Coloboma is a congenital defect of the eye, represented by missing pieces of tissue (gaps or notches) in the structures of the eye. The eye is made up of several protective layers and cavities. The outer layer consists of the sclera and cornea. The front part of the sclera is covered by a transparent membrane called the conjunctiva, which is involved in protecting the eyes and lining the eyelids. The cornea is a dome-shaped structure at the front of the eye that encloses the anterior chamber of the eye. It is transparent, allows light to enter the eye, and, together with the lens, helps focus and direct light onto the retina. The middle layer of the eye includes the iris, ciliary body, lens and choroid. The iris determines the color of a person's eyes, controls the size of the pupil and therefore the amount of light entering the eye. It separates the anterior and posterior chambers at the front of the eye. These chambers contain aqueous humor, which is important for nourishing the lens and cornea. The lens is a flexible, cutting structure that changes shape and therefore participates in focusing vision on near or distant objects. The vitreous humor is a gel-like substance that fills the back of the eye. It has a structural function and is involved in maintaining the shape of the eye, but it also helps transmit light to the retina. The choroid is a membrane located between the sclera and the retina. It outlines the back of the eye and is full of blood vessels. It is highly pigmented to absorb light and prevent scattering. The inner layer, the retina, lines the inside of the back of the eye and is the part that is sensitive to light. It is rich in photoreceptor cells and each photoreceptor is connected to...... middle of paper ......Willer, G.B., Smith, K., Gregg, R.G. and Gross, J.M.(2008) Zebrafish Blowout Provides Genetic Evidence of Patched1-mediated negative regulation of Hedgehog signaling within the proximal optic vesicle of the vertebrate eye, Developmental Biology 319(1):10-22Sanyanusin P., A.McNoe L., J.Sullivan M., Weaver R .Gray . Eccles MR (1995) PAX2 mutation in two brothers with renal coloboma syndrome, Human Molecular Genetics vol. 4, no. 11 2183-2184Torres M, Gómez-Pardo E., Gruss p. (1996) Pax2 contributes to inner ear patterning and optic nerve trajectory, Development 122, 3381-3391 Viringipurampeer IA, Ferreira T., DeMaria S., Yoon JJ, Shan X., Moosajee M., Gregory-Evans K. , Ngai J. , Gregory-Evans CY, (2012) Pax2 regulates a fashion-dependent molecular switch that drives tissue fusion during eye development, Human Molecular Genetics, vol. 21, no. 10 2357–2369