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Development of the Eye
Transcript of Development of the Eye
By Jenelle Corkill 4 Weeks On day 22 the developing eyes appears as a pair of optic grooves on the sides of the forebrain (diencephalon).
After the neural tubes close, the optic grooves form outpocketings of the forebrain, known as the optic vesicles.
The optic vesicles come in contact with nearby surface ectoderm where they induce it to form the lens placode (primordia of the lenses).
The lens placode later invaginates and develops into the lens vesicle.
The optic vesicle begins to invaginate and forms the double-walled optic cup. (1) Figure 1. Dorsal view of the cranial end of an embryo of approximately 22 days showing the optic grooves, the first indication of eye development.
(2) 5 Weeks By the fifth week of gestation, the lens vesicle loses contact with the surface ectoderm and sits in the mouth of the optic cup. (1)
The inner layer of the cup will form the neural retina, while the outer layers will form the pigmented retina. (3)
Additionally, the muscles which will move the eyes in the skull begin to develop. (1) Figure 3. Forebrain of an embryo (approximately 28 days) showing the covering layers of mesenchyme and surface ectoderm. (2) Figure 4. Lateral view of the brain of an embryo (approximately 32 days) showing external appearance of the optic cup. (2) Successive stages of the development of the optic cup and lens vesicle.
(2) 6-8 Weeks Invagination also takes place at the inferior surface of the optic cup and optic stalk, forming a gap called the choroid fissure (retinal fissure)
The choroid fissure contains vascular mesenchyme from which the hyaloid blood vessels that surround the back of the lens develop.
The hyaloid artery supplies the inner layer of the optic cup, the lens vesicle, and the mesenchyme in the the cavity of the optic cup. (1)
The hyaloid vein returns blood from these structures.
The lips of the choroid fissure fuse together, and the hyaloid vessels become enclosed within the primordial optic nerve
Nerve fibers from cells in the neural retina begin to form the optic nerves
More than a million nerve fibers will grow from each eye, connecting the eyes to the brain (6)
The mouth of the optic cup becomes a round opening that will be the future pupil. (1) Inferior surface of the optic cup and stalk showing progressive stages of closure of the retinal fissure and formation of the optic nerve
(2) Transverse sections of the optic stalk showing successive stages in closure of the choroid fissure and formation of the optic nerve
(2) The choroid fissure normally closes during the sixth week of development.
Defects in closure of the choroid fissure result in coloboma of the iris and/or retina.
The effects on vision range from mild to severe depending on the size and location of the gap.
The cause of colobomas can be associated with a mutation in the PAX2 gene (7) Coloboma Coloboma is a defect in the inferior sector of the iris or a notch in the pupillary margin, giving the pupil a keyhole appearance The retina develops from the walls of the optic cup
The outer thinner layer of the optic cup becomes the retinal pigment epithelium.
The inner thicker layer differentiates into the neural retina, the light-sensitive region of the optic part of the retina. This region contains photoreceptors (rods and cones) and the cell bodies of neurons and supporting cells (mantle layer). (8)
Fibroblast growth factor signaling regulates retinal ganglion cell differentiation (9) Development of the Retina As the neural portion of the retina develops, it differentiates into distinct cell layers.
The iris (colour part) regulates the amount of light entering the eye
The iris develops from the rim of the optic cup
epithelium of the iris represents both layers of the optic cup
The connective tissue framework (stroma) of the iris is derived from neural crest cells that migrate into the iris.
The dilator pupillae and sphincter pupillae muscles of the iris are derived from neural ectoderm of the optic cup. (1) Development of the Iris The vitreous body forms within the cavity of the optic cup
The vitreous body is composed of vitreous humor (a transparent jelly-like substance)
The primary vitreous humor is derived from mesenchymal cells of neural crest origin.
The secondary vitreous humor (believed to arise from the inner layer of the optic cup) consists of primitive hyalocytes (vitreous cells), collagenous material, and traces of hyaluronic acid. (2) Development of the vitreous body The cornea is induced by the lens vesicle.
The outer epithelial layer of the cornea is derived from surface ectoderm
The inner layers of the cornea are derived from neural crest cells (10) Development of the Cornea Mesenchyme surrounding the optic cup (largely of neural crest origin) is induced by the retinal pigment epithelium and differentiates into an inner vascular layer, the choroid, and an outer fibrous layer, the sclera (2) Development of the Choroid and Sclera During the sixth week eyelids begin to develop from neural crest cell mesenchyme and from two cutaneous folds of ectoderm that grow over the cornea.
The upper and lower eyelids grow over the cornea and fuse together (by the beginning of the tenth week)
The eyelids will separate again between the 26th to 28th weeks
while the eyelids are fused together, the lacrimal glands begin to develop
Lacrimal glands develop from a number of solid buds from the surface ectoderm
The lacrimal glands are small at birth and do not function fully until approximately 6 weeks
Therefore, a newborn infant does not produce tears when it cries until it is 1 to 3 months old.
Eyelashes are also derived from the surface ectoderm (2) Development of the Eyelids and Glands Induction & Lens Development Experiments where done first on frog and chick embryos and more recently on mouse embryos (it is assumed similar results would be found with human embryos) Conclusion: Optic Cup induces Lens Development (4) Example:
Before formation of the optic vesicle two events take place:
1. Chordamesoderm induces the neural tube and defines the anterior regions
2. Endodermal-Mesoderm induces lens ectoderm (later induced by the optic vesicle to become the lens placode) (5) Inductive interactions Mediate Eye Development Sonic Hedgehog and Eye Development -SHH mediates the division of the single eye field into two bilateral fields
-Though not yet proven it is believed that SHH emitted from the prechordal plate suppresses Pax6 which causes the eye field to divide into two.
-Experiments with mice have found that when the SHH gene is mutated, the result is cyclopia; formation of a single eye in the center of the face. (15) Three major genes involved in controlling eye development are: PAX6, SOX2, and OTX2 -These 3 genes fulfil multiple tasks in several cell types and at different stages of eye development
-They function in other organs too (mostly in the brain) PAX6 is the most researched of the Pax (Paired box) genes
It is considered the "master control" gene for the development of eyes and other sensory organs
Mutations in PAX6 result in abnormal eye development in all species studied
PAX6 function was first identified through aniridia-associated (no iris) mutations
PAX6 is expressed on the surface ectoderm and neural ectoderm early in development, then in the differentiating cells in the cornea, lens, ciliary body, and retina through development.
PAX6 is believed to have a role in determining cell fate in the morphogenesis of various human ocular tissue. (12) PAX6 SOX2 Expressed by retinal progenitors during development.
Associated with the ability of progenitor cells to differentiate into retinal neurons.
Highly expressed by human Müller stem cells (hMSCs)
SOX2 mutation causes bilateral anophthalmia (congenital absence of most eye tissues)
genetic mistakes in SOX2 is not passed down from the parents, but seem to rise spontaneously in either the egg or sperm (14)
It is the first gene activated in the anterior neuroectoderm, and its function is required for the formation of the anterior brain region (Mice with defected Otx2 don't form forebrain or midbrain)
OTX2 is downregulated in the eye field as Rx (transcription factor) is expressed
OTX2 is critical in the development of the retinal pigment epithelium
Mutations in OTX2 can cause eye disorders including anophthalmia (no eye) and microphthalmia (small eyes) (11) OTX2 Interaction of the genes Otx2, PAX6, Six3, Six6, and RX is believed to define the field of retinal progenitors, promote their evagination from the ventral forebrain and lead to formation of the optic cups. (11) Successive developmental stages of the lens, retina, iris, and cornea development:
(2) 5 weeks 6 weeks 20 weeks Newborn References 1.) Sadler, T. (2003). Langman’s Medical Embryology (9th ed). Philadelphia, Pennsylvania: Lippincott Williams & Wilkins.
2.) Moore K.L., Persaud T.V.N. (2007). The Developing Human (8th ed). Philadelphia, Pennsylvania:
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8) Purves, D., Augustine, G.J., Fitzpatrick, D. (2001). The Retina. Neuroscience 2nd edition. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK10885/
9.) Guillemot, F., Cepko, C.L. (1992). Retinal fate and ganglion cell differentiation are potentiated by acidic FGF in an in vitro assay of early retinal development. Development, 114(3):743-54. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/1377624
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11.) Zilinski, C., Brownell, I., Hashimoto, R., Medina-Martinez, O., Swindell, E.C., Jamrich, M. (2004). Expression of FoxE4 and Rx Visualizes the Timing and Dynamics of Critical Processes Taking Place during Initial Stages of Vertebrate Eye Development. Developmental Neuroscience,26;294-307. Doi:10.1159/000082271
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13.) Aniridia Eye Diagrams. Retrieved from http://www.visionfortomorrow.org/aniridia-eye-diagrams/
14.) Fantes, J., Ragge, N.K., Lynch S.A., McGill, N.I., Collin, J.R., Howard-Peebles, P.N., Hayward, C., Vivian, A.J., Williamson, K., Van Heyningen, V., FitzPatrick, D.R. (2003). Mutations in SOX2 cause anophthalmia. Nature Genetics 33(4): 461-3.
15.) Chiang, C., Litingtung, Y., Lee, E., Young, K.E., Corden, J.L., Westphal, H., Beachy, P.A. (1996). Cyclopia and defective axial patterning in mice lacking Sonic hedgehog gene function. Nature 383, 407 - 413 (03 October 1996); doi:10.1038/383407a0
16.) Sonic the Hedgehog. Retrieved from http://www.sonicthehedgehoggame.org/
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