Anatomy and Development of the Eye PDF

1 Anatomy and CHAPTER 1 Development of the Eye ANATOMY OF THE EYE z Formation of lens vesicle z The eyeball...

1 Anatomy and CHAPTER 1 Development of the Eye ANATOMY OF THE EYE z Formation of lens vesicle z The eyeball z Formation of optic cup z Visual pathway z Changes in the associated mesoderm z Orbit, extraocular muscles and z Development of various ocular appendages of the eye structures z Structures derived from the embryonic DEVELOPMENT OF THE EYE layers z Formation of optic vesicle and z Important milestones in the development optic stalk of the eye Coats of the eyeball ANATOMY OF THE EYE The eyeball comprises three coats: outer (fibrous This chapter gives only a brief account of the anatomy coat), middle (vascular coat) and inner (nervous coat). of eyeball and its related structures. The detailed 1. Fibrous coat. It is a dense strong wall which anatomy of different structures is described in the protects the intraocular contents. Anterior 1/6th of relevant chapters. this fibrous coat is transparent and is called cornea. Posterior 5/6th opaque part is called sclera. Cornea is THE EYEBALL set into sclera like a watch glass. Junction of the Each eyeball (Fig. 1.1) is a cystic structure kept cornea and sclera is called limbus. Conjunctiva is distended by the pressure inside it. Although, firmly attached at the limbus. generally referred to as a globe, the eyeball is not a 2. Vascular coat (uveal tissue). It supplies nutrition sphere but an ablate spheroid. The central point on to the various structures of the eyeball. It consists of the maximal convexities of the anterior and posterior three parts which from anterior to posterior are : iris, curvatures of the eyeball is called the anterior and ciliary body and choroid. posterior pole, respectively. The equator of the eyeball lies at the mid plane between the two poles 3. Nervous coat (retina). It is concerned with visual (Fig.1.2). functions. Dimensions of an adult eyeball Segments and chambers of the eyeball The eyeball can be divided into two segments: Anteroposterior diameter 24 mm anterior and posterior. Horizontal diameter 23.5 mm Vertical diameter 23 mm 1. Anterior segment. It includes crystalline lens (which is suspended from the ciliary body by zonules), Circumference 75 mm and structures anterior to it, viz., iris, cornea and two Volume 6.5 ml aqueous humour-filled spaces : anterior and posterior Weight 7 gm chambers. 4 Comprehensive OPHTHALMOLOGY Fig. 1.1. Gross anatomy of the eyeball. z Anterior chamber. It is bounded anteriorly by z Posterior chamber. It is a triangular space the back of cornea, and posteriorly by the iris containing 0.06 ml of aqueous humour. It is and part of ciliary body. The anterior chamber is bounded anteriorly by the posterior surface of about 2.5 mm deep in the centre in normal adults. iris and part of ciliary body, posteriorly by the It is shallower in hypermetropes and deeper in crystalline lens and its zonules, and laterally by myopes, but is almost equal in the two eyes of the ciliary body. the same individual. It contains about 0.25 ml of 2. Posterior segment. It includes the structures the aqueous humour. posterior to lens, viz., vitreous humour (a gel like material which fills the space behind the lens), retina, choroid and optic disc. VISUAL PATHWAY Each eyeball acts as a camera; it perceives the images and relays the sensations to the brain (occipital cortex) via visual pathway which comprises optic nerves, optic chiasma, optic tracts, geniculate bodies and optic radiations (Fig. 1.3). ORBIT, EXTRAOCULAR MUSCLES AND APPENDAGES OF THE EYE (FIG. 1.4) Each eyeball is suspended by extraocular muscles Fig. 1.2. Poles and equators of the eyeball. and fascial sheaths in a quadrilateral pyramid-shaped  ANATOMY AND DEVELOPMENT OF THE EYE 5 bony cavity called orbit (Fig. 1.4). Each eyeball is z Visceral mesoderm of maxillary process. located in the anterior orbit, nearer to the roof and Before going into the development of individual lateral wall than to the floor and medial wall. Each eye structures, it will be helpful to understand the is protected anteriorly by two shutters called the formation of optic vesicle, lens placode, optic cup eyelids. The anterior part of the sclera and posterior and changes in the surrounding mesenchyme, which surface of lids are lined by a thin membrane called play a major role in the development of the eye and conjunctiva. For smooth functioning, the cornea and its related structures. conjunctiva are to be kept moist by tears which are produced by lacrimal gland and drained by the lacrimal passages. These structures (eyelids, eyebrows, conjunctiva and lacrimal apparatus) are collectively called ‘the appendages of the eye’. DEVELOPMENT OF THE EYE The development of eyeball can be considered to commence around day 22 when the embryo has eight pairs of somites and is around 2 mm in length. The eyeball and its related structures are derived from the following primordia: z Optic vesicle,an outgrowth from prosencephalon (a neuroectodermal structure), z Lens placode, a specialised area of surface ectoderm, and the surrounding surface ectoderm, z Mesenchyme surrounding the optic vesicle, and Fig. 1.3. Gross anatomy of the visual pathway. Fig. 1.4. Section of the orbital cavity to demonstrate eyeball and its accessory structures. 6 Comprehensive OPHTHALMOLOGY FORMATION OF OPTIC VESICLE AND OPTIC STALK The area of neural plate (Fig. 1.5A) which forms the prosencepholon develops a linear thickened area on either side (Fig. 1.5B), which soon becomes depressed to form the optic sulcus (Fig. 1.5C). Meanwhile the neural plate gets converted into prosencephalic vesicle. As the optic sulcus deepens, the walls of the prosencepholon overlying the sulcus bulge out to form the optic vesicle (Figs. 1.5D, E&F). The proximal part of the optic vesicle becomes constricted and elongated to form the optic stalk (Figs. 1.5G&H). FORMATION OF LENS VESICLE The optic vesicle grows laterally and comes in contact with the surface ectoderm. The surface ectoderm, overlying the optic vesicle becomes thickened to form the lens placode (Fig. 1.6A) which sinks below the surface and is converted into the lens vesicle (Figs. 1.6 B&C). It is soon separated from the surface ectoderm at 33rd day of gestation (Fig. 1.6D). FORMATION OF OPTIC CUP The optic vesicle is converted into a double-layered optic cup. It appears from Fig. 1.6 that this has happened because the developing lens has invaginated itself into the optic vesicle. In fact conversion of the optic vesicle to the optic cup is due to differential growth of the walls of the vesicle. The margins of optic cup grow over the upper and lateral sides of the lens to enclose it. However, such a growth does not take place over the inferior part of the lens, and therefore, the walls of the cup show deficiency in this part. This deficiency extends to Fig. 1.5. Formation of the optic vesicle and optic stalk. Fig. 1.6. Formation of lens vesicle and optic cup.  ANATOMY AND DEVELOPMENT OF THE EYE 7 some distance along the inferior surface of the optic In the posterior part of optic cup the surrounding stalk and is called the choroidal or fetal fissure fibrous mesenchyme forms sclera and extraocular (Fig. 1.7). muscles, while the vascular layer forms the choroid and ciliary body. DEVELOPMENT OF VARIOUS OCULAR STRUCTURES Retina Retina is developed from the two walls of the optic cup, namely: (a) nervous retina from the inner wall, and (b) pigment epithelium from the outer wall (Fig. 1.10). (a) Nervous retina. The inner wall of the optic cup is a single-layered epithelium. It divides into several layers of cells which differentiate into the following three layers (as also occurs in neural tube): Fig. 1.7. Optic cup and stalk seen from below to show CHANGES IN THE ASSOCIATED MESENCHYME The developing neural tube (from which central nervous system develops) is surrounded by mesenchyme, which subsequently condenses to form meninges. An extension of this mesenchyme also covers the optic vesicle. Later, this mesenchyme differentiates to form a superficial fibrous layer (corresponding to dura) and a deeper vascular layer Fig. 1.8. Developing optic cup surrounded by mesenchyme. (corresponding to pia-arachnoid) (Fig. 1.8). With the formation of optic cup, part of the inner vascular layer of mesenchyme is carried into the cup through the choroidal fissure. With the closure of this fissure, the portion of mesenchyme which has made its way into the eye is cut off from the surrounding mesenchyme and gives rise to the hyaloid system of the vessels (Fig. 1.9). The fibrous layer of mesenchyme surrounding the anterior part of optic cup forms the cornea. The corresponding vascular layer of mesenchyme becomes the iridopupillary membrane, which in the peripheral region attaches to the anterior part of the optic cup to form the iris. The central part of this lamina is pupillary membrane which also forms the tunica vasculosa lentis (Fig. 1.9). Fig. 1.9. Derivation of various structures of the eyeball.