2012 Group Project 1
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:
History of Discoveries
Brief History of Discoveries Reference:
Ancient Egyptians – First to document cataracts. [Edwards, 1996]
535 BC - Ancient Greek philosopher Alcmaeon conducted dissection of humans for the first time in recorded history. This included dissection of the eye. However, not much is known about which anatomical features he discovered.
384- 322 BC- Aristotle performed dissections of animal embryos.
340 BC- Lens is thought to have been discovered by Hippocrates, due to his descriptions of the contents of the internal eye There has been studies in chick development later on by followers of Hippocrates. They claimed that eyes were visible in early embryogenesis.
23-79 AD – Pliny the Elder however, said that the eye is the last of the organs to develop in the womb (Magnus, 1998)
- More info to be added soon ***
1907 - Images from Historic Textbook: Atlas of the Development of Man
Formation of Primary Optic Vesicle:
Development of Lens
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. 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.
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).
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. Figure 2 then 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. 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. This is demonstrated in Figure 3. 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.
The retinal component of the eye is formed when the optic vesicle folds in upon itself, forming the optic cup (see Figure 4). In doing so it creates two layers - an inner wall and an outer wall of the optic cup (Figure 5). These two layers of the optic cup will give rise to the two layers of the retina - the inner neural retina, and the outer pigmented epithelium. Note the existence of the space between the two layers of the retina. This is known as the intraretinal space and disappears by the 7th week of development, however the two layers never completely fuse and can become separated as a result of physical trauma to the head - leading to a detached retina and loss of vision.
The inner wall of the optic cup which will give rise to the neural retina consists of a layer of pseudostratified cells (see Figure 6) that later differentiate into rod, cone, bipolar, ganglion, horizontal, amacrine and glial cells of the retina (Figure 7). The outer wall of the optic cup consists of a layer of cuboidal cells that contain melanin - the light absorbing pigment. The function of this layer is to absorb light and prevent internal reflection of light within the eye, which would impair our ability to form distinct images. Interestingly, in some animals such as cats, this layer actually reflects light intentionally to increase the amount of light available to the eye in low-light conditions. This is why cats seem to have eyes that glow in the dark.
The inner wall itself is divided into two components - the inner neuroblastic layer and the outer neuroblastic layer (see Figure 6). The outer neuroblastic layer forms the rod and cone cells while the inner neuroblastic layer forms the remaining cell types found in the retina - the bipolar, ganglion, horizontal, amacrine and glial cells (Figure 7). The organisation of the retina is interesting in that incoming light passes through several layers of these neural retina cells before it is detected by rod and cone cells at the back of the retina, and then nerve signals are passed back through the layers of neural retina cells that the light just passed through moments before - a seemingly strange design that the eye does not share with man-made light-capturing devices such as a camera (imagine putting the wires in front of the image sensor!).
Differentiation of the neuroblastic layers into neural retina cells occurs in a pattern both within the layers and across the retina. Cells differentiate from the inner neuroblastic layer to the outer neuroblastic layer, and differentiate from the central retina to the peripheral retina. The macula begins to form in the 6th month when ganglion cells start to form multiple rows, and the primitive fovea begins to form shortly afterwards in the 7th month as a depression in the macula. It is not until several months after birth that this area becomes exclusively populated by cone cells and becomes the fovea centralis - the area of the retina with the highest visual acuity.
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 ciliary processes form slightly posterior to the iris, developing from part of the anterior rim of the optic cup. It is thought that the folded structure of the ciliary processes is brought about by intraocular pressure and specific signalling pathways. While the ciliary muscles and the endothelial cells of the ciliary blood vessels are chiefly formed by mesenchymal cells, the neural crest and neuroectoderm also contribute to their development . 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. Napier and Kidson (2007) summarised numerous genes that have been associated with ciliary body development, however their direct roles have not been well documented .
The iris is a thin layer that develops at the end of the third month of development and is derived from the anterior rim of the optic cup. The stroma of the iris develops from cells of neural crest cell origin. The muscles that are responsible for the dilation and constriction of the pupil (dilator pupillae and sphincter pupillae muscles) form from the neuroectoderm of the optic cup. These cells are initially epithelial cells that then transform into smooth muscle cells. The final colour of the iris is not evident until the postnatal period.
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.
There are both anterior and posterior aqueous chambers of the eye which contain aqueous humour. A space develops in the mesenchyme situated between the lens and cornea to form the anterior aqueous chamber. The mesenchyme located superficially to this chamber forms the mesothelium as well as the transparent portion of the cornea.
The posterior chamber develops from a similar space in the mesenchyme, however it is located between the iris and the lens. The anterior and posterior chambers are able to communicate with one another once the papillary membrane vanishes and the pupil is formed. This channel is known as the scleral venous sinus .
The cornea is the transparent, most anterior portion of the eye and consists of 5 layers. The epithelium of the external surface of the cornea is derived from surface ectoderm, while the mesenchyme is derived from the mesoderm. The endothelium is made up of differentiated neural crest cells that were initially from the optic cup . The final transparent structure develops because hyaluronidase removes hyaluronic acid, and thyroxine causes dehydration of the stroma .
Choroid and Sclera
The choroid and sclera are adjacent layers that surround the eye and act to vascularise and protect the eye respectively. They are formed from neural crest and mesoderm-derived mesenchyme which condenses around the optic cup and lens vesicle between weeks 5 and 7 of development to form a primitive eyeball structure known as the optic globe.
The eyelids are ectodermal in origin and are an extension of the skin which covers and protects the eye.
Lacrimal glands initially develop from budding of the conjunctival epithelium near the superolateral portion of the eye. The mesenchymal cells that surround these buds are of neural crest origin, and the budding continues until the mature gland is formed. These glands are responsible for the production of tears however they do not start to function until 1-3 months after birth.
This research studied c-Jun N-terminal Kinase (JNK) activation in the retina of newborn rats
Advanced Cell Technology- A biotechnology company which is currently running two clinical trials that utilise human embryonic stem cell derived retinal pigmented epithelial cells. These trials are examining the possibility of using these cells to treat stargardt's macular dystrophy and dry age-related macular degeneration.
Accommodation- changing the focal length of the lens in order to focus on an object
Ectoderm- outermost layer of germ cells in an early embryo
Endoderm- innermost layer of germ cells in an early embryo
Glial cells- non-neuronal cells that provide structure and protection to neurons as well as producing myelin
Macula- a highly pigmented, oval-shaped area located near the centre of the retina. Important for visual acuity
Mesoderm- middle layer of germ cells in an early embryo
Mesothelium- the epithelial layer of the mesoderm
Myelinisation- development of a myelin sheath around a nerve fibre
Neural tube- hollow structure that results from the folding of the neural plate and eventually forms the central nervous system
Neuroblast- immature cells that differentiate to form either glial cells or neurons
Neuroectoderm- portion of the ectoderm that develops to form the central and peripheral nervous systems
Optic placode- ectodermal placode that gives rise to the lens of the eye
- <pubmed> 1222340</pubmed>
- Schoenwolf: Larsen’s Human embryology, 4th ed. Churchill Livingstone, An Imprint of Elsevier. 2008
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Moore: The Developing Human, 9th ed. Saunders, An Imprint of Elsevier. 2011
- Moore: The Developing Human, 9th ed. Saunders, An Imprint of Elsevier. 2011
- Schoenwolf: Larsen’s Human embryology, 4th ed. Churchill Livingstone, An Imprint of Elsevier. 2008
- <pubmed> 9882499</pubmed>
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