close gives rise to ocular coloboma.