2012 Group Project 1

From Embryology

Vision Development

Introduction

Eyes are an important sensory organ shared across many different species and allow organisms to gather useful visual information from their environment. The visual system uses light from the environment and processes this information in the brain for visual perception. The visual system is complex, and is made up of various structures that work together to form vision. Each of the structures in the eye have specific tasks which contribute to the visual system.

The main anatomical structures of the eye are as follows:

-Cornea

-Sclera

-Iris

-Ciliary body

-Choroid

-Retina

-Anterior chamber

-Posterior chamber

Research history

Development

The eye itself is formed from several components; notably the optic placode of the head ectoderm, the optic vesicle from the neural tube, and mesenchyme from the mesoderm and neural crest cells. The optic placode contributes the lens to the eye, the optic vesicle gives rise to layers of the retina, while the mesenchyme will produce the ciliary body, iris, choroid and sclera.[1] Cells from the neural tube will also produce the optic nerve, which receives nerve impulses from the retina of the eye. Eyes initially form as laterally paired structures and migrate medially in the human embryo. In other animals such as birds and lizards, the eyes do not migrate and develop laterally on the head. The optic placodes become prominent on the surface of the embryo at approximately Stage 14 of development.


A cross section showing the organisation of the developing brain, the optic vesicle and the lens (optic) placode.[2]
A Stage 14 embryo showing the location of an otic placode.[3]

















Optic Nerve

The optic nerve consists of nerve fibres that transmit information from the retinal photoreceptor cells to the brain. The optic nerve is formed from the optic stalk. The optic stalk develops as the optic vesicle migrates from its origin in the neural tube to its destination - the surface ectoderm - where it will fuse with the optic placode (also known as the lens placode, which will contribute the lens to the eye).


Fig. 1: Early formation of the optic vesicle from the neural groove.


As can be seen in figure 1 above, the optic vesicle forms from the neural tube. However, note that the neural tube has not yet closed, and is still the neural groove at this point.


Fig. 2: The optic vesicle at a later stage, showing the optic stalk.


Figure 2 now shows the optic vesicle at slightly later stage in the same simplified cross-section of the embryo, as it migrates from the neural tube to the surface ectoderm. Note the presence of the optic stalk which links the optic vesicle to the neural tube. Later in development, this primitive structure will become the optic nerve, which will link the eye to the brain.


The nerve fibres themselves will initially originate from the retinal ganglion cells in the eye during week 6.[4] After two weeks, these fibers will have grown along the inner wall of the optic stalk and have reached the brain. They grow both in length and width, with the nerve fibres filling the hollow optic stalk to form the solid optic nerve. More than one million nerve fibers will eventually make up the optic nerve, along with glial cells which arise from the inner wall of the optic stalk itself. Myelinisation of the optic nerve begins much later in development at around 7 months, beginning at the optic chiasm and moving towards the eye. The optic chiasm forms just before the nerves reach the brain, and is where half the nerve fibres from each eye will cross over to the opposite side of the brain.


Fig. 3: A recognisable brain and eye structure in later development.


Figure 3 demonstrates this. Note the crossing over of the optic nerves just before they enter the brain, at the optic chiasm. This organisation is now much more familiar, with the eyes near the ectoderm and the optic nerve leading through the mesoderm to the brain buried deep in the embryo.


Retina

The retinal component of the eye is formed when the optic vesicle folds in upon itself, forming the optic cup.


Ciliary Body

The ciliary body consists of ciliary processes and three portions of fibres that constitute the ciliary muscles. It functions to maintain normal eye physiology as well as playing a direct role in accommodation.


During development, the proximal portion of the optic cup rim morphs to form the ciliary processes. It is thought that the folded structure of the ciliary processes is brought about by intraocular pressure[5]. While the ciliary muscles and the endothelial cells of the ciliary blood vessels are chiefly formed by mesenchymal cells[6], the neural crest and neuroectoderm also contribute to their development [7]. The normal development of the ciliary body is dependent on the correct expression of bone morphogenetic protein (BMP)-4, which is a member of the transforming growth factor-β superfamily[8]. Napier and Kidson (2007) summarised numerous genes that have been associated with ciliary body development, however their direct roles have not been well documented [9].

Iris

Lens

The lens has its origin from the optic placode, which develops on the ectodermic surface of the embryo and migrates both medially and inwards into the embryo. The lens allows accommodation of the eye, and adjusts its thickness in order to focus on near or far objects.


Aqueous Chambers

Cornea

Choroid and Sclera

Eyelids

The eyelids are ectodermal in origin and are an extension of the skin which covers and protects the eye.


Lacrimal Glands

Extraocular muscles

Current research

Useful links

Glossary

Image gallery

References


External Links

External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.

--Mark Hill 12:22, 15 August 2012 (EST) Please leave the content listed below the line at the bottom of your project page.


2012 Projects: Vision | Somatosensory | Taste | Olfaction | Abnormal Vision | Hearing