2017 Group Project 4
|2017 Student Projects|
- 1 Anatomy of the Eye
- 2 Eye Development
- 2.1 Timeline
- 2.2 Carnegie Stages
- 2.3 Development of the eye components
- 2.4 Common Abnormalities
- 2.5 Further Research
- 2.6 Glossary
- 2.7 References
- 2.8 External links
Anatomy of the Eye
The eye is a complex sensory structure which allows a variety of species to intake and process visual information from the world around us.
To discuss the key anatomical components of the eye, we will break down the eye functionally:
Supporting Structures (Orbit, Extraoccular Muscles, Lacrimal Gland)
The orbit of the eye consists of a framework of bones and connective tissue which provide structural support and protection to the sensitive human eye. 7 bones contribute to the orbit of the eye: frontal, lacrimal, sphenoid, zygomtatic, ethmoid, maxilary, palatine. Several openings exist in the orbital structure, 2 key fissures are the superior orbital fissure and inferior orbital fissure and the posterior optic canal. These openings allow crainial nerves passageway.
A set of 6 extraoccular muscles allow for a strong voluntary control of the movement of the eye. These muscles include superior recuts, inferior rectus, lateral rectus, medial rectus, inferior oblique, superior oblique muscles. These muscles are some of the smallest in the human body and are designed to produce fast, controlled motion to focus on an object of interest.
The eyelid is the anterior covering of the eye, consisting of a thin fold of skin. It is controlled by the levitator palpabrae superioris muscle. Superio-laterally to the eyelid is the lacrimal gland, which serve to create a film of tear, an important component to keeping the anterior surface of the eye moist. The eyelid assists in this process by opening and closing, or blinking, which spreads the tear film across the surface of the eye. Tear film and debris caught in the tear film are excreted through the lacrimla duct which is located on the infero-medial surface of the eyelid.
Week 3 - 4
- Optic vesicles
- Lens placode
Week 5 - 6
- Optic cup
- Lens vesicle
- Hyaloid artery
Week 7 - 8
- Anterior Chamber
Week 9 - 15
- Cillary Body
Week 8 - 10
Development of the eye components
The eyes are derived from four sources:
- The neuroectoderm of the forebrain forms
- Posterior layers of the iris
- The optic nerve.
- The surface ectoderm of the head forms
- The lens of the eye
- The corneal epithelium.
- The mesoderm between the neuroectoderm and the surface ectoderm forms
- The fibrous and vascular coats of the eye
- The neural crest cells forms
- Corneal endothelium
Overview of eye development
The eye starts to develop at 22 days. The optic grooves (sulci) appears in the neural folds at the cranial end of the embryo. When the neural fold fuse to form the forebrain, the optic grooves will form optic vesicles. The optic vesicles are continuous cavities from the cavity of the forebrain and project from the wall of the forebrain and into the mesenchyme. The optic vesicle extends from the diencephalon and will come in contact with the surface ectoderm of the head. This induces the formation of a lens placode. The surface ectoderm near the optic vesicles will thicken and form the lens placodes. The lens placodes will sink into the surface ectoderm and form lens pits. The edges of the lens pits will travel towards each other and fuse to form round lens vesicles, which will later lose connection with the surface ectoderm. The optic vesicles do also keep developing - they will form double-walled optic cups which are connected to the brain by the optic stalk. The two layers of the optic cup will differentiate in different directions. The cells of the outer layer will produce melanin pigment and later become the pigmented retina. The cells of the inner layer of the optic cup will proliferate fast and develop glia, ganglion cells, interneurons and light-sensitive photoreceptor neurons. These cells are in the neural retina. The ganglion cells of the retina are neurons that send signal to the brain. The axons of the ganglion cells of the neural retina will grow in the wall of the optic stalk. The cavity in the optic nerve will start disappearing, and instead, the axons of the ganglion cells will form the optic nerve. The optic stalk is now the optic nerve 
The optic cups will fold inwards around the lens while the lens vesicles have grown inwards so they have fully lost their connection with the surface ectoderm, which locates them in the cavities of the optic cups. The retinal fissures (linear grooves) will develop and cover the ventral surface of the optic cups and down to the optic stalk. The retinal fissures contain vascular mesenchyme and hyaloid blood vessels will develop here. The hyaloid artery supplies the structures in the eye with blood and the hyaloid vein will return the blood from these structures.
Formation of the optic vesicle
It is a specific area of the neural ectoderm that will become the optic vesicle - this happens because of a group of transcription factors - Six3, Pax6, and Rx1. These transcription factors are expressed in the most anterior tip of the neural plate. This area will split into bilateral regions and form the optic vesicles. The Pax6 protein has shown to be especially important for the development of the lens and retina. This protein is important for photoreceptive cells in all phyla. Pax 6 is also present in the murine forebrain, hindbrain, and nasal placodes, but the eyes are most sensitive its absence .
The sonic hedgehog gene is important for the separation of the single eye field into two fields. If this gene is inhibited, the eye field will not split which will result in cyclopia, a single eye in the center of the face .
Human lens induction occurs at around 28 days and is completed around day 56. The surface ectoderm will thicken near the optic vesicle and create the lens placode and later form the lens vesicle . Lens cells come from ectoderm and differentiate into either lens fibers or the lens epithelium. The anterior monolayer of epithelial cells of the lens will create the lens epithelium, which makes the sheet of cuboidal epithelium covering the anterior surface of the lens. The posterior lens vesicle cells will produce the linear primary fibre cells aligned parallel to the optic axis. These fibres will create the lens mass and form the embryonic lens nucleus. The lens epithelial cells will keep proliferating and produce new cells which generate a secondary lens fiber cells. This rows of cell will form the outer shells and keep the lens growing throughout life. This makes the eye lens unique - it will have an addition of new cells inside the surrounding capsule all the time .
The ciliary body is a muscular and secretory tissue and is located directly behind the lens. The ciliary body forms part of the anterior segment of the eye and is an important regulator of eye physiology and the vision. The ciliary body produces the aqueous fluid which fills the eyes and nourishes the lens and cornea - this aqueous fluid function to keep the eye in a pressurized and inflated state, which is important the vision.The ciliary body also synthesizes collagenIX and tenasin-C .
The ciliary body extends from the iris root (anteriorly) to the ora serrata (posteriorly). It consist of ciliary muscles and ciliary processes. Each ciliary process (fold) is covered by a double-layered secretory epithelium; the outer pigmented and the inner unpigmented (closes to the lens). The epithelial layers of the ciliary body comes from the retina of the optic cup. We see two different ciliary body epithelium: the inner non-pigmented ciliary epithelium which is connected with the neural retina and the outer pigmented ciliary epithelium which is connected with the retinal pigmented epithelium. The epithelium of the iris is the further anterior extension. The epithelial layers are associated with a stroma containing the ciliary muscle. The ciliary muscle is complex, but can be divided into three portions; anterior, posterior and internal .
In the chick eye development, we see the mesenchyme grow together on the margin of the optic cup and will form the stroma of the ciliary body and the iris, which is located more anteriorly. It is unknown how if the lens has a role in inducing the secretory ciliary body epithelium and the muscular iris epithelium .
The Iris develops at the end of the third month of development. The iris is a thin layer and derives from the anterior rim of the optic cup. In the anterior of the eye, the optic epithelium is nonneural and matures as ciliary body and iris epithelia.
Choroid and Sclera
We could talk briefly in this sections about the causes of short/long-sightedness and common causes of blindness at a developmental level - z3416557
5117343 In the news, media, websites starting point: Macular Research: https://www.cera.org.au/research/macular-research/ > Bionic Eye - https://theconversation.com/artificial-vision-what-people-with-bionic-eyes-see-79758 Corneal Research: https://www.cera.org.au/research/corneal-research/
Retinopathy of prematurity (ROP)
Optic Nerve Hypoplasia
z5075309 - <pubmed>26956898</pubmed>