DEVELOPMENT OF THE EYE The eyes develop from three (3) sources 1) neuroectoderm of the forebrain, 2) surface ectoderm of the head, and 3) mesoderm located between the neuro ectoderm and surface ectoderm. 1. The formation of the optic vesicles and lens placodes, respectively, are induced by which tissues? lens placode; neuroectoderm fovea lentis; surface ectoderm mesenchyme adjacent to brain; optic vesicles optic sulci; fovea lentis lens placode; optic vesicles Right. No, the answer is C. Eye development is first seen during Stage 10 (about 22 days), when the optic sulci appear. As the neural folds fuse, the optic sulci evaginate to form the optic vesicles; this evagination is induced by the mesenchyme adjacent to the brain. The bulb-like optic vesicles grow outward through the mesenchyme and become flattened on their outer edges as they approach the inner side of the ectoderm. The lens placode is formed from surface ectoderm, and its formation is induced by the optic vesicles; the inducing agent is probably a chemical emanating from the optic vesicle. The lens pit or (fovea lentis) forms as an invagination of the lens placode. 2. What is the fate of the hyaloid vessels? the proximal portions degenerate; the distal portions persist as the choroid plexus the proximal portions persist as the central artery and vein of the retina; the distal portions degenerate both portions of both the hyaline artery and hyaline vein persist as the ophthalmic artery both portions degenerate and are absorbed by the optic nerve the hyaloid artery persists as the ophthalmic artery; the hyaloid vein degenerates Very good. No, the answer is B. The hyaloid artery is a branch of the ophthalmic artery; it supplies the inner layer of the optic cup, the lens vesicle, and the mesenchyme of the optic cup. The hyaloid vein drains deoxygenated blood from these structures. The hyaloid vessels eventually become enclosed in the optic nerve; their distal portions degenerate, but the proximal portions persist as the central artery and vein of the retina. 3. The dilator and sphincter pupillae muscles are derived from endoderm lining the optic cup mesoderm in the outer layers of the optic cup pigment epithelium of the neuroectoderm mesenchymal stroma around the optic cup mesoderm at the base of the future eyelid Correct. No, the answer is C. In this most unusual event, these smooth muscle cells are derived from the neuroectoderm of the outer layer of the optic cup; specifically, these muscles form as epithelial cells and are transformed into smooth muscle fibers. These findings have been confirmed by electron microscopy. 4. Which of the following events are a part of normal lens development? the lens vesicle cavity is obliterated by lens fibers elaborated by the cells of the posterior wall of the vesicle cells along the equator of the future lens elaborate meridianally arranged fibers low columnar cells of the anterior wall become the anterior epithelium of the adult lens cells from the posterior wall form the permanent adult fibers organized in an anterior-posterior direction Very good. No. All foils are correct. The low columnar cells of the anterior wall of the lens vesicle do not change appreciably they become the anterior epithelium of the adult lens. The cells from the posterior wall elaborate lens fibers ( by elongation and differentiation) oriented in an anterior-posterior direction. Later, by mitosis, elongation, and differentiation, the cells at the equator of the lens form fibers arranged meridianally these cells show concentric layers of differentiation; the psoterior cells lose their nuclei as the lens matures. Fiber formation continues throughout life. 5. The vitreous body is formed from which embryonic tissue type(s)? endoderm neuroectoderm ectoderm mesenchyme Correct. No. The second and fourth foils are correct. The vitreous body forms within the cavity of the optic cup; it is partly formed from the mesenchyme which enters the optic cup and partly from the neuroectoderm of the optic cup. 6. The iris is continuous with the double-layered epithelium of the ciliary body forms from the inner layer of the optic cup; the outer layer of the optic cup differentiates to form the cornea is continuous with the neural layer of the retina develops from endoderm Correct. No. The first and third foils are corrrect. The iris is formed from both layers of the optic cup and is continuous with the double-layered epithelium of the ciliary body and with the neural layer of the retina. The iris forms from the edge of the optic cup; it eventually binds inward to partially cover the lens. The iris develops from the endoderm of the optic cup. 7. Infants have a vacant stare for the first few weeks of life because they do not see well. This is most likely due to mental retardation incomplete development of the macula lutea bilateral optic atrophy incomplete development of the fovea centralis Very good. No. All foils are correct. During the first weeks of life, all infants see poorly, because the macula lutea and the fovea centralis (area of the retina responsible for visual acuity) are incompletely developed; both structures should be fully developed within a month. If the vacant stare persists beyond a month, it may indicate mental retardation or a visual defect such as bilateral optic atrophy. 8. The retina develops from the outer layer of the optic cup develops from the inner layer of the optic cup develops as an outgrowth of the diencephalon develops from mesencephalon Correct. No. The first three foils are correct. The retina is formed from both the outer and inner layers of the optic cup, which is an outgrowth of the diencephalon. The outer, thinner layer becomes the pigment epithelium, and the thicker inner layer becomes the neural layer. These two layers are separated during embryonic and early fetal periods by the intraretinal space; this space gradually disappears as the retina matures. 9. The eyelids form as the surface ectoderm of the optic cup separates and turns inward; this occurs during the tenth week of development form from two ectodermal folds which meet and fuse during the tenth week of development contain a core of mesenchyme which differentiates into muscle and tarsal plates A and C are correct B and C are correct Very good. No, the answer is E. The eyelids develop from two ectodermal (cutaneous) folds which meet and adhere by about the tenth week; they remain adherent until about the twentysixth week. These ectodermal folds contain cores of mesenchyme which give rise to the muscles and tarsal plates of the eyelids. The eyelashes and glands are derived from surface ectoderm. 10. The lacrimal glands develop from mesoderm develop from ectoderm develop from solid scleral buds which eventually become canalized to form the ducts and alveoli of the glands develop at the superior fornices of the conjunctival sacs Very good. No. The second and fourth foils are correct. The lacrimal glands develop from solid buds of surface ectoderm which branch and become canalized to form the ducts and alveoli of the glands; the glands form at the superolateral (superior fornices) of the conjunctival sacs. The lacrimal glands are small at birth and do not function fully for about six weeks hence, the newborn produces no tears when it cries. 11. Regarding congenital abnormalities of human eye development the most critical period is during days 1-16 days 22-50 days 50-150 days 150-term Correct. No, the answer is B. The type and severity of any congenital malformation depends on the embryonic stage during which development is disturbed. The most critical period for human eye development is during stages 10-20 (22-50 days; 3-6 weeks). 12. Absence of the sinus venosus sclerae causes a rise in intraocular tension is caused by recessive mutant genes causes congenital glaucoma causes buphthalmos Very good. No. All foils are correct. Congenital glaucoma (buphthalmos) is caused by the absence of or abnormal development of the sinus venosus sclerae; this condition is usually caused by recessive mutant genes, but can also be caused by a maternal rubella infection during early pregnancy. This malformation causes abnormal drainage of the aqueous humor, and, as a result, introcular tension rises; this pressure increase causes enlargement of the eye. 13. Congenital ptosis is relatively common is caused by abnormal development or failure of development of the levator palpebrae superioris muscle is caused by incomplete innervation of the levator palpebrae superioris muscle is transmitted as an autosomal dominant trait all of the above Very good. No, the answer is E. Congenital drooping of the eyelids (ptosis) is relatively common and is inherited as an autosomal dominant trait. It can result from abnormal development or failure of development of the levator palpebrae superioris muscle or from incomplete innervation of this muscle. 14. Probable causes of congenital cataracts noxious agents rubella virus congenital galactosemia enzymatic deficiencies Very good. No. All foils are correct. Congenital cataracts can be caused by noxious agents (such as the rubella virus) or by enzymatic deficiencies such as congenital galactosemia. The latter condition causes abnormal accumulation of galactose (from milk) in the infant's tissues and blood, and this causes cataract formation. 15. Colobomas are defects usually occurring in the lens may occur in palpebral, iridial and retinal forms results from a persistent pupillary membrane results from defective closure of the optic fissure Right. No. The second and fourth foils are correct. Colobomas may occur in the eyelid, iris and/or ciliary body and retina. Palpebral colobomas usually occur in the upper eyelid as a small notch. Iridial colobomas occur as an inferior defect in the iris, giving the pupil a keyhole appearance. The defect may be caused by genetic or environmental factors and is rarely associated with the first arch syndrome. 16. Absence of eyelid formation can be called anophthalmos microphthalmos analarophthalmos cryptophthalmos more than one of the above are correct Right. No, the answer is D. Cryptophthalmos is failure of eyelid development. Microphthalmos is a small eye on one side. Anophthalmos is failure of eyeball development with the eyelid forming normally. Analarophthalmos is a original term created by the author. 17. Administration of oxygen to premature infants may produce eye malformations including retinal vessel hyperplasia, detached retinal and abnormal development of the extrinsic eye muscle. True False Very good. No, the statement is true. 18. Which of the following is (are) derived from mesoderm? anterior epithelium of the cornea sphinctor and dilator pupillae lens choroid and sclera Very good. No. Only the fourth foil is correct. The anterior epithelium of the cornea forms from the surface ectoderm while the substantia propria and posterior epithelium (mesothelium or endothelium of the anterior chamber) form from mesenchyme. The sphincter and dilator pupillae unlike the ciliary muscles (mesodermally - derived) form from neuro-ectoderm. The lens forms from the surface ectoderm by thickening and vesicle formation. The vascular inner choroid and fibrous, outer sclera form from mesoderm. The sclera is continuous developmentally with the substantia propria of the cornea. 19. Which of the following structures is (are) derived from the optic cup? pigmented and neural layers of the retina ciliary body iris cornea Very good. No. The first three foils are correct. The pigmented and neural layers of the retina, form from the outer and inner layers of the optic cup respectively. The epithelium of the ciliary body consists of a pigmented portion derived from the outer layer of the optic cup (continuous with the pigmented epithelium of the retina) and a nonpigmented portion which represents the anterior prolongation of the neural layer of the retina. The iris also develops from both layers of the optic cup and is continuous with the double - layered epithelium of the ciliary body. The cornea forms from surface ectoderm and mesoderm (mesenchyme).