Embryology of the Eye - Manufacturing Vision
In our journey to understand the awesome phenomenon of sight, we spoke about the evolution of the eye and how it is that these incredible organs of vision came to exist. Our next step will be to understand how these complicated structures develop in our body.
The embryology of the eye is pretty amazing because it brings together the ectoderm and the mesoderm and forms structures that exist no where else in the body. So how does it happen?
The eye develops predominantly from the ectoderm. The mesoderm contributes supporting structures of the eye. The ectoderm that forms the eye has two parts. Neuroectoderm and surface ectoderm.
The neuroectoderm forms the brain parenchyma which differentiates into the forebrain, the mid brain and the hind brain. From the forebrain, two grooves develop which are known as the optic grooves. These grooves elongate to form the optic sulcci. The optic sulcci grow towards the overlying surface ectoderm to form optic vesicles. As the development proceeds, the optic vesicles move closer to the surface ectoderm and are connected to the forebrain by a small string like structure of neuroectodermal cells called the optic stalk.
Now while the neuroectoderm proliferates and forms the optic stalk and the optic vesicle, the surface ectoderm undergoes changes of its own. The changes in the surface ectoderm are actually induced by the underlying neuroectoderm. The surface ectoderm thickens in the area over the optic vesicle and this thickening is called the lens placode. The lens placode is drawn towards the optic vesicle beneath it and eventually the lens placode pinches off from the surface ectoderm and becomes a hollow group of cells called the lens vesicle. As the lens vesicle pinches off, the optic vesicle under it forms an invagination and forms the double layered optic cup in which the lens vesicle is housed.
So the basic structure of the eye is established.
The neuroectoderm forms the optic stalk and the optic cup and the surface ectoderm forms the lens vesicle which is housed in the optic cup.
Now how does development progress. Let us see what happens to the optic stalk, the optic cup and the lens vesicle separately.
The optic stalk forms the optic nerve. However there is peculiarity about the optic stalk that we should understand. The neuroectoderm, as it forms the optic cup and optic stalk Is deficient in its inferior part. There is a slit that is left open and this slit allows the migration of the blood vessels into the optic stalk. The blood vessel in the optic stalk is known as the hyaloid artery and it extends all the way upto the lens and provides blood supply during development. The proximal part of the hyaloid artery remains as the central retinal artery. The distal part disappears.
The optic cup, remember, is an invagination of the optic vesicle which means that it has two layers. The inner layer close to the lens vesicle and the outer layer close to the developing fore brain. Between these two layers lies the inter retinal space. Embryologically, the anterior one fifth of the optic cup and the posterior four fifth of the optic cup give rise to different structures.
The anterior one fifth is also known as the rim, gives rise to the iris and the ciliary body. The posterior four fifth of the optic cup gives rise to the layers of the retina. The inner layer gives rise to the neural retina which eventually gives rise to the rods, cones, bipolar cells and ganglion cells finally that form the optic nerve. The outer layer gives rise to the pigment layer.
The lens vesicle which is a derivative of the surface ectoderm is a hollow ball which sits in the optic cup. The next step is for the hollow ball to be filled. The filling of this hollow ball takes place in two stages. The first stage, cells from the posterior part of the lens vesicle proliferate and they are called the primary lens fibers. The second stage, cells from the anterior part of the lens vesicle proliferate and they form the secondary lens fibers. When the filling is complete, the lens vesicle is turned into a biconvex structure.
The outer structures of the eye like the eyelids, the lacrimal apparatus, the conjunctiva and the epithelium of the cornea develop from the surface ectoderm.
So this establishes the inner structures of the eye. The structures surrounding this comes from the mesenchyme or mesoderm. As the optic cup and the lens vesicle are forming, the mesenchyme or the mesoderm forms two layers around the developing eye. Behind the lens vesicle the inner layer of mesenchyme forms the vascular choroid and the outer layer gives rise to the fibrous sclera. In front of the lens, the mesenchyme extends and again splits into two layers to cover the anterior chamber - the inner layer forms the iridopupillary membrane which is continuous with the choroid and the outer layer forms stroma of the cornea which is continuous with the sclera.
The development of the eye is an intricate and incredible series of events that result in a beautiful functioning system. Understanding the development of the eye and the origins of the various parts of the organ will allow us to better understand certain pathologies like developmental anomalies and cancers of the eye. Armed with this knowledge we will see the structure, function and dysfunction of the eye in the articles that follow.
Author: Narendran Sairam (Facebook)
Sources and citations
Ort, Victoria, and David Howard. “II. Development of the Optic Cup and Lens Vesicle.” The Development of the Eye, New York University, education.med.nyu.edu/courses/macrostructure/lectures/lec_images/eye.html.