Embryology of the Eye PDF

EMBRYOLOGY OF THE EYE 300L ANATOMY LECTURE BY DR RAJI K.A DEPARTMENT OF HUMAN ANATOMY FACULTY OF BASIC MEDICAL SCIENCES BAZE UNIVERSITY ABUJA OPTIC CUP AND LENS VESICLE The developing eye appears in the 22-day embryo...

EMBRYOLOGY OF THE EYE 300L ANATOMY LECTURE BY DR RAJI K.A DEPARTMENT OF HUMAN ANATOMY FACULTY OF BASIC MEDICAL SCIENCES BAZE UNIVERSITY ABUJA OPTIC CUP AND LENS VESICLE The developing eye appears in the 22-day embryo as a pair of shallow grooves on the sides of the forebrain With closure of the neural tube, these grooves form outpocketings of the forebrain, the optic vesicles. These vesicles subsequently come in contact with the surface ectoderm and induce changes in the ectoderm necessary for lens formation OPTIC CUP AND LENS VESICLE The optic vesicle begins to invaginate and forms the optic cup The inner and outer layers of this cup are initially separated by a lumen, the intraretinal space This lumen disappears soon after, and the two layers appose each other  Invagination also result in formation of the choroid fissure. Formation of this fissure allows the hyaloid artery to reach the inner chamber of the eye  During 7th week, the lips of the choroid fissure fuse, and the mouth of the optic cup becomes the future pupil. During these events, cells of the surface ectoderm begin to elongate and form the lens placode This placode subsequently invaginates and develops into the lens vesicle. The lens vesicle loses contact with the surface ectoderm and lies in the mouth of the optic cup RETINA, IRIS, AND CILIARY BODY The outer layer of the optic cup is known as the pigmented layer of the retina The posterior 4/5ths, the pars optica retinae, contains cells bordering the intraretinal space that differentiate into the photoreceptive rods and cones Rods are more numerous (120 million) and more sensitive than cones (6 to 7 million) but do not detect color like the cones.  Adjacent to the photoreceptive layer is the mantle layer which gives rise to neurons and supporting cells, including the outer nuclear layer, inner nuclear layer, and ganglion cell layer On the surface is a fibrous layer that contains axons of nerve cells of the deeper layers. Nerve fibers in this zone converge toward the optic stalk, which develops into the optic nerve. Hence, light impulses pass through most layers of the retina before they reach the rods and cones LENS Shortly after formation of the lens vesicle cells of the posterior wall begin to elongate anteriorly and form long fibers that gradually fill the lumen of the vesicle By the end of the 7th week, these primary lens fibers reach the anterior wall of the lens vesicle. Lens fibers are continuously being added to the central core CHOROID, SCLERA, AND CORNEA At the end of the 5th week, the eye primordium is completely surrounded by loose mesenchyme This tissue soon differentiates into an inner layer comparable with the pia mater of the brain and an outer layer comparable with the dura mater. The inner layer later forms the choroid The outer layer develops into the sclera and is continuous with the dura mater around the optic nerve CHOROID, SCLERA, AND CORNEA The posterior chamber is the space between the iris anteriorly and the lens and ciliary body posteriorly. The anterior and posterior chambers communicate with each other through the pupil and are filled with the aqueous humor produced by the ciliary process of the ciliary body. The clear aqueous humor circulates from the posterior chamber into the anterior chamber providing nutrients for the avascular cornea and lens. From the anterior chamber, the fluid passes through the scleral venous sinus (canal of Schlemm) at the iridocorneal angle where it is resorbed into the bloodstream. Blockage of the flow of fluid at the canal of Schlemm is one cause of glaucoma CHOROID, SCLERA, AND CORNEA The anterior chamber forms through vacuolization and splits the mesenchyme into an inner layer in front of the lens and iris, the iridopupillary membrane, and an outer layer continuous with the sclera, the substantia propria of the cornea The anterior chamber itself is lined by flattened mesenchymal cells.  Hence the cornea is formed by (1) an epithelial layer derived from the surface ectoderm; (2) the substantia propria or stroma, which is continuous with the sclera; and (3) an epithelial layer, which borders the anterior chamber The iridopupillary membrane in front of the lens disappears completely.  OPTIC NERVE The optic cup is connected to the brain by the optic stalk, which has a groove, the choroid fissure, on its ventral surface. In this groove are the hyaloid vessels. The nerve fibers of the retina returning to the brain lie among cells of the inner wall of the stalk. During the 7th week, the choroid fissure closes, and a narrow tunnel forms inside the optic stalk  As a result of the continuously increasing number of nerve fibers, the inner wall of the stalk grows, and the inside and outside walls of the stalk fuse (Fig. C). Cells of the inner layer provide a network of neuroglia that supports the optic nerve fibers. OPTIC NERVE The optic stalk is thus transformed into the optic nerve. Its center contains a portion of the hyaloid artery, later called the central artery of the retina. On the outside, a continuation of the choroid and sclera, the pia arachnoid and dura layer of the nerve, respectively, surround the optic nerve. Clinical correlates Coloboma Coloboma may occur if the choroid fissure fails to close. Normally, this fissure closes during the seventh week of development When it does not, a cleft persists. Coloboma iridis: a cleft is in the iris only Sometimes it may extend into the ciliary body, the retina, the choroid, and the optic nerve. Occasionally coloboma involves the eyelid Clinical correlates Persistent iridopupillary membrane This is when iridopupillary membrane persist instead of being resorbed during formation of the anterior chamber. Clinical correlates Congenital cataracts Congenital cataracts cause the lens to become opaque during intrauterine life. If the mother is infected after the 7th week of pregnancy, the lens escapes damage, but the child may have hearing loss as a result of cochlear abnormalities. Clinical correlates Others are Anophthalmia Congenital aphakia Aniridia Cyclopia Synophthalmia