2012 Group Project 1

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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
Basic structure of the human eye

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Research History

Brief History of Discoveries Reference:[1]


      • More info to be added soon ***
Time Discovery Image
Ancient Egyptians First to document cataracts. [Edwards, 1996] image
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. image
384- 322 BC Aristotle performed dissections of animal embryos. Image
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. image
23-79 AD Pliny the Elder however, said that the eye is the last of the organs to develop in the womb (Magnus, 1998) First Row Column 3
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340 BC text image
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1907 - Images from Historic Textbook: Atlas of the Development of Man

Formation of Primary Optic Vesicle:

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Development of Lens

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Development, Structure and Function of Ocular Components

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









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).[3]


Fig. 1: Early formation of the optic vesicle from the neural groove.
Fig. 2: The optic vesicle at a later stage, showing the optic stalk.


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.[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. 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.


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


Retina

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.[3] 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.[4]

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.


Fig. 4: Mechanism of optic cup formation.
Fig. 5: Layers of the optic cup in retina development.


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).[4] 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.


Fig. 6: Cross-section of the primitive retina showing cell types and layers.
Fig. 7:Cross-section of a developed retina showing cell types and layers.


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 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[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. [5]

Iris

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.[4] 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[9].

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. The study of lens development was one of the first to highlight the importance of inductive signaling in development, with Spemann's pioneering work at the start of the 20th century, finding that the absence of retinal development resulted in the absence of lens formation.[3] Indeed, it has been consistently shown that the interaction of the migrating optic vesicle with the surface ectoderm of the head is vital in producing differentiation of the lens.[10] The mechanism of interaction is complex but basically involves upstream genes switching on downstream genes, with the genes eventually producing specialised proteins which constitute the lens. The whole process starts with the signaling molecules from the optic cup initiating a thickening of the surface ectoderm of the head (Figure 8). It is thought that this region of specific ectoderm is responsive to the signaling molecules, as lens formation is incomplete or absent when ectoderm from the lateral portion of the embryo (i.e. non-head ectoderm) is exposed to the same inductive signaling processes.[11] Pax6 has been shown to be one of the major genes required for differentiation of the lens, which in turn switches on transcriptional genes such as Sox 1, 2 and 3 among others - producing water-soluble proteins called crystallins - responsible for giving the lens its transparency and refractive properties.[12]


Fig. 8: The importance of the optic cup in lens differentiation.
Fig. 9: The lens placode separates from the ectoderm and migrates into the mesoderm forming the lens vesicle.


The lens placode invaginates from the head ectoderm and migrates into the mesoderm (Figure 9). Once this structure (now known as the lens vesicle) is in place opposite the optic cup, the combined structure is referred to as the optic globe and resembles a recognisable eye structure. The lens continues to differentiate further, as mentioned above, through the formation of crystallin proteins, which give the lens its unique properties and allows for the fine control over the degree of refraction that takes place.

Aqueous Chambers

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.[9]

Cornea

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 [9]. The final transparent structure develops because hyaluronidase removes hyaluronic acid, and thyroxine causes dehydration of the stroma. [4]

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.[4] Blood vessels first start to appear in the choroid layer at approximately week 15, and arteries and veins can be distinguished by week 23.[13] Inductive processes are thought to play a vital role during formation of the choroid and sclera; with the retinal pigmented epithelium inducing differentiation of the surrounding mesenchyme while at the same time the neural crest-derived mesenchyme contributing components to the retinal pigmented epithelium such as melanocytes.[14] In addition to having functional roles themselves, the primitive choroid and sclera also contribute components to the developing ciliary body and cornea (Figure 10). In the adult eye, the choroid is continuous with the ciliary body and the sclera with the cornea.


Fig. 10: The choroid and sclera derives from mesenchyme surrounding the optic cup.

Eyelids

The eyelids are ectodermal and mesodermal in origin and are an extension of the skin which covers and protects the eye. Eyelid formation can be first noted during week 5 when small grooves develop in the surface ectoderm (Figure 11).[15] These small grooves deepen and extend into the mesoderm and the primitive eyelid structures grow towards one another, eventually fusing together during week 8.[9] It is not until week 26-28 that the eyelids will separate again. The ectoderm reflects over the developing cornea to form the conjunctival sac, a space that is filled by secretions from the lacrimal gland in order to allow smooth motions of the eyelid over the eye and also to clean the cornea and prevent accumulation of particles on the eye that may disrupt vision.


Fig.11: Small grooves in the ectoderm of the head - the precursors to an eyelid.

Lacrimal Glands

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[16]. These glands are responsible for the production of tears however they do not start to function until 1-3 months after birth.

Current Research

<pubmed>22496813</pubmed>

This research studied c-Jun N-terminal Kinase (JNK) activation in the retina of newborn rats

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<pubmed>20544023</pubmed>

<pubmed>20459797</pubmed>

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[17].



<pubmed>20652025</pubmed>

The lipoprotein receptor-related protein 5 (LRP5) has a significant function in the development of retinal vasculature.[18] Research has shown that mutations of the LRP5 causes loss of function, due to incomplete development of retinal vessel network, in both humans and mice. The authors investigated how mutations occur in the LRP5, which leads to abnormal development of the retinal vasculature. They have studied retinal endothelial cells in mutant mice in their study. Their results showed that in retina with mutated LRP5, endothelial cells in the retinal vasculature primarily produced cell clusters in the inner-plexiform layer instead of migrating into deeper layers of the retina to form normal retinal vasculature. The authors also discovered that there was a decrease in Slc38a5, which is a “a Müller cell-specific glutamine transporter”, in mice with mutated LRP5. Their results lead the authors to conclude that normal LRP5 is very important in the development of normal retinal vasculature due to their role in causing migration of retinal endothelial cells in the deeper layers of the retina. LRP5 is also important for retinal interneurons and Müller cells to function correctly.


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Useful 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.

Visualisation of eye development in the embryo

Embryonic Development of the eye

Glossary

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

Image Gallery


References

  1. http://www.vetmed.vt.edu/education/curriculum/vm8054/eye/EMBYEYE.HTM
  2. 2.0 2.1 http://embryology.med.unsw.edu.au/embryology/index.php?title=ANAT2341_Lab_6_-_Early_Embryo
  3. 3.0 3.1 3.2 <pubmed>11687490</pubmed>
  4. 4.0 4.1 4.2 4.3 4.4 4.5 Schoenwolf: Larsen’s Human embryology, 4th ed. Churchill Livingstone, An Imprint of Elsevier. 2008
  5. 5.0 5.1 <pubmed>16959249</pubmed>
  6. <pubmed>16249499</pubmed>
  7. <pubmed>12127103</pubmed>
  8. <pubmed>1222340</pubmed>
  9. 9.0 9.1 9.2 9.3 Moore: The Developing Human, 9th ed. Saunders, An Imprint of Elsevier. 2011
  10. <pubmed>15558475</pubmed>
  11. <pubmed>9216064</pubmed>
  12. <pubmed>9609835</pubmed>
  13. Development of the Choroid and Related Structures, K. Sellheyer, Eye (1990) 4, 255-261
  14. <pubmed>1628748</pubmed>
  15. <pubmed>7364662</pubmed>
  16. <pubmed> 9882499</pubmed>
  17. <pubmed>22281388</pubmed>
  18. <pubmed>20652025</pubmed>




--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